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Renal sclerosing perivascular epithelioid cell tumor (PEComa)/angiomyolipoma with extensive rhabdoid cell features

To the Editor: Perivascular epithelioid cell (PEC) was proposed as the prototypic cell of PEComas, which has unique morphologic (smooth muscle, epithelioid, and lipid rich histology), immunohistochemical [coexpression of myogenic (smooth muscle actin) and melanocytic (HMB45) markers], ultrastructural, and genetic features, and has no known normal cellular counterpart. PEComas include renal and hepatic angiomyolipomas (AMLs), clear cell sugar tumor of the lung, lymphangioleiomyomatosis, and clear cell myomelanocytic tumor of the falciform ligamentum teres. Pure epithelioid PEComas (so-called epithelioid AMLs) of the kidney are related to their better known triphasic typical AML counterparts and often have disease progression. Recently, Hornick and Fletcher reported 13 sclerosing PEComas with extensive stromal hyalinization as a distinct and indolent variant of epithelioid PEComa. Neoplasms with rhabdoid cell features have been reported in many anatomic sites, and pediatric malignant rhabdoid tumor of the kidney represents the prototypic lesion. These neoplasms may exist as pure malignant renal or extrarenal rhabdoid tumors as well as composite neoplasms with rhabdoid cell features in ‘parent’ neoplasms. INI1 (hSNF5/ SMARCB1/BAF47) is a highly conserved factor, from yeast to humans, in an ATP-dependent chromatin-modifying complex-SWI/SNF. In humans, loss of INI1 expression was first described in malignant rhabdoid tumor, and since then, has been identified in several malignant neoplasms which occasionally show rhabdoid cell features. In general, INI1 loss and rhabdoid histology are associated with aggressive tumor behavior. A 51-year-old woman without any notable past or family history including tuberous sclerosis was admitted to our hospital with the chief complaint of right abdominal bloating. Abdominal computed tomography (CT) scan revealed an enhancing broad-based mass which emanated from the right kidney. The tumor measured 4.0 cm in a largest diameter. Under a diagnosis of renal cell carcinoma, right partial nephrectomy was performed for the tumor. The patient is currently alive without recurrence or metastasis of the tumor at 16 months after surgery. On gross examination, the right kidney tumor was well circumscribed, the size of which was 4.0 × 3.0 × 1.4 cm. The tumor was surfaced by perinephric adipose tissue on one side and by kidney parenchyma on the opposite side. The cut surface of the tumor was tan-white, homogeneous and solid without evidence of hemorrhage or necrosis (Fig. 1a). Microscopically, the tumor was predominantly composed of cords, nests, and sheets of polygonal epithelioid cells with round to oval, relatively uniform nuclei and eosinophilic cytoplasm, embedded in densely sclerotic stroma (Fig. 1b). Approximately 40% of tumor cells showed rhabdoid cell features which were characterized by vesicular nuclei, occasionally prominent nucleolus, and abundant intracytoplasmic hyaline inclusions (Fig. 1c). A spindle cell component was observed in about 10% of tumor cells (Fig. 1d). Tumor cells with clear cytoplasm or significant nuclear pleomorphism were not identified. The tumor cells were frequently associated with the walls of blood vessels (Fig. 1a). A lipomatous component was not seen. Tumor necrosis and mitotic figures were absent. No perinephric fat invasion by tumor cells was observed. The background kidney had mild chronic interstitial inflammation and scattered globally sclerotic glomeruli. Immunohistochemically, the epithelioid tumor cells including those with rhabdoid cell features and spindle tumor cells showed similar immunoreactivity for the examined antibodies: diffusely positive for α−smooth muscle actin (SMA), estrogen receptor (ER), progesterone receptor (PgR), and INI1, focally immunoreactive with HMB45 (Fig. 2), and negative for PAX8. Sclerotic stroma was diffusely immunoreactive for INI1 and negative for α−SMA, ER, PgR, PAX8, and HMB45. Typically, epithelioid PEComa/AML is characterized by cohesive nests or compartmentalized/diffuse sheets of large, polygonal epithelioid tumor cells with abundant eosinophilic cytoplasm and occasional atypical nuclei, admixed with markedly pleomorphic cells with nuclear pleomorphism, hyperchromatism, and the presence of prominent nucleoli. In contrast to classic AMLs which have typical triphasic features, epithelioid PEComas/AMLs are potentially malignant lesions. In a large series of 41 pure epithelioid PEComa of the kidney, recurrence and metastasis of the tumor were seen in 17% and 49% of patients, respectively, and 33% of patients died of disease. Recently, Hornick and Fletcher reported 13 cases with epithelioid PEComas with extensive stromal hyalinization as a distinctive variant of ‘sclerosing PEComa’. Those cases showed a striking prediction for the retroperitoneum of middle-aged women, and an indolent clinical course. No case in their series involved renal parenchyma. Histologically, those tumors purely consisted of Pathology International 2014; 64: 247–250 doi:10.1111/pin.12155 bs_bs_banner