Images in Radiology - Intradiploic Epidermoid Cyst

A 35-year-old lady, presented with approximately 5 cm X 4 cm sized swelling in the left parietal region since childhood. There was history of progressive increase in the size of the swelling since childhood. There was no history of trauma. There was history of pain associated with swelling since last two months. On examination, the swelling was soft, non-tender and could be moved over the underlying bone. The skin over the swelling could not be retracted. Frontal and lateral radiograph of the skull showed an approximately 5 cm X 4 cm , well defined osteolytic lesion in the left posterior parietal bone (Figure 1). The inner table was scalloped and the outer table was everted (Figure 2). There was no evidence of calcification or periosteal reaction. Rest of the skull was unremarkable. For better delineation of the lesion, plain and contrast enhanced axial computed tomographic (CT) scan was performed. Plain CT scan revealed, a 5 cm sized soft tissue density lesion seen within the diploic space of the left posterior occipital bone with attenuation values ranging between 20 Hounsfield unit (HU) to +20 HU (Figure 3). This lesion was seen to protrude outside the margins of the skull and the dura matter was seen separately from the lesion. There was no calcification within the mass. On contrast there was no appreciable enhancement. Rest of the cerebral parenchyma was normal.