A case of pararenal PEComa with extensive bone metaplasia

Dear Editor, The group of perivascular epithelioid cell tumours (PEComas; a family of rare mesenchymal neoplasms arising at different visceral and soft tissue sites) has a wide spectrum of morphology. We present a case with a very unusual feature: wide bone metaplasia. The patient was a 63-year-old hypertensive woman complaining from left flank pain and cystitis. Laboratory data were normal, except E. coli in urine. Results of an abdominal simple X-ray showed a 4-cm calcified round tumour at the upper left renal pole, very evident with the computed tomography (CT) scan (Fig. 1). It was excised through laparoscope. The mass was covered by fatty tissue and had good cleavage from adjacent kidney. The gross specimen was a 28-g, 4-cm-sized, smoothsurfaced tumour with very loose attachments to the fat nearby. It showed haemorrhagic and cystic areas alternating with a few white solid ones. The consistency was so hard that it required decalcification. Microscopically, it was composed by aggregates of clear or light eosinophilic epithelioid cells with relatively bland nuclei and some giant cells, among fine arborizing capillaries or around thick-walled, often hyalinized blood vessels, alternating with hypocellular collagenous–hyalinized areas, sometimes calcified. Endochondral ossification and lamellar bone was frequent (Fig. 2). Occasionally, mature adipose tissue was found. Cellular areas were 21% of the total. There was no mitosis or necrosis. The tumour was entirely circumscribed by a fine connective capsule. Fatty tissue and adrenal gland had no abnormalities. Immunohistochemically, neoplastic cells expressed vimentin, frequent human melanoma black (HMB)45 (more intense in larger epithelioid–clear cells, with paranuclear accentuation; Fig. 3) and melan-A (same staining pattern), also S100, smooth muscle antigen, clusters of differentiation (CD)10, CD68 and occasionally pancytokeratin. They were desmin, myogenin, α-inhibin, epithelial membrane antigen, CD117, cyclin D1, CD34 and CD31 negative. Ki 67 was inapparent. This immunohistochemical profile is specific of PEComa. Other clear cell tumours of that area can be excluded [4]. HMB45-positive large neoplastic cells (proliferating PEC cells) were sometimes located very close to endothelial lined spaces. In sum, this unusual case belongs to the PEComa family, a recently defined group of tumours originated from perivascular epithelioid (PEC) cells [1, 2] at multiple sites, encompassing angiomyolipoma, lymphangiomyomatosis, clear cell “sugar” tumour and rare tumours in other locations. They have a wide range of clinical, pathological and immunohistochemical features: clearly malignant [3, 5], some of uncertain malignant potential and a few benign. This case would fulfil the criteria of the Virchows Arch (2008) 452:349–350 DOI 10.1007/s00428-008-0581-1