A rare subtype of papillary kidney tumors ( case report )

Biphasic squamoid papillary RCC or biphasic alveolo-squamoid RCC was originally reported as a separate entity by Peterson et al in 2012 (2). A recent review lists about 100 cases reported in the literature (1). A slight male predominance has been described with an age of onset ranging from 39 to 79 years, similar to conventional papillary type 1 carcinomas. These tumors occur mainly on native kidneys, but two unifocal cases of appearance on grafts have been described (3). They can occur in association with other types of RCC (papillary or clear cell carcinoma). Some rare multifocal cases have been reported, including one case from a mother and her son with a MET mutation context (4). C-MET is known to play a major role in the development of papillary renal cell carcinomas, mostly type 1, some authors have described an association between c-Met expression and clinical outcome and c-Met could be a prognostic marker (5). Moreover, a case of biphasic squamoid papillary RCC has been reported in a patient with Birt-Hogg-Dubé syndrome (6). In one of the largest series of 21 cases described by Hes et al in 2016, the authors describe the aggressive potential of this entity with 3 patients displaying distant metastases and two patients, who died as a result of the spread of the disease (7). In another study in 2017, the 24-month median follow-up of 23 patients revealed one patient alive with disease and one dying of disease (6). Macroscopically, these tumors are solid, whitish or brown. Mostly their size is small but bigger lesions up to 15 cm have been described (1). From a histological point of view, these generally solid growing tumors display as main characteristic two different cell populations. The first contingent consists of large cohesive cells with abundant eosinophilic cytoplasm and high nuclear atypia with nucleoli. They are mostly grouped in clusters consisting of small nests creating alveolar aspects. These cells are designated as “squamoid” because of their resemblance to squamous cells, but there is no evidence of squamous differentiation such as intercellular bridges or positivity of p63 or CK5/6 immunohistochemistry (1). The other population which form small amphophilic or clear cells, sometimes with cuboid scant cytoplasm, resemble to cells seen in type 1 papillary renal carcinoma. These cells are arranged around nests, which display glomeruloid-like aspect. Hes et al and Trpkov et al have observed images of emperipolesis within the contingent of large “squamoid” cells. Emperipolesis is defined by the presence of intact cell within the cytoplasm of another cell. Emperipolesis is unusual in RCC but has been described to be a morphologic hallmark of biphasic squamoid papillary RCC (6,7), but can also be found in high grade clear RCC with a syncytial-type multinucleated giant tumor cell component (8,9). In addition, necrosis, During this last decade the kidney cancer classification has undergone a real revolution. Increased understanding of underlying processes has taken place, many new types have been described and characterised and allow a better patient’s management. Some of these entities have been introduced into the latest WHO classification 2016, some have recently been recognized and are still under evaluation (1). Biphasic squamoid papillary renal cell carcinoma (RCC) is a rare type of renal tumor, recently described (2). This lesion is not yet introduced into the WHO classification 2016.