Renal mucinous cystadenoma in the context of lynch syndrome and colonic neuroendocrine neoplasm: a case report.

Renal mucinous cystadenoma (RMC) is an exceptionally rare finding with a poorly understood pathogenesis. Links between RMC and other malignancies are not well described, nor are there known associations with familial cancer disorders. We present the first case of RMC associated with Lynch syndrome (LS) and neuroendocrine neoplasm. A 56-year-old woman presented with iron deficiency leading to a diagnosis of a colonic mixed neuroendocrine-non-neuroendocrine carcinoma. After initial surgery, she experienced local recurrences at 6 and 12 months, treated with resection and adjuvant chemotherapy. Tumours displayed deficient MMR proteins with BRAF positivity, and germline testing confirmed LS. Surveillance subsequently revealed a complex cyst arising from a horseshoe kidney, for which she underwent a partial nephrectomy. Histopathology confirmed this to be renal mucinous cystadenoma arising from the pelvicalyceal system. This case underscores the need for further investigation into RMC pathogenesis and its potential association with LS.