Metanephric adenoma and solid variant of papillary renal cell carcinoma: common and distinctive features

AIMS: To evaluate morphological and immunohistochemical (IHC) features helpful in distinguishing metanephric adenoma (MA) from solid papillary renal cell carcinoma (s-PRCC). METHODS AND RESULTS: We present a detailed study of 21 MA and 23 s-PRCC. The two entities exhibited significant similarities, both being well-circumscribed tumours composed of tightly packed small cells arranged in solid sheets or ill-defined tubules, often presenting glomeruloid bodies, psammoma bodies and dystrophic calcification, and showing overlapping immunoreactivity for S100, CD57 and CK7. Conversely, most MA were non-encapsulated, whereas most s-PRCC showed a thick fibrous pseudocapsule; MA cells had scanty cytoplasm and a high nuclear:cytoplasmic ratio in comparison to s-PRCC, where occasional tumour cells showed abundant cytoplasm and high nuclear grade. Polypoid branching fronds were common in MA, but absent in s-PRCC; multifocality and papillary hyperplasia/adenoma were seen only in s-PRCC. MA were positive for WT1 and negative for EMA and alpha-methylacyl-CoA racemase (AMACR); s-PRCC were positive for EMA and AMACR and negative for WT1. CONCLUSIONS: Despite overlapping features, careful morphological and architectural evaluation should result in accurate diagnosis of most MA and s-PRCC. In challenging cases, IHC stains for WT1, EMA and AMACR may help in distinguishing these two entities.