Low-grade oncocytic
tumor (LOT) has recently been described as a distinct renal
tumor. LOT shows consistent morphologic features and a CK7-positive/CD117-negative immunophenotype. To examine the clinicopathological, immunohistochemical, and molecular features of LOT, we searched our institutional archives and identified seven cases of LOT. All patients were female, with a mean age of 66 years (range 44-79 years). The average
tumor size was 3.2 cm (range 1.6-5.5 cm). Macroscopically, the
tumors showed tan-brown and solid cut surfaces. Microscopically, the
tumors showed compact nested to solid growth pattern, three cases with areas of edematous stroma containing loosely connected small clusters, cords or dispersed single
tumor cells. The
tumor cells had uniformly round to oval nuclei with eosinophilic cytoplasm, and showed perinuclear halos. Two cases focally had nuclear irregularities and binucleated cells were occasionally seen in three cases. Immunohistochemically, diffuse positivity for CK7 and lack of CD117 expression were present in all cases. All of the
tumors were negative for CD10, CK20,
vimentin, CA9, TFE3, TFEB, HMB45, and
Melan-A. All
tumors were positive for MTOR and negative for
Cathepsin-K. FH and SDHB were retained. Next generation sequencing identified genetic variations in the MTOR pathway related genes: TSC1 (4/7), TSC2 (5/7), and MTOR (1/7). All patients were alive and without
disease progression, after a mean follow-up of 43 months (range 6-89 months). LOT is an uncommon eosinophilic
renal neoplasm with unique morphological and characteristic immunophenotypic features, and may represent an emerging separate renal entity characterized by mutations in the
TSC/MTOR pathway.