A Case of Spontaneous Regression of Recurrent Desmoid Tumor Originating From the Internal Obturator Muscle After Delivery

Desmoid-type fibromatoses are pathologically benign but locally aggressive tumors. We report the case of a desmoid tumor that disappeared spontaneously after recurrence. A 21-year-old woman was referred to our hospital because of left lower limb weakness during menstruation. The following day this weakness had disappeared but menstrual colic remained; consequently, the patient underwent an internal examination that revealed an intrapelvic tumor. Magnetic resonance imaging demonstrated an enhanced mass (diameter, 8 cm) arising from the internal obturator muscle and attached to the urinary bladder. The tumor was diagnosed as a desmoid-type fibromatosis after histologic evaluation of a transvaginal biopsy; marginal resection was carried out at < 1 month after the first hospital admission. The patient experienced recurrence at 2 years after surgery, which was confirmed as two enhanced masses (diameter, 1 cm) using magnetic resonance imaging. Eleven months later, the diameters of these masses had increased to 1.8 cm; however, there was no further increase in size beyond this point. The patient delivered successfully at 5 and 7 years after surgery; at 8 years, the recurrent tumors had disappeared completely as confirmed by magnetic resonance imaging. This case involving recurrence is rare for two reasons. The first was that no change in the size of the tumors occurred during pregnancy and after delivery, and the second was that the patient experienced complete remission of the recurrent tumors after only simple observation. Thus, it is important to recognize that even a recurrent desmoid tumor can sometimes exhibit spontaneous regression.