Abstract

A 61-year-old Japanese woman had recurrent low-grade endometrial stromal sarcoma (ESS) after primary treatment 9 years earlier. Initial and recurrent tumors showed the same configuration that the polycystic part showed in the solid tumor ultrasonographically. The central part of both the initial and recurrent tumors showed typical histologic findings, but bizarre cells were detected in the peripheral layer of the recurrent tumor. They resembled glandular cells, Comet cells of high-grade ESS, fibroblasts, decidual cells, and predecidual endometrial stromal cells at the middle to late secretory phases. The change and differentiation of low-grade ESS by therapy and environment seem to be the same as the differentiation in mesenchymal stem cells. We speculate that recurrence and prognosis of low-grade ESS are related by extrauterine development but not by mitotic activity or DNA content.