TEST YOURSELF: ANSWER 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