Case of the Month

A normal-appearing, six-week-old female presented with a history of volvulus on the second day of life. “Strictured” areas of bowel were noted during ostomy takedown and resected. What is the most likely diagnosis? A. Hirschsprung’s disease
B. Intestinal atresia
C. Meconium ileus
D. Meckel’s diverticulum
E. Duplication cyst Click to register your guess We will reveal the answer in next month’s issue! Do you think you have a good case of the month? Email it to edit@thepathologist.com

Answer to last issue’s Case of the Month… D. Biphasic papillary renal cell carcinoma Biphasic papillary renal cell carcinomas (BPRCC) have papillary architecture and a dual cell population, which can also be found in other kidney tumors (1). Histologically, they resemble type 1 papillary renal cell carcinoma (PRCC) with the addition of interspersed nests of large, eosinophilic, squamous-like cells. However, they tend to be more aggressive compared with classic type 1 PRCC (2). These tumors are mostly composed of papillae covered by a single layer of small cells with regular nuclei and scant cytoplasm. Interspersed are aggregates of large, eosinophilic, squamous-like cells with higher-grade nuclei, which may comprise more than half of the tumor volume (3). Nevertheless, neither intercellular bridges nor keratin pearl formation – the distinguishing features of squamous differentiation – are present; hence the designation “squamoid” in the alternative name of this tumor: biphasic squamoid alveolar renal cell carcinoma (1,2). Moreover, squamoid cells are negative on IHC for p63 and CK5/6, confirming the lack of their squamous differentiation. Consistently, large squamoid cells exhibit the phenomenon of emperipolesis (sometimes referred to as “cells eating cells”), microscopically seen as intact adjacent cells penetrating and sitting within their cytoplasm (2). This phenomenon can be seen in some hematological disorders, such as Rosai-Dorfman disease, but has not been described in other kidney tumors. The immunohistochemical profile of BPRCC mostly matches type 1 PRCC. Tumor cells are positive for cytokeratin 7, racemase, vimentin and PAX8; additionally, the squamoid component is typically positive for cyclin D1 staining. BSARCC are negative for cytokeratin 20, cytokeratin 5/6, c-Kit, p63. They stain variable for carboanhydrase IX and CD10 (2,3). Genetically, these tumors also match the PRCC profile. Gain of chromosome 7 and 17 and loss of chromosome Y were the most frequent findings in a cohort of 11 cases (2,3). Differential diagnosis of BPRCC includes:

• Urothelial carcinoma with squamous differentiation. They are usually located more centrally and related to the urothelium of the renal pelvis and calices. Urothelial metaplasia or dysplasia or carcinoma in situ is often found. Tumors are CK20 positive.
• Chromophobe renal cell carcinoma with sarcomatoid and squamous transformation. These tumors are c-Kit positive and vimentin negative.

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