Case of the Month

Answer to March/April's Case of the Month

d. Low-grade oncocytic tumor

Low-grade oncocytic tumor (LOT) of the kidney is an emerging entity, recently recognized by the Genitourinary Pathology Society (GUPS) (1) and mentioned in the latest WHO Classification of Tumours in the chapter, “Other oncocytic tumors of the kidney (2).”

LOT usually affects older patients (median age 66 years) and is solitary and sporadic. Grossly, it is a small, circumscribed, tan to brown solid tumor with or without cystic areas (Figure 1). Its prognosis is generally good.

Histologically, LOT shows compact nests, solid sheets, or a trabecular growth and is composed of bland oncocytic cells with round low-grade nuclei, usually with perinuclear clearing but without membrane irregularity (Figure 2). Often, it has characteristic sharp areas of edema where cells become discohesive, stretched out, or interconnected in short strands (Figure 3).

Main differential diagnoses include oncocytomas, hybrid oncocytic tumors, and other renal cell carcinomas (RCC) such as the eosinophilic subtype of chromophobe RCC (ChRCC), eosinophilic solid cystic RCC (ESC-RCC), and succinate dehydrogenase-deficient RCC (SDH-RCC).

The nuclei characteristics are useful to distinguish LOTs from oncocytomas, which lack a perinuclear halo, and from ChRCC, which have additional nuclear membrane irregularities. The immunohistochemical profile of diffuse cytokeratin 7 positivity (Figure 4) with a negative to weak CD117 staining (only dispersed mast cells) (Figure 5), which is typical of LOT, is also useful. Colloidal iron staining has been reported as either negative, only apical/luminal, or bar-/block-like (3) (Figure 6).

Distinction from SDH-RCC, hybrid oncocytic tumor in the context of Birt-Hogg-Dubé syndrome, or ESC-RCC is generally possible on clinical and morphologic grounds alone, but further SDHB immunohistochemistry studies or detection of FLCN or TSC gene mutations could be warranted. These tumors are also associated with TSC and mTOR mutations.

Submitted by Tiago Oliveira, anatomical pathology resident, and Dolores López-Presa, anatomical pathologist at Hospital de Santa Maria, CHULN - Lisbon, Portugal.