Low-grade oncocytic tumor (LOT) - a new renal entity ready for a prime time: An updated review

Low-grade oncocytic tumor (LOT) of kidney has been recently proposed as a new renal entity. LOT was identified in the spectrum of oncocytic renal tumors with overlapping features between oncocytoma and eosinophilic chromophobe renal cell carcinoma, or it has been labelled as one of those entities in prior studies and in practice. LOT is often a single, relatively small tumor, found in a non-syndromic setting, but rare examples of multiple LOTs or admixed with other tumors have been found in patients with tuberous sclerosis complex. LOT typically has solid architecture, and it is composed of eosinophilic cells, with round to oval 'low-grade' nuclei, lacking irregularities and showing focal perinuclear halos. Sharp transition into edematous stromal areas, with scattered or loosely arranged cells are frequently found. LOT has a consistent immunohistochemical profile with diffuse reactivity for cytokeratin 7 and absent (or rarely weak) expression for CD117, a profile different from oncocytoma and eosinophilic chromophobe renal cell carcinoma. Similarly, in contrast to those entities, it also lacks or shows only weak expression for FOXI1. Recent studies have shown that LOT has a molecular/genetic profile different from other renal tumors, with frequent alterations affecting the MTOR/TSC pathway genes. LOT demonstrates either disomic pattern or deletions of 19p13, 19q13 and 1p36, and lacks complete chromosomal losses or gains. In all published studies to date, LOT has shown benign behavior. In this review, we summarize the evidence from recently published studies, which strongly supports the conclusion that LOT is a distinct and unique renal entity.