Microsatellite instability in a pleomorphic rhabdomyosarcoma in a patient with hereditary non‐polyposis colorectal cancer

AIMS: To describe the rare occurrence of a pleomorphic sarcoma with microsatellite instability in a patient with hereditary non-polyposis colorectal cancer (HNPCC). METHODS AND RESULTS: A soft tissue tumour was removed from the upper leg of a patient who had previously been shown to harbour a germ-line MSH-2 mutation. The tumour was analysed with immunohistochemistry and molecular methods. The morphology and immunohistochemical findings were in keeping with a pleomorphic rhabdomyosarcoma. Microsatellite instability was documented in the tumour with molecular methods and in addition loss of MSH-2 expression in the tumour cells was confirmed by immunohistochemistry. CONCLUSIONS: Although sarcomas do not form part of the HNPCC diagnostic criteria, they may occur in this mismatch repair syndrome and, moreover, may well be caused by the underlying genetic defect.