Tudor Racaru

36-year-old man with sudden severe headache

DiagnosisGiant intradiploic epidermoid cyst. A large arachnoid cystof the middle fossa of the brain is visible inferior to theintradiploic epidermoid cyst.DiscussionEpidermoid cyst (EC) is a rare, benign, slow-growinglesion of the skull. The origin of ECs of the skull isdebated. According to the congenital theory, ECs arise fromectodermic inclusions during neural tube closure (weeks 3–5of embryogenesis). Epithelial cells become trapped in thebone by the laterally migrating otic or optic capsule [1–4].Acquired ECs of the skull are believed to develop frominclusions of the epithelium after trauma [1, 2, 5].Epidermoid cysts of the skull are detected incidentallyby imaging for other reasons [1].Large ECs of the skull may become symptomatic,causing compression of the brain and neurological effects,including headache, seizures, optic nerve or venous sinuscompression, and intracranial hypertension [1, 5, 6]. In ourcase, the cause of the patients headache was nonspecific; itcould have been attributed to direct meningeal irritation bythe lesion or, less likely, to chronic changes in intracranialpressure, related to the mass.Late complications of ECs include a fistulous tract thatopens to the skin, suprainfections, and meningeal syndromesubsequent to cyst rupture [4, 7]. Rarely, intradiploic ECsundergo malignant transformation into squamous cellcarcinoma [8].On computed tomography (CT), ECs of the skullusually appear as well-defined, heterogeneous masseswith densities ranging from −20 to +20 Hounsfield units(HUs), interspersed with higher-density areas that corre-spond to protein or cholesterol deposits (Fig. 1; see TestYourself: Question) [9].Epidermoid cysts typically have low signal intensity onT1-weighted MRI and high signal intensity on T2-weightedMRI. The T1-weighted MRI signal may increase focally,corresponding to cholesterol deposits (Fig. 2; see TestYourself: Question) [1, 10].Most ECs fail to show enhancement after injection ofcontrast media (Fig. 2) [10].In our case, a large arachnoid cyst of the middle fossawas visible inferior to the EC.The EC and CSF of the arachnoid cyst could not bedifferentiated based on the density by CT (the averagedensity was approximately 12 HU for both lesions; Fig. 1),whereas EC was easily detected by MRI (Fig. 2). ECs areheterogeneous and hyperintense relative to cerebrospinalfluid (CSF) on fluid attenuation inversion recovery(FLAIR) images (Fig. 2) [11]. Further, unlike arachnoid

Bone mass of the calvarium

Plasmacytoma of the calvarium is very rare, accounting for the 0.7 % of all plasmacytomas [1–5]. The most common locations are the thoracic and lumbar spine (30–40 % of cases). Involvement of the hip, femurs, and ribs is less frequent [6]. Plasmacytoma of the calvarium can be indistinguishable from other lytic lesions, especially metastatic disease. Meningioma, eosinophilic granuloma, and dermoid cysts can have similar appearances, although usually in a different clinical context [10, 11]. Histopathological examination and laboratory tests are necessary to make a definitive diagnosis of plasmacytoma (Fig. 1). The typical features on imaging are a lytic lesion on Xray examination (Fig. 1 question) [1, 6]. On computed tomography (CT), plasmacytoma presents as a lytic punched-out lesion with interspersed residual thickened bony trabeculae. Cortical thinning and focal destruction are typical (Fig. 2 question) [2]. On magnetic resonance imaging (MRI), bone plasmacytoma of the calvarium presents as an intramedullary lesion of low to intermediate intensity on T1weighted images and high intensity on T2-weighted images. Contrast enhancement is usually mild [4–9]. In the presented case, the calvarium plasmacytoma presents a “mini-brain” appearance, due to the presence of low-intensity hypertrophic residual trabeculae interspersed in a relatively homogeneous mass, mimicking the brain sulci. Major et al. first described the “mini-brain” appearance of plasmacytoma in a series of ten patients with spinal