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Table of Contents
REVIEW ARTICLE
Year : 2021  |  Volume : 4  |  Issue : 3  |  Page : 113-129

Bony calvarial and skull base tumors: Pandora's box


Department of Neurosurgery, National Institute of Mental Health and Neurosciences, Bengaluru, Karnataka, India

Date of Web Publication02-Nov-2021

Correspondence Address:
Dr. Srinivas Dwarakanath
Department of Neurosurgery, II Floor, Faculty Block, National Institute of Mental Health and Neurosciences, Hosur Road, Bengaluru - 560 029, Karnataka
India
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Source of Support: None, Conflict of Interest: None


DOI: 10.4103/IJNO.IJNO_417_21

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  Abstract 


The calvaria or skull vault, in general, has a limited spectrum of oncological disorders which are both unique to it and ubiquitous throughout the body. Lesions of the calvarium may originate from the bony structures or may be secondary to an invasion of scalp-based lesions or brain-based lesions into the skull vault.A PubMed/Medline search was conducted using the MESH keywords “Calvarial tumors”, “Bony tumors”, “Skull base bony lesions” and all reported cases and series were collected, and relevant details extracted from the same. Data were extracted concerning the type of study, the number of cases, follow-up duration, the primary treatment offered, details of adjuvant therapy, overall survival, and recurrence rates where relevant. All data were then tabulated and reviewed. Cases showing relevant information on treatment modalities and their effect on outcomes and survival were only included. A total of 1020 results were found on the PubMed database (until March 2021) and after duplicates removal, a total of 9 studies were found to be relevant for calvarial tumors with 1 case series and 4 retrospective reviews of intraosseous meningiomas which included 2 review papers as well. Most of these results were either retrospective case series or case reports along with a single systematic review of intraosseous meningioma. The primary treatment modality in almost all cases was either surgery or biopsy followed by adjuvant radiotherapy. The skull base and cranial vault is a Pandora's box of malignancies with varied shapes and appearances. Presentation is either due to pressure symptoms or swellings, and whenever accessible, complete excision offers the best management. Imaging findings are of germinal importance and help differentiate lesions, which often decides management. When in doubt, a biopsy may offer the best solution. Bony reconstruction and adjuvant therapy with close follow-up often decide prognosis.

Keywords: Analysis, calvarium, skull vault, tumors


How to cite this article:
Dwarakanath S, Deora H. Bony calvarial and skull base tumors: Pandora's box. Int J Neurooncol 2021;4, Suppl S1:113-29

How to cite this URL:
Dwarakanath S, Deora H. Bony calvarial and skull base tumors: Pandora's box. Int J Neurooncol [serial online] 2021 [cited 2021 Nov 27];4, Suppl S1:113-29. Available from: https://www.Internationaljneurooncology.com/text.asp?2021/4/3/113/329812




  Introduction Top


The calvaria or skull vault, in general, has a limited spectrum of oncological disorders, which are both unique to it and ubiquitous throughout the body. Embryologically, the skull vault and skull base could not be more different. The bony calvaria while formed by intramembranous ossification comprises of flat bones: Both frontal bones, both parietal bones, the squamous part of both temporal bones, and the interparietal part of the occipital bone, in contrast to the skull base (chondrocranium), which is formed by endochondral ossification. This can help identify lesions that may be unique to the skull base, i.e. chordoma and chondrosarcoma. The skull vault is in turn composed of two cortical tables; the inner and outer tables, and the diploe or marrow space between them. Lesions of the calvarium may originate from the bony structures or may be secondary to an invasion of scalp-based lesions or brain-based lesions into the skull vault.

Like any other tumor, the benign and malignant nature of the same needs to be differentiated [Table 1]. In general, benign tumors have well-defined borders with a narrow transition zone; sclerotic margins are frequently present [Table 2]. On the other hand, malignant tumors have poorly defined margins, a wide transition zone, aggressive periosteal reaction, and often have a soft tissue component; these lesions cause dramatic bony destruction with intracranial or extracranial extension. Skull lesions can be lytic or sclerotic and single or multiple with varied composition [Table 3]; they may arise from osteogenic, chondrogenic, fibrogenic, vascular, and/or other elements of bone. They can also be classified as per their pattern of bony involvement: lytic, sclerotic, or transdiploic (with soft-tissue components overcoming calvarial tables) [Table 4]. Finally, pseudo lesions are frequent in this location (e.g. arachnoid granulations, vascular canals) which can lead to confounding findings and a dilemma in diagnosis.
Table 1: Characteristics of benign versus malignant lesions of the skull

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Table 2: Benign skull lesion and their appearance on computed tomography and magnetic resonance imaging

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Table 3: A summary of magnetic resonance imaging characteristics of the pathology for large solitary lytic skull vault lesions in adults

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Table 4: Classification of pattern of bone involvement

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  Methods Top


A PubMed/Medline search was conducted using the MESH keywords “Calvarial tumors”, “Bony tumors”, “Skull base bony lesions” and all reported cases and series were collected, and relevant details extracted from the same. Data were extracted concerning the type of study, the number of cases, follow-up duration, the primary treatment offered, details of adjuvant therapy, overall survival, and recurrence rates where relevant. All data were then tabulated and reviewed. Cases showing relevant information on treatment modalities and their effect on outcomes and survival were only included.


  Results Top


A total of 1020 results were found on the PubMed database (until March 2021) and after duplicates removal, a total of 9 studies were found to be relevant for calvarial tumors with 1 case series and 4 retrospective reviews of intraosseous meningiomas which included 2 review papers as well. Most of these results were either retrospective case series or case reports along with a single systematic review of intraosseous meningioma. The primary treatment modality in almost all cases was either surgery or biopsy followed by adjuvant radiotherapy; however, differences were clear. For ease of understanding, the results were divided among 4 types: pseudolesions, lytic, sclerotic, and transdiploic.

The purpose of this review is for the plebian and patrician neurosurgeon to have a practical understanding of the presentation, imaging findings, and treatment for all calvaria and skull base lesions. This is accompanied by the need for special imaging sequences and adjuvant treatment needed for particular cases.[1]


  Discussion Top


Pseudolesions

These are quite frequent and may be referred to the neurosurgeon for opinion. Thus, knowledge of these pseudo-lesions is necessary as it may prevent unnecessary intervention.

Arachnoid granulations (pacchionian granulations)

These granulations are the most common lesions, which consist of arachnoid granulations protruding into venous sinuses and diploe. They can grow or become hypertrophic with age and appear to be causing mass effect on imaging. They appear as nonenhancing, well-circumscribed bubbly filling defects in venous sinuses or lytic indentations through the inner table with a sclerotic rim when affecting the bone. Computed tomography (CT) and magnetic resonance imaging (MRI) signal corresponds to that of cerebrospinal fluid (CSF). They are usually located near the transverse or superior sagittal sinuses. Such lesions reaching a size of more than 1 cm are termed giant arachnoid granulations (GAGs) which have been previously reported to cause venous hypertension and headaches. Dural arteriovenous fistula can result due to venous hypertension induced by GAG (although this is very rare) and may need symptomatic management.[2]

Meningeal herniations

Herniations can happen for any layer of the brain and are usually seen in areas of devoid vasculature such as the sphenoid wing, lamina cribosa, occipital region, and sphenoid sinus walls. If just comprised of meninges, these may be asymptomatic, but if accompanied by the gliotic brain (encephalocele), they can lead to low-pressure headaches, seizures, or even CSF leaks.[3] They can be atretic in nature[4] containing dysplastic neuronal structures, normally occurring in the parietal midline and often accompanied by other midline structure anomalies. Often, the best management is the repair of the same with excision of the gliotic brain, but this is purely for cosmetic reasons.

Dilated vascular canals

These are normal connections between dural and scalp veins that increase with age. Arachnoid granulations, meningeal herniation, and dilated perivascular canals often coexist in the elderly and may be misinterpreted as multiple lytic lesions, such as myeloma or metastases. Thus, a visual memory of these channels and knowledge of their benign nature is necessary to prevent unnecessary interventions.

Hyperostosis frontalis interna

Hyperostosis frontalis interna is a benign process of unknown etiology, characterized by the thickening of the internal plate of the skull, which most commonly affects the frontal bone. It is much more common in women (16.4% females and 7% males) and correlates with age, obesity, parity, and diffuse idiopathic skeletal hyperostosis.[5]

Bilateral thinning of the parietal bones

Bilateral thinning of the parietal bones is a rare idiopathic entity of unknown origin. This acquired, slowly progressive anomaly found in middle-aged people is more frequently observed in women and is probably associated with osteopenia.[6]

Enlarged parietal foramina

Enlarged parietal foramina are symmetric bilateral congenital bone defects in the parietal calvarium associated with gene deletions in chromosome 11p. They may be isolated or syndromic. If this is the case, they are associated with cortical dysplasia and vascular and skeletal anomalies.[7],[8]

Sinus pericranii

Sinus pericranii is an aberrant vascular connection between intracranial and extracranial veins through a bone defect. It is usually located near the dural sinuses and presents as a soft, palpable scalp mass. This abnormality can be congenital, in which case it is often obvious, but may also be acquired through trauma or iatrogenesis and might be subtle in these cases. Identification is vital to avoid harmful biopsy.

Lytic lesions

An algorithmic approach for diagnosing lytic skull or skull base lesions is ideal when faced with such a case [Figure 1]. Apart from the obvious deformity, these cases may present with pressure symptoms due to the mass effect on critical structures such as nerves or vessels.
Figure 1: An algorithmic approach to lytic bony lesions of the skull and skull base

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Fibrous dysplasia

Fibrous dysplasia is a bone disease of unknown origin that can occur in monostotic or polyostotic forms. In either form, the disease may involve the skull, with a preference for the frontal, ethmoid, sphenoid, and maxillary bones. The presenting symptoms are most commonly facial deformities, proptosis, or headaches. Malignant or sarcomatous degeneration can occur in fibrous dysplasia and has been reported to occur in 0.5% of cases. Visual disturbance occurs in a significant number of cases [Table 5] and along with hearing impairment, is the most common neurological deficit caused by fibrous dysplasia.
Table 5: Cases of fibrous dysplasia presenting with visual loss reported in literature

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In the polyostotic form, it may be a part of the McCune-Albright spectrum (MAS), which is the result of an early embryonic postzygotic somatic activating pathogenic variant in GNAS (encoding the cAMP pathway-associated G-protein, Gsα), is characterized by involvement of the skin, skeleton, and certain endocrine organs. However, because Gsα signaling is ubiquitous, additional tissues may be affected.[9] In most individuals, the diagnosis of MAS is based on the finding of two or more typical clinical features. In individuals whose only clinical finding is monostotic fibrous dysplasia, identification of a somatic activating pathogenic variant in GNAS by molecular genetic testing is required to establish the diagnosis. Variant detection depends on the level of mosaicism in the tissue and the sensitivity of the technique.

Surgery is regarded as the treatment of choice for preventing or arresting visual loss in cases of fibrous dysplasia. Specifically, surgery to decompress the optic nerve within the optic canal has been recommended, both for the treatment of visual loss once it has occurred and prophylactically to prevent visual loss. This can be achieved both via a craniotomy [Figure 2] and decompression or via a more direct endonasal approach.
Figure 2: (a) Plain computed tomography scan axial cuts show diffuse enlargement of the fronto-orbital region with compression of the optic nerve foramina; (b and c) Axial and sagittal plain computed tomography scans showing decompression of the optic nerve

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Epidermoid and dermoid cysts

These are benign cutaneous tumors that usually arise at birth and result due to the entrapment of the surface ectoderm along the lines of embryonic fusion. Dermoid cysts are mature teratomas lined with epithelium that may contain various mature tissue-types, typically located near the midline. The skull and scalp are common locations and children may present with a swelling early in their childhood or young adult lives and these lesions comprise about 15.4%–58.5% of all scalp and skull masses in pediatric patients. Since these are benign lesions, conservative management may be considered, though malignant transformation and symptoms due to their expansive nature often necessitate excision. Once the case presents with a swelling, imaging can reveal these lesions to arise from the bone itself or affect it by contiguity from the adjacent intra- or extracranial tissues. They are well-defined, expansile lesions that can cause adjacent non-aggressive bone lysis or re-modeling. The cystic content of the epidermoid can be heterogeneous with calcification and bleeding, but its features are often similar to a fluid, with the most salient finding being intense diffusion restriction. Dermoid characteristically contain fat and will be bright on T1 images. In the largest reported series to date of 234 cases,[10] Prior et al., have elucidated some important findings. Dermoid and epidermoid cysts tend to present early in life, as 81.6% of our patients were younger than 3 years of age and 89.7% younger than 5 years of age. There may be slight female preponderance. The most common locations were the frontal, occipital, and supraorbital regions. Midline scalp cysts present a higher probability of intracranial extension because they are on a line of embryonic fusion. The rate of malignant tumors including metastatic. ones, ranged from 0%–25% in different series[10] and though rare, malignancies may develop into long-lasting scalp dermoids.[11],[12]

Eosinophilic granuloma/Langerhans cells histiocytosis

Histiocytic disorders are a group of diseases that occur when there is an over-production of white blood cells known as histiocytes that can lead to organ damage and tumor formation. Langerhans cell histiocytosis (LCH) and Rosai–Dorfman disease comprise the spectrum of histiocytic disorders. In both diseases, the diagnosis is made microscopically, the key features in the former being the Langerhans cells and in the latter, the emperipolesis. They have characteristic immunohistochemical and ultrastructural features.

LCH is a focal pseudo tumoral growth of clonal cellular elements in the reticuloendothelial cell system.[13],[14] LCH starts in early childhood with the clonal proliferation of dendritic histiocytes. The incidence of LCH is reported to be 2–20 cases per one million pediatric population.[15] The pathogenesis of LCH is still unclear. There is debate as to whether LCH is an inflammatory disorder or a tumor, further complicating the optimal treatment strategy. The disease has an excellent prognosis in the localized form, but patients with multisystemic involvement carry a guarded prognosis. Currently, “LCH” remains the preferred nomenclature, although new terminology may evolve when the precise origin of the CD-207+ cells in LCH is fully understood. The historical terms histiocytosis-X, Letterer-Siwe disease, Hand-Schüller-Christian disease, and diffuse reticuloendotheliosis should be abandoned. The term “eosinophilic granuloma” is sometimes used to describe the pathology of an individual lesion, particularly isolated lytic processes in bone.[14] The central nervous system (CNS) is rarely involved, and the LCH involvement is frequently limited to the skeleton only. Intraparenchymal involvement is rare and tends to be limited to the hypothalamus and posterior pituitary.[15],[16] In LCH, the most commonly involved organs are bone, skin, and pituitary in 80%, 33%, and 25% of patients, respectively. Skull bones are most frequently involved in 20% to 40% of cases [Table 6].[13],[14],[15],[16],[17],[18],[19],[20],[21],[22],[23],[24],[25],[26],[27] The CNS involvement is mostly in the spectrum of multisystemic involvement and with a male preponderance. Involvement of the temporal bone has been described in 15% to 61% of patients, usually in association with multisystemic involvement. The typical age of diagnosis of LCH is 5–15 years, and the disease is characterized by osteolytic lesions in mono or multifocal involvement.[16]
Table 6: Compilation of studies for the treatment of Langerhans cell histiocytosis reported in English and Japanese literature

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On imaging, endosteal scalloping (or erosions) and slight expansion beyond inner and outer tables determine the characteristic “beveled edges” and “hole within a hole” signs. A central “button sequestrum” is classically described. A telltale somewhat lenticular morphology, craniocaudally oriented, may also be seen. MRI shows a T2-hyperintense lesion with variable degrees of contrast enhancement, and possible bone and soft tissue edema.

The different treatment options of chemotherapy and radiation therapy have never been compared directly in any prospective or retrospective trial. Cladribine has been found effective for CNS lesions. LCH demands a multidisciplinary team approach for its management. Usually, the disease extent is evaluated and divided into single system LCH, low-risk multisystem LCH, and multisystem LCH with risk organ (bone marrow, liver, and/or spleen) involvement. The term “CNS-risk” areas include the mastoid, sphenoid, orbital, ethmoid, or temporal bones and denote increased risk of involvement of the CNS. Thus, a child with a single non-CNS-risk skull lesion will have a low recurrence rate (4%–10%) following curettage. However, single skull lesions of the orbit, mastoid, temporal, or sphenoid bones (CNS-risk) may be better served with a combination of vinblastine and prednisolone,[28],[29],[30],[31] although bone marrow suppression remains a valid concern. Cases with pure CNS involvement may present as white matter lesions, endocrine abnormalities, or neurodegenerative syndrome. It will need chemotherapy that crosses the blood-brain barrier, resection, radiation therapy, or a combination of these. Although not compared in a randomised control trial (RCT), chemotherapy remains the first choice with Cladribine, Clofarabine, Cytarabine, and MAP2K inhibitors being tried empirically.

Intraosseous hemangioma

Primary hemangioma is most commonly seen in the spine though it may be present in the skull as well. It may be divided into three types: cavernous, capillary, and mixed. In the skull, the cavernous type is the most common comprising 10% of all benign tumors and 0.2% of cranial tumors of the skull.[32] It can also present in other parts such as the clivus, frontal, parietal, or petrous bone and the symptoms will vary with the location. However, the frontal bone is the most common with 44.1% occurring in this area.

Cavernous hemangioma may be an inherited disease, and a mutation in KRIT1/CCM1 gene which encodes a protein that affects endothelial cell structure and function is correlated with the pathogenic mechanisms of cavernous hemangioma.[33] Recent studies have reported that trauma and ionizing radiation, particularly treatment with stereotactic radiosurgery for brain tumors, may be related to the development of cavernous hemangioma. Hemangiomas of the skull typically occur in patients in the fourth and fifth decades of life, and the incidence rate in men and women is approximately 1:2–3.[34] They originate from the plate barrier and spread along both sides of the barrier, which is indicative of an osteolytic mass. Hemangiomas of the skull are predominantly solitary, and rarely invade the entire skull [Figure 3]. Cavernous hemangiomas consist of clusters of dilated blood vessels, which are separated by fibrous septa, whereas capillary ones are rich in small vascular channels without a significant number of fibrous septa. Radiological evaluation includes standard skull X-rays, CT scans, and MRI. X-ray is the most basic method of examination, while CT is the most commonly used method of inspection. However, hemangiomas of the skull do not have typical radiological features. Nonenhanced CT typically reveals a mass, which represents an osteolytic lesion, with intense enhancement after intravenous (IV) contrast administration. CT consists of pronounced thickened trabeculae (”polka-dot” or “spoke-wheel appearance”). MRI better demonstrates contrast enhancement and tissue characterization with internal T1 and T2 hyperintensities due to fat and vascular content, respectively. Although rare, the enlarged bony trabeculae may grow beyond the cortical bone, producing a hair-on-end-like periosteal reaction or even dural or soft tissue contrast-enhancing components, resembling a more aggressive or malignant lesion.
Figure 3: (a-c) T1 coronal contrast enhanced, T2 coronal and axial MRI images showing a T2 hetero-intense lesion enhancing on contrast in the parietal region; (d) postoperative plain computed tomography scan axial images showing a wide margin of excision with replacement with mesh

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For a single mass, complete resection is beneficial, and the cosmetic results of most cases were satisfactory and revealed no recurrence within a short period.[35] Embolization combined with craniotomy may be the superior treatment in large hemangiomas. However, multiple lesions may require combined radiotherapy, particularly for diffuse cranial hemangiomas. Gene detection in cases of hemangioma of the skull could be a prominent topic in future studies.

Aneurysmal bone cyst

An aneurysmal bone cyst (ABC) is a rare vascular and benign tumor-like lesion of bone.[36] They account for 1%–2% of all primary bone tumors and cranial location is described in 3%–6% of all cases, and are very rare in the skull base. ABCs are fibro-osseous lesions that occur predominately in children. ABCs are nonneoplastic expansile bone lesions characterized by cystic cavities, usually filled with blood. These lesions can expand rapidly, resulting in cranial nerve palsy, pain, or other changes.

They can present with myriad symptoms like ptosis, loss of vision, cranial nerve palsies, signs of raised intracranial pressure, seizures, and cerebellar signs. Rarely, spontaneous intracerebral hemorrhage has been described. Axial CT head can reveal a multi-loculated, expansive, osseous lesion with occasional fluid levels that may enhance peripherally after contrast. MRI findings include multiple fluid levels within multiple cysts resulting from unclotted blood, separate from the soft tissue and medullary bone. Some cysts appear hyperintense on T1-weighted sequences due to methemoglobin.

Staging of ABCs according to Enneking classification is:[37]

  • Stage I (latent), ABC remains static or heals spontaneously
  • Stage II (active), grows progressively but without cortical destruction
  • Stage III (aggressive), progressive growth with cortical destruction.


Pathologically, ABC usually involves both the inner and outer tables of the skull symmetrically and almost always has intracranial extension. ABC may be divided into the classic (containing cysts; 95%) and solid (5%) varieties.Microscopically, ABCs appear as blood-filled cavernous spaces with a paucity of endothelial cells, separated by septae composed of spindle-celled fibrous tissue. En bloc resection is the treatment of choice with the lowest risk of recurrence. Reported recurrence rates vary from 20% to 70% with incomplete resection. But, the primary challenge in the management of cranial ABCs involves the inherent difficulty in accessing and completely excising the lesion, especially involving the skull base, orbit, and paranasal sinuses. In these cases, partial excision or intralesional curettage with adjunctive therapies, such as embolization, cryotherapy, or radiotherapy, should be considered.

Giant cell tumor/osteoclastoma

Giant cell tumor (GCT) in a primary intramedullary neoplasm and comprises 5% of skeletal tumors and is composed of numerous multinucleated osteoclast-like giant cells scattered throughout a mass of ovoid or spindle mononuclear stromal cells.[38] It is considered a benign tumor and is locally destructive due to the osteolytic properties of osteoclast-like giant cells that express the markers involved in bone resorption activity. Although the giant cells are a significant part of this tumor, the stromal cells constitute the actual neoplastic component.

Among 3902 GCT cases in three large series,[39],[40],[41] only 20 occurred in the skull. In the skull base, among 104 case reports from 1969 to 2017, most of the cases were localized to the sphenoid (49 cases) and then clivus (12 cases).[38] This may be because, like long bones, the sphenoid and the temporal bone are generated through endochondral bone formation; whereas, the other skull bones are produced by an intramembranous formation. Due to the location, these cases may present with multiple cranial nerve involvement with subsequent headache, decreased vision, visual field defect, diplopia, ophthalmoplegia, and deafness. Imaging shows a solid or mixed solid-cystic mass with bone expansion, prominent cortical thinning (often partially broken or disappeared), and well-defined but non-sclerotic margins. Solid components tend to be NECT hyperdense and T2 hypointense attributed to hypercellularity, hemorrhage, or fibrous matrix. The tumor is highly vascularized and shows intense contrast enhancement. An associated secondary ABC should be suspected when cystic areas are found. The gold standard for the management of this tumor contemplates surgical resection, followed by adjuvant treatment with chemotherapy or radiotherapy. However, no effective chemotherapeutic agents have yet been identified and some patients developed osteosarcoma transformation at sites of previous irradiation. To date, only one case of clival GCT has been treated with Denosumab after surgical resection by endoscopic endonasal transsphenoidal surgery and the treatment proved to be effective in preventing tumor growth.[42]

Metastasis

These are the most prevalent neoplastic bone lesions especially in breast and prostate cancers, in which there is a prevalence of 70% in postmortem studies.[43] Prior cancer history is usually forthcoming and symptoms shall range from pain to a combination of cranial nerve or lobar symptoms. Skull base metastasis are rare tumors that comprise about 4% of all malignant neoplasms and may be the presenting feature in 1/3 of tumors. Primary cancers comprise breast (20%–30%), prostate (14%–38.5%), lymphatic system (8%), lung (6%) and others (27%).[44] Symptomatology can be divided as per Laigle-Donadey et al.[45] into six different types of syndromes: orbit (12.5%), parasellar and sellar (29%),  Gasserian ganglion More Details (6%), jugular foramen (3,5%), occipital condyle (16%), and others (33%).

The initial workup should include positron emission tomography-CT/MRI of the whole body for accurate staging, along with CT and MRI of the lesion itself. The lesions are usually focal, lytic (with varying degrees of bone destruction and soft-tissue components), narrow zones of transition, and periosteal reaction. Expanding lytic (or blowout) metastases are typical of renal-cell or thyroid papillary carcinomas. Permeative patterns of bone destruction with possible disproportionate soft-tissue components may be observed in highly cellular, small-round cell tumors that can grow through the Haversian canals (PNET, small-cell lung cancer). Hypervascular tumors such as renal-cell, thyroid cancer, melanoma, and hepatocarcinoma may present avidly contrast-enhancing and highly perfused metastases. Finally, prostate and breast tumors are the usual suspects with blastic metastases. In some cases, a biopsy can be made to confirm the diagnosis.

Treatment usually involves complete excision. In inaccessible locations and where the primary is under control, radiotherapy/radiosurgery may be given as adjunctive treatment after surgical excision of the accessible portions. Gamma-Knife radiosurgery is often used as a secondary treatment after surgery (residual tumor) or when the tumor size is relatively small and multiple. It has demonstrated a good efficacy in the control of tumor growth attaining a good functional status in about 61%–67% of the cases. However, complete surgical resection remains the best and most decisive modality of management whenever possible and the Karnofsky performance scale (cut-off 70) remains a good yardstick for the same.

Myeloma/plasmacytoma

This is a part of the larger spectrum of round cell tumors which can affect the skull vault or base. Myeloma is a hematological disorder that generally affects elderly patients (median age 68–70), rarely those under 40. It accounts for 1%–2% of all cancers and around 17% of hematologic malignancies, with an annual incidence of 4–5 cases per 100,000. It most frequently presents with multifocal pain and has a poor prognosis. The most common and well-known imaging pattern of bone involvement in myeloma is multiple punched-out lytic lesions. However, the disease can also present with diffuse osteopenia-like medullary bone-infiltration; exceptionally, and exclusively associated with POEMS syndrome, the lesions can be sclerotic. On CT, the lesions are slightly dense intrinsically. MRI demonstrates intermediate signal and often markedly restricted diffusion, with avid contrast-enhancement. The axiom that “multiple aggressive lytic lesions in the elderly patient are myeloma or metastases until proven otherwise” is a well-known medical postulate. Some authors add that multiple metastases rarely affect the mandible, in contrast to multiple myeloma.

Cytological studies, either by a biopsy (excisional/incisional) or needle, will display CD138-positivity, a marker seen in most CNS myeloma cases. A high percentage of proliferation marker Ki-67 may also be seen which is suggestive of an aggressive growth pattern and has been associated with an increased risk of extraosseous relapse of myeloma. From our extensive review of literature, we could not define a standard guideline for treatment and usually, a multimodal approach is attempted. Systemic treatment, alone or combined with radiotherapy, resulted in a significant improvement of survival in patients when compared to no systemic therapy. Intrathecal agents have been used in CNS MM with conflicting results, as intrathecal agents are often used in combination with systemic therapies, and to this date, did not prove to be as efficient as monotherapy. Whole-brain radiation is another therapeutic option in CNS MM but its practical application is limited due to associated toxicity.[46]

Sclerotic lesions

These usually include either osteomas, osteosarcomas, or blastic metastasis. Prostate and breast tumors have been classically associated with blastic metastases, but neuroendocrine tumors, urothelial carcinomas, and small-round cell tumors should also be considered.

Osteoma

Cranial osteomas are benign tumors that form in mature osseous tissues,[47] usually arising from the inner or outer table of the skull. Intracranial osteomas located in the subdural or subarachnoid space and without the involvement of the bony structure are very rare. In a recent review by Li et al.,[47] a total of 18 such lesions [Table 7] were found to have an intracranial extension. The age of intradural osteoma patients ranges from 16 to 64 years (mean ± standard deviation: 42.2 ± 15.8 years) and most are female (female: male ratio of 5:1). Although the ethnicity of the patients is rarely mentioned, it is interesting that the majority of case reports (14 cases) were from Asian countries. Headache or dizziness was the most frequent complaint.
Table 7: All cases reported in literature of osteoma with intracranial extension[47]

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High-density appearance on CT [Figure 4] with a lucent dural line at the bone window of CT indicates the dura between the skull and the osteoma, which can differentiate an intradural osteoma from meningeal ossification. MRI shows hyperintensity of the lesion on T1-weighted imaging indicating the existence of adipose tissue within the mass. The gold standard treatment of intradural osteoma remains surgical excision.
Figure 4: (a) Plain computed tomography scan axial images showing intensely hyperintense lesion of the frontal bone with mass effect; (b) Plain computed tomography scan axial images showing a wide margin of excision with replacement with mesh

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Osteosarcoma

Primary osteosarcoma of the skull is rare, with an incidence of only 1%–2% of all skull tumors. Fewer than 150 cases have been reported. Reports on the cases of primary pediatric osteosarcoma of the skull are even rarer with approximately 15 cases reported. Approximately 6% of osteosarcomas occur in the head and neck region, with the majority occurring in the mandible. The known risk factors include bone dysplasias including Paget's disease, fibrous dysplasia, enchondromatosis, and hereditary multiple exostoses, in addition to Li–Fraumeni syndrome (germline TP53 mutation), retinoblastoma, and Rothmund–Thomson syndrome. The only known environmental risk factor is radiation exposure [Figure 5]. Unlike extremity tumors, skull osteosarcomas are often painless. These patients frequently present with headaches, cranial nerve palsies, exophthalmos, visual impairments, or increased intracranial pressure. Generally, osteosarcoma appears as a bone-forming rather than a lytic lesion. Although the methods of treatment are widely varied, surgical resection is still the mainstream. Complete surgical excision with wide surgical margins have been associated with improved survival; however, some studies have reported that chemotherapy increased survival rates by 5 years for patients with localized tumors, from 20% to 60%–70%. Nonetheless, low-grade osteosarcoma can be treated with wide resection without chemotherapy.[48]
Figure 5: (a): Axial cuts of contrast enhanced MRI showing an enhancing dural based lesion of the left frontal bone and underlying dura; (b) Axial cuts of contrast enhanced MRI showing the area of previous radiation exposure for an anaplastic oligodendroglioma operated 7 years back; (c) Plain computed tomography scan axial images showing a wide margin of excision with replacement with mesh

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Ewing sarcoma

Dr. James Ewing first defined EWS in 1921 as a hemangioendothelioma of the bone. It is a malignant bony tumor in patients in the first two decades of life, second only to osteosarcoma.[49] Primary calvarial involvement is seen in <1% of cases. Compared with primary calvarial involvement, secondary from other sites are more common (10%–30%). The most commonly affected age group is 5–13 years. 90% of primary cranial EWS patients are under 20 years of age. It commonly presents with localized, painless skull swellings and sometimes with lymphadenopathy. Intratumoral hemorrhage is seen in some cases of EWS. We suggest fine needle aspiration cytology from lymph nodes or the tumor itself for diagnostic purposes.

Parietal and frontal bones are commonly involved, while occasionally mastoid, occipital, and sphenoid bones may also get involved. Rarely, EWS may diffusely affect the anterior skull base in a carpeting fashion, clinically leading to multiple cranial nerve involvement. CT scan may show focal osteoblastic activity with the erosion of inner or outer tables. MRI shows the tumor's heterogeneous structure, with evidence of solid and cystic areas, hemorrhage, and necrosis [Figure 6]. The management of primary EWS involves a multimodal treatment protocol. Total or subtotal excision [Figure 7] followed by postoperative chemotherapy and radiotherapy has improved the overall results with a 2–5-year progression-free survival of 50%–80%.[50]
Figure 6: (a): Axial cuts of contrast enhanced MRI showing an enhancing dural based lesion of the left temporal region; (b) Coronal T2 cuts of MRI showing the extradural nature of the lesion with heterogenous hyperintensity

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Figure 7: (a): Axial cuts of T2 weighted images of MRI showing no residue on follow-up imaging (b) Coronal T2 cuts of contrast enhanced MRI showing no residue on follow-up imaging

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Transdiploic lesions

These lesions include primarily dural-based lesions which can invade the bone such as meningiomas, hemangiopericytomas, or lymphomas [Table 8]. Other described examples are metastasis or plasmacytomas.
Table 8: Magnetic resonance spectroscopy and perfusion findings in case of transdiploic tumors

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Meningioma

Primary intraosseous meningioma is best defined as a meningioma with the largest component contained within the calvarium with no or only minimal dural component. In a recent systematic review of intraosseous meningiomas,[51] 111 patients were found with 58% females and 42% males. The mean patient age was 51 years and the frontal bone was the most common tumor location, occurring in 26.1% of the cases. Surgical resection was the predominant modality of treatment in 97.2% of the cases, and gross total resection was achieved in 84% of cases that reported the extent of resection. There were no recurrences for grade I meningiomas. However, all grade III meningiomas recurred and 33.3% of grade II meningiomas showed recurrence within a mean postoperative follow-up interval of 20 months.

Overall, meningiomas are the most common primary brain tumors with 33.8% of all primary brain and CNS tumors, with a (2.3:1) female predilection, and a median age at diagnosis of 65 years. Intradiploic and primary extraosseous meningiomas, however, account for <2% of all meningiomas. Neurofibromatosis type 2 and schwannomatosis predispose to multiple meningiomas. They may also present as large primarily intracranial lesions with bony involvement [Figure 8] and need extensive bony removal [Figure 9] and [Figure 10] and reconstruction. Despite the increasing use of stereotactic and fractionated radiosurgery in neuro-oncology, surgical resection is still the predominant modality that leads to sustained benefit and reduced recurrence rates.
Figure 8: (a) Coronal T2 cuts of contrast enhanced MRI showing a large parasagittal meningioma with transdural extension and bony hyperostosis, with involvement of the contralateral side and sinus, as MR venogram (b) is showing no flow and the development of venous collaterals

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Figure 9: Sagittal cuts of T2 weighted images of MRI showing (a): Preoperative appearance of the meningioma with transdural extension and bony hyperostosis with involvement of the sinus; (b) Appearance after excision and mesh artifacts

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Figure 10: (a) Postoperative contrast enhanced coronal cuts of MRI showing residue in the sinus which was left intentionally and will be subject to radiosurgery; (b and c) Intraoperative picture of the bony hyperostosis with a wide craniotomy done followed by reconstruction using parent bone and mesh

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Hemangiopericytoma

Hemangiopericytoma or solitary fibrous tumor is a rare dural-based lesion (which is often a differential diagnosis for meningiomas) accounting for <1% of all CNS tumors, with a median age of 40–60 years, with a slight male predilection. On imaging, it presents as an extraaxial, avidly enhancing mass, with flow voids and a possible “dural tail.” This appearance makes it difficult to differentiate it from the much more common meningioma. However, unlike the latter, hemangiopericytoma does not calcify and does not present with hyperostosis since bone involvement is purely lytic. When the dural-base is narrow, it shows the classical “mushroom” appearance. HPCs are more aggressive, tend to recur even after gross total resection, and occasionally have extracranial metastases. Unlike meningiomas, which frequently show hyperostosis of the adjacent bone and may have intratumoral calcifications on CT scans, these tumors occasionally have bone erosion and lack calcifications and hyperostosis. Complete excision with adjuvant radiotherapy and a close follow-up for recurrence and metastasis is the standard of care.[52]

Lymphoma

Lymphoma can affect all ages, with a peak incidence between 50 and 60 years of age, while it is exceptional under 10 years of age. Primary bone lymphoma accounts for 7% of all bone malignancies and 5% of all extranodal lymphomas. Imaging characteristics of calvarial lymphoma are those of small-round cell tumors. It can be lytic, sclerotic, or mixed, but most specifically, permeative with transdiploic spread through the Haversian canals, with little or no bone destruction and with a disproportionate soft tissue component. Soft tissue component tends to be CT hyperdense, T2WI hypointense, and highly diffusion-restricting, due to hypercellularity. Contrast enhancement is intense and homogeneous. Among the reported cases in literature, the average patient age was 60 years (ranging from 13 to 84 years). The presentation was a subcutaneous scalp mass (90%), seizures (10%), focal neurological deficit (20%), headache (30%), and proptosis (10%). All of the bones from the cranial vault were affected, but the parietal area was more predominantly involved (37.5%). When the diagnosis of cranial vault lymphoma is confirmed, a thorough physical examination, complete blood count, CT of the chest and abdomen, and bone marrow biopsy must be performed to exclude systemic involvement. Extranodal disease limited to a single site responds favorably to local treatments such as surgery or focal radiation. If complete resection is achieved, one can consider clinical and radiological follow-up without any additional treatment. Multimodality treatment consisted of cranial radiotherapy (total doses ranged from 30 to 50 Gy) associated with chemotherapy.[52]

Chordomas and chondrosarcomas

Chordomas and chondrosarcomas are rare bony tumors of the skull base with a combined annual incidence of approximately 1/100,000 and a peak incidence in the fourth to sixth decades of life.[53] Despite differences in origin and prognosis, their similar anatomical locations, clinical presentation, and radiological findings often lead the two conditions to be considered together [Figure 11]. Chordomas arise from the remnants of primitive notochord at the spheno-occipital synchondrosis, in comparison to chondrosarcomas which arise from mesenchymal cells or the embryonic rest of the cartilaginous matrix of the cranium. A histological distinction can be made with immunohistochemistry as chordoma is positive for epithelial markers cytokeratin and epithelial membrane antigen, whilst chondrosarcomas are negative for both. Radiologically, chordomas tend to be centered on the clivus, with chondrosarcomas typically centered on the petro-occipital fissure. MRI is the best technique to assess the soft tissue extent of the tumor and visualize its dural extension. Chordomas and chondrosarcomas are typically heterogeneously bright on T2-weighted images, which can aid in their diagnosis. Most tumors are associated with heterogeneous contrast enhancement. CT scan is more effective for demonstrating bone lesions. Calcification is not uncommon with a chondrosarcoma. The characteristic ring-forming calcifications seen on CT scans can be correlated with the histologic pattern of calcification. Diffusion-weighted MRI may aid diagnosis. Both tumors tend to be locally invasive, and given their proximity to the important structures of the skull base, they can present a serious management challenge. Due to their rarity and the limitations of the published literature to case series, detailed risk stratification and prognostic data are not available. However, the prognosis is reasonable with a 5-year survival of 65% for chordomas and 81.8% for chondrosarcomas. Surgery is the main treatment modality with several studies suggesting that maximizing resection reduces the risk of recurrence and improves survival. Complete resection is challenging due to the locally aggressive nature of these tumors and the proximity to important skull base structures. Adjuvant postoperative radiotherapy is usually administered, with a further reduction in recurrence. This may take the form of fractionated high-dose photon therapy, proton beam therapy, or hypofractionated stereotactic modalities.[54]
Figure 11: (a) Plain computed tomography sagittal cut showing a hyperintense lesion forming a base on the clivus with extension in the retropharyngeal space; (b and c) Magnetic resonance contrast enhanced and T2 weighted sagittal cuts showing heterogenous enhancement with T2 hyperintensity of the lesion

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Miscellaneous lesions

Paget's disease

These occur frequently in elderly males and can present as inflammation or erythema. Imaging via CT scans shows cortical bone expansion and thickening, as well as the coexistence of lytic and sclerotic lesions in variable degrees depending on the phase of the disease (osteoclastic vs osteoblastic). Osteoporosis circumscripta cranii is considered an early stage (lytic phase) of the disease with a characteristic imaging pattern consisting of geographic, large, patched, radiolucent areas involving both medullary and cortical bones, and usually located in frontal and occipital regions. Medical treatment has been recommended by the Endocrine Society and the European Society of Endocrinology guidelines for patients with an active disease with a risk of future complications. The current recommended treatment is a single 5-mg IV dose of zoledronate for patients without contraindications In some cases, this may lead to pressure symptoms causing hydrocephalus which can be managed either by shunt placement or excision of the lesion.[55]

Osteopetrosis

Osteopetrosis is an uncommon hereditary disorder that results from defective osteoclasts. Bones become sclerotic and thick while being weak and brittle. There are many classifications, but the classically known “benign” or “adult” osteopetrosis (an autosomal dominant variant) is the less severe form of osteopetrosis. A subtype of this entity (often known as type 1) usually affects the cranial vault with diffuse thickening and sclerosis resulting in the typical appearance of “bone within a bone.” Clinically, it often presents with cranial nerve compressive palsies and may need decompression.

Osteomyelitis

Ostemomyelitis or Pott puffy tumor is a subperiosteal abscess due to associated frontal skull osteomyelitis [Figure 12]. This usually occurs as a rare complication of frontal sinusitis or trauma to the forehead that presents with a forehead tender swelling associated with fever, headaches, nasal discharge, or symptoms of increased intracranial pressure. Including this in the differential diagnosis of patients with this presentation is important. Broad spectrum antibiotics initially followed by culture specific ones are the treatment of choice. The options include penicillin or vancomycin, third generation cephalosporin, and metronidazole. The length of treatment varies but is prolonged and includes 4–8 weeks of IV antibiotic therapy, including in those patients who had surgery performed. Some small extradural collections are often treated with IV antibiotics, but surgical excision of the infected area is highly recommended. This reduces both the infectious load and gives a good culture specimen too.
Figure 12: (a) Computed tomography axial bone window showing destruction of the bone of the left frontal region with islands of normal bone in between, suggestive of osteomyelitis of the skull secondary to trauma; (b) Postoperative images after removal of the infected bone

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  Conclusion Top


The skull base and cranial vault is a Pandora's box of malignancies with varied shapes and appearances. Presentation is either due to pressure symptoms or swellings, and whenever accessible, complete excision offers the best management. Imaging findings are of germinal importance and help differentiate lesions which often decides management. When in doubt, a biopsy may offer the best solution. Bony reconstruction and adjuvant therapy with close follow-up often decide prognosis.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.



 
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    Figures

  [Figure 1], [Figure 2], [Figure 3], [Figure 4], [Figure 5], [Figure 6], [Figure 7], [Figure 8], [Figure 9], [Figure 10], [Figure 11], [Figure 12]
 
 
    Tables

  [Table 1], [Table 2], [Table 3], [Table 4], [Table 5], [Table 6], [Table 7], [Table 8]



 

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