The morphologic changes seen in desmoid-type fibromatosis after radiotherapy have not been well studied, and the degree of cytologic atypia, cellularity, mitotic activity, and lesional necrosis found in desmoid-type fibromatosis treated with ionizing radiation has not been thoroughly assessed. Therefore, we evaluated a series of primary and locally recurrent desmoid-type fibromatoses resected at variable intervals after radiation therapy (XRT) and compared their histopathologic and immunophenotypic features with paired pathologic specimens that were obtained before radiotherapy. The morphologic characteristics of desmoid-type fibromatoses resected before and after radiotherapy were not significantly different in 7 of the 8 cases studied. One case resected 191 months after XRT showed morphologic evidence of fibrosarcomatous transformation, with zonal necrosis, hypercellularity, severe nuclear atypia, and increased mitotic activity. No significant differences in the immunophenotype of desmoid-type fibromatosis were observed after XRT. In rare cases of recurrent desmoid-type fibromatosis, the differential diagnosis may include radiation-induced fibrosarcoma. Our data suggest that histomorphologic alterations attributable to the effects of ionizing radiation in desmoid-type fibromatosis are minimal. Therefore, the presence of cytologic features of malignancy in this setting warrants careful examination and consideration of the rare occurrence of postradiation sarcoma. Lesser degrees of nuclear atypia seen in isolation do not necessarily indicate a poor prognosis in irradiated desmoid-type fibromatosis.

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