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  In this Article
 »  Abstract
 »  Skull Vault Char...
 » Osteoma
 » Osteoid Osteoma
 » Osteoblastoma
 »  Hemangioma/venou...
 »  Hemangiosarcoma/...
 »  Dermoid and Epid...
 »  Langerhan Cell H...
 » Tuberculosis (Tb)
 »  Bacterial, Funga...
 » Sarcoidosis
 » Fibrous Dysplasia
 » Metastases
 » Multiple Myeloma
 » Ewing Sarcoma
 » Paget Disease
 » Conclusion
 »  References
 »  Article Figures
 »  Article Tables

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Table of Contents    
Year : 2020  |  Volume : 68  |  Issue : 4  |  Page : 732-740

Skull Imaging-Radiographs and CT revisited

1 Department of Musculoskeletal Radiology, The Royal Orthopedic Hospital, Bristol Road South, Northfield, Birmingham, UK
2 Department of Radio Diagnosis, Global Superspeciality Hospital, Vijayawada, Andhra Pradesh, India
3 Department of Radiology, Nizam Institute of Medical Sciences, Hyderabad, Telangana, India

Date of Web Publication26-Aug-2020

Correspondence Address:
Dr. Rajesh Botchu
Department of Musculoskeletal Radiology, The Royal Orthopedic Hospital, Bristol Road South, Northfield, Birmingham
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Source of Support: None, Conflict of Interest: None

DOI: 10.4103/0028-3886.293481

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 » Abstract 

Although rare, skull vault lesions include a vast array of pathology encompassing infection, benign, and malignant bone tumors. Given the large range of potential diagnoses, it is crucial to identify imaging features to differentiate one from another, ensuring early diagnosis. Radiographs are still valuable in modern radiology but have largely been superseded by computed tomography (CT) due to its high spatial resolution. Both are especially important in developing countries where access to magnetic resonance imaging (MRI) may be limited.There are currently several publications outlining imaging appearances of skull vault lesions. However, the majority of literature is dated, with the last dedicated textbook published in 1980 (Principles of X-ray diagnosis of the skull). Despite overlapping features, a few lesions have “aunt minnie,” type classical characteristics, which we will highlight. Most vault lesions also appear as a spectrum depending on location and the exact stage of the disease. A small subset within each disease entity also has atypical features not widely discussed in the current literature. In this pictorial review, we hope to focus on radiographic and CT imaging appearances to help differentiate between various skull vault lesions.

Keywords: Radiographs, skull, vault
Key Messages: Skull vault lesions are frequently encountered on plain radiographs. In the majority, an exact diagnosis or limited differential can be given based on location and morphology. CT is useful in evaluating the degree of bone destruction, skull tables and diploe involvement as well as internal matrix characteristics.

How to cite this article:
Rajakulasingam R, Botchu R, Vemuri VN, James SL, Subbarao K, Davies A M. Skull Imaging-Radiographs and CT revisited. Neurol India 2020;68:732-40

How to cite this URL:
Rajakulasingam R, Botchu R, Vemuri VN, James SL, Subbarao K, Davies A M. Skull Imaging-Radiographs and CT revisited. Neurol India [serial online] 2020 [cited 2021 May 18];68:732-40. Available from:

Skull vault lesions are typically detected incidentally on imaging or as part of staging for alternative pathology. Even though the use of radiographs is declining with the advent of computed tomography (CT), they can be useful in identifying four key features: skull vault thickening or thinning, bony sclerosis or lysis, focal or generalized nature, and finally singularity or multiplicity of lesions. Radiographs are also commonly used in the setting of long term follow up. CT will further add to this information and can better assess extension, matrix pattern, and pathognomonic patterns. Magnetic resonance imaging (MRI) is ideal for assessing the bone marrow and diploic space involvement. In this pictorial review, we will focus solely on radiographic and CT appearances. As most vault lesions are either sclerotic or lucent, we will focus on differentiating features and pathognomonic patterns to aid diagnosis.

The skull vault is composed of a calvarium made of two cortical layers, the inner and outer table with an intervening diploic space [Figure 1]. The scalp is the immediate layer just superficial to the outer table. The extra/epidural space separates the dura from the inner table layer.
Figure 1: The skull has an outer and inner table of cortical bone. The central cancellous bone is called the diploe. The skull bones all ossify separately and become united at the skull sutures as shown here in the midline

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Calvarial lesions may originate from within the cortical tables; invade the skull vault from the brain parenchyma or skull base. The differential diagnosis for calvarial lesions is crucial to decide if biopsy, conservative, or surgical intervention is indicated. As a rule of thumb, there are features on radiographs, which are useful in differentiating benign from malignant lesions and these can be applied to the skull. Benign lesions tend to be well demarcated with surrounding healthy bone. On the contrary, malignant tumors are ill-defined with varying degrees of infiltration into the surrounding tissues. Clinical history is usually combined with lesion morphology to help in further characterization. [Table 1] outlines some helpful features to differentiate benign and malignant lesions.
Table 1: General imaging appearances to differentiate benign from malignant bony lesions

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 » Skull Vault Characteristics Top

The skull vault has a rather unique anatomy giving rise to certain pathognomonic appearances on imaging in response to disease. These features include:

  1. The vault bones are flat and thinned, so any destructive process tends to involve the whole bone thickness at an earlier stage
  2. The vault is immediately adjacent to the scalp layer and brain parenchyma, thus affected by pathology arising from these areas. The dura membrane is also closely apposed to the vault. This implies that the vault appearance can somewhat reflect the nature of adjacent intracranial pathology
  3. The diploic space is highly vascular with a further large collection of small arteries and veins in the scalp and meningeal layer. This is likely responsible for the high frequency of metastases, especially those with a good blood supply. Vascularity assists in the formation of viable granulation tissue favoring an osteoclastic over an osteoblastic response. The pericranium is structurally different from the periosteum around a long bone, explaining why new bone formation around an infective or destructive skull vault lesion is rare.

This pictorial review will not cover MRI appearances, pseudo lesions, or anatomical variants mimicking pathology. Instead, we will focus on the most common skull vault lesions ranging from infection, benign and malignant bone tumors. Within each subset, we will discuss the imaging features to help differentiate one from the other.

 » Osteoma Top

This is the most common primary calvarial bone tumor. They can originate from the dura, skull base, skull vault, and brain parenchyma.[1] The latter has no connection to the dura and are very rare. Dural osteomas tend to arise from the falx and are usually incidental. Skull base osteomas typically arise in the main nasal sinuses. Skull vault osteomas can arise from either the outer table (exostotic) or the inner table (enostotic) and are normally asymptomatic. They only require excision if causing mass effect or adjacent complications eg: mucocele formation. If multiple, Gardner syndrome should be suggested.

Radiologically osteomas are classified as either ivory/dense, mature/spongy, or mixed.[2] Ivory osteomas contain compact bone lacking a Haversian system [Figure 2]. They have a wide base on imaging with dense bone and are slow-growing. Mature osteoma, also referred to as osteoma spongiosum resembles normal trabecular bone, is pedunculated and relatively faster growing [Figure 3].[2] The mixed type has a mixture of both the ivory and spongy forms. Most calvarial osteomas are the mature type arising from the outer table presenting with a palpable lump. The enostotoic variety is very rare and presents mainly with features of raised intracranial pressure. These appear as a lobulated mass attached to the inner table via a stalk. Ossified meningiomas and metaplastic dural ossification are differentials for enostotic osteomas.[3] Meningiomas show contrast enhancement and do not have a bony stalk. There is usually concomitant osseous change as well. Metaplastic dural ossification tends to be multicentric, located on both sides of the dural falx, and displaying a plaque-like or nodular morphology.[3]
Figure 2: Frontal (a) and lateral (b) X-ray showing an ivory osteoma in the right frontal sinus. Note its dense bony calibre due to the absence of a haversian system

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Figure 3: Axial CT. Mature osteoma arising from the left skull vault. Note that it has the same density as the underlying trabecular bone

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 » Osteoid Osteoma Top

This is a benign bone-forming tumor typically occurring in adolescents. They classically cause night pain relieved by salicylate analgesia. In the skull they are usually seen in the frontal-ethmoid region. Unlike in tubular bones, skull vault osteoid osteomas tend to have less surrounding sclerotic reaction.[4] They can show a lucent nidus, which over time may develop a sclerotic peripheral rim [Figure 4]. The sequestrum-like density, as seen in [Figure 4], is thought to occur in ageing lesions.[5]
Figure 4: Sagittal (a) and axial (b) CT. Osteoid osteoma within the diploic space of the frontal skull vault. There is a central lucent nidus containing a focal area of calcified osteoid

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 » Osteoblastoma Top

This is usually seen in young adults with a strong male predilection.[6] In the skull, osteoblastomas are usually seen in the temporal bone but are very rare.[6] Imaging typically shows a very well defined, intramedullary, osteolytic lesion over 3cm in size.[7] They do appear expansile with small internal calcific foci and matrix mineralization [Figure 5]. Older patients tend to present with more mineralization and less lytic changes.[7]
Figure 5: Lateral radiograph. Lobulated lucent skull vault lesion showing matrix mineralisation. Note the relatively large size, over 3cm making this lesion more likely to be osteoblastoma and not an osteoid osteoma

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Some lesions even show a central nidus of sclerotic bone with a surrounding halo, as seen in osteoid osteoma. The key difference is that osteoid osteomas are <2 cm in size and cortically based.[8] Osteoblastomas are on average 4 cm and centrally based in the medullary cavity.[8]

Most osteoblastomas remain confined to the skull vault but some do appear aggressive and penetrate the cortex mimicking a malignant process. The lesions although benign are rapidly growing and maybe symptomatic. Patients can present with chronic pain or even a pathological fracture through the lesion.[8] CT typically shows a well-demarcated lesion with a sclerotic border, matrix mineralization, and variable enhancement on postcontrast imaging.[8]

 » Hemangioma/venous Malformations Top

Calvarial hemangiomas are now classified as venous malformations, representing slow growing benign vascular tumors. They usually occur in the fourth to fifth decades of life with a strong female predilection. On radiographs they appear as well defined lytic lesions.[9] Parts of the bone can be sclerotic due to a thickened trabeculation giving a characteristic sunburst or honeycomb pattern radiating from the centre [Figure 6].[9] There tends to be the absence of periosteal reaction. Unlike osteomas, calvarial hemangiomas tend to arise in the diploic space with expansion of the outer table with little or no involvement of the inner table.[10]
Figure 6: Lateral radiograph (a) and axial CT image (b) shows a sunburst or honeycomb trabeculation pattern classical of skull vault hemangioma

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The key CT imaging features include analysing for areas of fat and calcification within the lesion. Rarely, hemangiomas may become aggressive, with an intra or extracranial soft-tissue component that may simulate a malignant neoplasm.

 » Hemangiosarcoma/pericytoma Top

These tumors are vascular soft tissue tumors, which can affect the capillary blood cells coursing through skull vault. They are usually seen in the scalp of elderly patients.[11] They present as an eccentric osteolytic lesion with a bubbly or permeative appearance.[11] Angiosarcoma can show prominent signal voids implying high flow vessels and fluid levels.[11] They usually present as a well-demarcated lytic, hypervascularized, and hemorrhagic mass [Figure 7].[11] These neuroradiological features are not pathognomonic; however, and may also be seen in other sarcomas or metastasis.
Figure 7: Axial CT. Hypervascular well demarcated extra axial mass penetrating through the left skull vault. Note the compression of the underlying left lateral ventricle. Although not specific, imaging appearances favor a hemangiopericytoma

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Diagnosing malignant vascular tumors on radiographs can be very difficult and usually additional imaging with MRI is needed. Even then, most have non-specific signal characteristics. They typically however do not have calcification or a dural tail commonly seen in meningiomas.[12] A soft tissue component is also more suggestive of an aggressive tumor.

 » Dermoid and Epidermoid Tumor Top

Calvarial dermoid tumors are usually seen in children and are in fact cysts lined by epidermal components, such as sweat glands and hair follicles. In the skull they are normally midline and anterior. On radiographs and CT, dermoid cysts appear as expansile lucent lesions usually with an extra osseous mass extending into the adjacent midline soft tissues.[13] They expand the inner and outer table with a peripheral sclerotic border.[13] They typically have some amount of internal fat content; rarely a fat fluid level is visible on CT.

External orbital angular dermoid is a painless and slow growing subcutaneous mass usually found at the upper outer quadrant of the orbit or lacrimal fossa [Figure 8].[14] They are usually seen as lucent round lesions with peripheral sclerotic rim overlying the zygomatico-frontal suture on radiographs.[14]
Figure 8: Frontal radiograph. Lucent lesion within the upper outer quadrant of the orbit consistent with orbital angular dermoid cyst. It is usually centred over the fronto-zygomatico suture and contains a mixture of fat and fluid content

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Epidermoid cysts have a similar appearance to dermoid cysts on radiographs but typically appear in the third to fourth decade of life. On CT they will have the attenuation and signal characteristics similar to water. Unlike dermoid cysts they do not have significant fat content, although the exact ratio of keratin (protein) to cholesterol (lipid) will vary. Moreover, due to a thin squamous lining, epidermoid cysts tend to only have a faint sclerotic outline on imaging with little or no calcification.

In comparison, dermoid cysts show a thicker border with varying amounts of internal calcification. Most epidermoid cysts are intradural, but a small proportion can be intradiploic, occurring in the diploic space [Figure 9].[15] Imaging shows a lobulated/round area of lucency with scalloped sclerotic margins expanding and possibly eroding the inner and outer table. As these lesions are chronic, they can show prominent internal calcification unlike classical epidermoid cysts.[16]
Figure 9: Lateral radiographs of skull showing lucent intra diploic lesion with a well circumscribed but thin sclerotic rim consistent with a intra diploic epidermoid cyst (arrow). Dermoid cysts will tend to show a thicker sclerotic rim with more dense internal calcification

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 » Langerhan Cell Histiocytosis (Lch) Top

Another differential for lucent skull vault lesions, especially in children is LCH. It can affect the adult population, but normally before the age of fifty. It usually affects the orbit, maxilla, mandible, or temporal bone. The lesion is normally lucent on imaging with a well or poorly defined non-sclerotic border. Sclerosis tends to only be present in the healing phase of the disease.[15]

A few classical imaging appearances have been described, the most common being multiple lucent lesions with a round/oval shape giving a punched out appearance.[17] The lesion causes asymmetric destruction of the inner and outer skull vault, giving rise to a bevelled edge appearance [Figure 10]. Frequently, this gives rise to a fragment of intact bone within the lytic lesion, termed “button sequestrum.”[18] Over time, multiple lesions may enlarge and coalesce giving a geographic appearance to the skull vault [Figure 10]. LCH may also have an associated soft tissue mass, which could be visualized on both CT and MRI.[18]
Figure 10: Frontal (a) and lateral (b) radiographs demonstrates lucent lesion on the left within the skull vault showing a bevelled edge. Note the lack of a sclerotic rim. Over time (c) multiple lucent lesions may coalesce giving rise to a geographic skull

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 » Tuberculosis (Tb) Top

Before the advent of chemotherapy, calvarial TB was usually seen in people less than 20 years old, but rarely in infants.[19] Patients usually present with a painless swelling or sometimes a discharging scalp sinus. The most common sites for TB skull involvement are the frontal and parietal bones followed by the sphenoid and occipital bones.[19] Three distinct patterns of skull involvement on X-ray have been described on radiographs. The most common is termed “circumscribed lytic,” morphology.[19] Here, there are multiple punched-out lytic lesions throughout the skull vault affecting the inner and outer table [Figure 11]. No significant periosteal reaction is seen. As most tuberculous lesions are osteolytic with a punched-out appearance there is common overlap with metastases, myeloma, and bacterial osteomyelitis, and hence positive mantoux test and raised ESR need to be correlated with.[19]
Figure 11: Lateral radiograph (a) and axial CT (b) demonstrates multiple lytic lesions in the skull vault with surrounding sclerotic margins consistent with TB. CT image (b) shows a midline subgaleal collection consistent with potts puffy tumor

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The second type is called “spreading type.” These refer to lesions causing widespread destruction of the inner table with associated extra dural granulation tissue. The least common is called “circumscribed sclerotic type,” and represents generalized areas of sclerosis thought to imply secondary infection.[20]

Potts puffy tumor represents the sequale of usually frontal bone osteomyelitis with a subperiosteal abscess. The abscess is usually well-defined and convex in appearance [Figure 11].[21] The underlying cause is usually frontal sinusitis and secondary haematogenous spread of infection.[21] CT can assist in imaging the extent of bone destruction, scalp swelling, and degree of intracranial involvement.

 » Bacterial, Fungal and Parasitic Infection Top

Bacterial skull vault infection tends to have a rather non-specific spectrum of imaging appearances ranging from lucent lytic to focal sclerosis. Some infections do have more classical radiological appearances. Actinomycosis is a rare bacterial infectious disease resembling fungal involvement. CT and radiographs tends to show gross skull vault sclerosis indicating a chronic form of osteomyelitis [Figure 12].[22] The frontal and parietal bones are normally affected, and there maybe an associated epidural mass.[22]
Figure 12: Lateral radiograph (a) and axial CT (b) showing dense asymmetrical skull vault thickening and sclerosis consistent with Actinomycosis infection

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Hydatid cysts caused by the parasite echinococcus are usually intracranial. Most are “silent,” and only cause symptoms due to their mass effect or once ruptured. They are usually unilocular but can be multilocular. Calcification is rarely seen in [Figure 13]. Most cysts are located just below the cortex but can protrude to the calvarium eroding the skull vault.[23] Cysts which affect the skull vault usually present with direct involvement of the bony spongiosa. Osseous hydatid cysts can then extrude into the extra dural space, or even spread via healthy dura matter to form intracerebral cysts.[23]
Figure 13: Lateral radiographs demonstrate multi loculated cystic lesions within the skull vault consistent with hydatid cysts. Note the lack of internal calcification

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A typical CT appearance of hydatid cysts are a round or ovoid lesion with a “double layered arcuate calcification.”[24] This is considered specific for hydatid cysts caused by echinococcus infestation rather than any other cyst forming disease.

 » Sarcoidosis Top

Skull vault sarcoidosis is a very rare manifestation of the disease. If extensive it may cause hydrocephalus due to raised intracranial pressure. Other possible symptoms includes various cranial nerve palsies especially if the optic nerve is involved.[25]

Most of the skull vault lesions consistent with sarcoidosis are reported to be purely lytic, with no surrounding sclerosis or periosteal reaction [Figure 14].[25] Despite being lytic, they do not erode the inner or outer table unlike metastases or other more aggressive skull vault pathology.[25]
Figure 14: Multiple ill defined lytic lesions in the skull with no periosteal reaction or surrounding sclerotic rim consistent with sarcoidosis

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 » Fibrous Dysplasia Top

Cranio facial fibrous dysplasia is commonly seen in adolescents and young adults. It may present with cranial asymmetry, facial deformity, proptosis, and visual impairment. It can be monostotic or polyostotic, with skull vault involvement being reported in both forms. It usually affects the frontal or temporal bones and may cross suture lines.[26] The lesion is often intradiploic, expansile with a classical ground glass matrix on radiographs and CT. Unlike LCH or Pagets disease the outer table is preferentially affected more than the inner table. The diploic space is usually expanded on imaging with a displaced outer table.[26]

Radiologically three distinct patterns have been described. The first is mixed lytic and sclerotic pattern.[27] In this form, there can be a very prominent mixture of dense sclerotic and lucent areas of fibrosis giving an appearance similar to Pagets disease, termed “pagetoid,” fibrous dysplasia [Figure 15].[28] The most common imaging appearance is homogenously sclerotic is the second pattern and the least common is predominantly cystic.
Figure 15: Frontal (a) and lateral (b) skull radiographs showing typical features of fibrous dysplasia. However, a large lytic area is seen in the posterior parietal region consistent with sarcomatous transformation

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 » Metastases Top

Skull vault metastasis is rare but is usually secondary to breast, lung, renal, prostate, and thyroid malignancy in adults and neuroblastomas or sarcomas in children. When a large single expanded lytic lucent lesion is seen, thyroid, or renal metastases should be the main differential diagnosis in the appropriate age group.[29] Prostate carcinoma metastasis is the main differential for a sclerotic vault lesion in a male, with a density lower than that of intra osseous meningioma [Figure 16]. The predilection of neuroblastoma, prostate carcinoma, and breast cancers to spread to the extradural space is well known.[30] They spread hematologically to the skull and by direct extension to the epidural space and dura.
Figure 16: Lateral radiograph showing a very rare sclerotic metastases from prostate cancer

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Metastases are usually lytic, sclerotic, or mixed pattern depending on the primary tumor [Figure 17]. In most cases, they present as multiple well-circumscribed lucent lesions with extension into the adjacent soft tissues.[31] Diffuse infiltration is very rare. Even though most skull vault metastases have no specific features, the correlation between imaging pattern (e.g., lytic and sclerotic) and clinical history for example, sex, age, primary malignancy) is normally sufficient to give an exact or differential diagnosis.
Figure 17: Lateral radiographs (a) showing multiple lytic metastases from breast carcinoma. Frontal radiograph (b) showing widespread vault infiltration with lytic breast adenocarcinoma metastases

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The main differential for multiple lytic skull vault lesions other than metastasis is multiple myeloma. In the latter, most lucent lesions tend be small and rather homogenous in size. This is in contrast to metastases having multiple ill defined lesions of varying sizes and shapes.[32]

 » Multiple Myeloma Top

Multiple myeloma and solitary plasmacytoma are found mainly within the axial skeleton bones containing hematopoietic marrow, such as the skull. Seventy is the average age of presentation. Solitary plasmacytoma may occur without any evidence for multiple myeloma.[31] A few distinct patterns within the skull have been described on radiographs:[31]

  • Normal bone marrow-this occurs in roughly 25% of patients, whereby no radiographic changes are seen
  • Focal infiltration- multiple small round osteolytic lesions. They are relatively uniform in size with sharp and non-sclerotic margins, classically termed “punched out,” lesions
  • Diffuse infiltration-Widespread small punched out lytic lesions throughout the vault [Figure 18]. This gives a mottled appearance and when combined with the more “cystic,” areas of osteopenia, the appearance is described as “pepper pot.” Some lytic lesions may coalesce together into larger osteolytic segments
  • Diffuse osteopenia-this is normally very rare in the skull.

The most common appearance of multiple myeloma in the skull are multiple punched out lesion with a sharp non-sclerotic border.[33] If abutting the cortex there maybe endosteal scalloping [Figure 18]. Over time, a few lesions may coalesce into larger lytic segments. The disseminated diffuse osteopenia or focal plasmacytoma form is rare in the skull.[33]
Figure 18: Axial CT (a) and lateral radiograph (b) demonstrating multiple lesions of similar size throughout the skull vault consistent with widespread multiple myeloma. The mixture of lytic and cystic osteopenic areas give rise to term “pepper pot,” skull

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 » Ewing Sarcoma Top

Ewing sarcoma of the skull is a very rare presentation of the disease, and usually holds a fatal prognosis. Dural invasion leading to hydrocephalus and raised intracranial pressure can cause symptoms such as headaches and scalp swelling. Men are usually affected more and roughly 90% of cases occur in the first two decades of life, with a peak incidence between 5 and 13 years.[34]

Unlike other lytic skull vault lesions, for example, metastases, fibrous dysplasia, and LCH, Ewing sarcoma commonly presents with a large epidural mass out of proportion to the amount of osseous vault bony erosion.[34] The epidural mass is usually best seen on CT, showing heterogeneous enhancement on post contrast studies. The classical onion peel lamellated periosteal reaction as seen elsewhere in the body is rarely seen in the skull.[35] Ewing sarcoma has the ability to disperse via the haversian canals without causing macroscopic bony destruction. The outer periosteum and cortex is affected first, unlike most other malignant bony lesions, which typically affect the endosteum first [Figure 19].[36] This gives rise to an appearance termed cortical saucerization.
Figure 19: Lateral radiograph. Large extra osseous component associated with destruction of the posterior skull vault typical of Ewing sarcoma. The mass is normally out of proportion to the amount of underlying bony destruction

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Ewing sarcoma does not produce osteoid tissue hence soft tissue calcification usually represents debris from periosteal new bone formation. According to Resnicket al. soft tissue calcification and and extrinsic erosion (saucerization) is rarely ever present in skull vault lesions and is seen in ~6% of Ewing sarcomas of bone.[37]

 » Paget Disease Top

Paget disease is typically seen in patients over the age of forty. It historically has a strong Western European and American predilection, and is rarely seen in Asia.[38] Skull vault disease gives arise to bony enlargement, which may encroach neural foramina and compress cranial nerves with associated symptoms.

Paget disease is characterized by excessive and abnormal bony remodelling. The classical triad in the skull vault includes thickening of the cortex, accentuation of the trabecular pattern, and bony expansion. The disease begins as a destructive process only involving the outer table, giving rise to the classical appearance termed osteoporosis circumscripta.[38] Here, there are well-defined lytic areas usually in the frontal or occipital bone with the absence of peripheral sclerosis. The lesions may cross suture lines giving a geographic lytic skull vault appearance. Later on, osteoblastic activity supervenes preferentially affecting the inner table.[38] This results in sclerosis and thickening of the diploe. Discrete islands of osteosclerosis form between the inner and outer table giving rise to the “cotton wool,” appearance.[39] By this stage, there is prominent diploic space widening as a result of inner and outer table involvement [Figure 20] and [Figure 21].
Figure 20: Lateral radiographs demonstrating multiple lucent and sclerotic lesion throughout the skull consistent with fibrous dysplasia. The amount of sclerosis can be prominent, giving rise to the name Pagetoid fibrous dysplasia

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Figure 21: Lateral radiograph (a) showing large areas of lucency involving the frontal regions typical of osteolysis circumscripta. Lateral radiograph (b) showing widening of the diploic space with areas of sclerosis and vault thickening consistent with the 'cotton wool,' appearance in Paget disease

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There are only a handful of case reports discussing sarcomatous degeneration of skull vault Paget disease, usually osteosarcoma. Most cases in the skull show focal areas of lysis surrounded by vault thickening. Other features include a radiolucent focus with speckled regions of calcification which disrupts the skull vault [Figure 22].
Figure 22: Lateral radiograph (a) showing typical appearances of multifocal uncomplicated Paget's disease. Lateral radiograph (b) taken several years later shows evidence of malignant mineralisation over the posterior parietal area consistent with Paget's osteosarcoma

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Owing to the underlying distortion of the pagetic bone it is usually difficult to detect early sarcomatous transformation. This may prove challenging on radiographs, often termed “pseudo sarcoma,” and is usually only diagnosed after a negative biopsy.[40] CT is normally required to differentiate Pagetoid lucencies and focal bony proliferation from sarcomatous transformation. Specifically, cortical defects out of keeping with the background Pagetoid changes should raise immediate suspicion. CT can further assess full tumor delineation, brain parenchyma, and dural involvement.

 » Conclusion Top

The art of interpreting skull radiographs is slowly being lost due to the decline in the number of examinations being performed and seen by the radiologist. We have presented the radiological appearances of the most frequently encountered skull vault lesions with particular attention to diagnostic clues and some CT appearances. This may be essential if radiographs are the only modality available in developing countries where access to MRI is limited.

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Conflicts of interest

There are no conflicts of interest.

 » References Top

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  [Figure 1], [Figure 2], [Figure 3], [Figure 4], [Figure 5], [Figure 6], [Figure 7], [Figure 8], [Figure 9], [Figure 10], [Figure 11], [Figure 12], [Figure 13], [Figure 14], [Figure 15], [Figure 16], [Figure 17], [Figure 18], [Figure 19], [Figure 20], [Figure 21], [Figure 22]

  [Table 1]


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