We report here a case of leiomyosarcoma in the sacrum with a differential diagnosis of metastasizing leimyoma of uterus, since the patient had a past history of resection of uterine leimyoma 19 years ago. The sacral tumor was an osteolytic lesion, 8x6 cm in size on radiological examination. Microscopically, tumor cells consisted of spindle shaped cells with moderate cellular atypia. The tumor cells invaded into the surrounding muscle tissue. Immunohistochemistry revealed that the tumor cells were positive for α-SMA and vimentin, and they were enclosed by type IV collagen, suggesting the presence of the basement membrane. The labeling index of Ki67 in the tumor cells was 25 %. Re-examination of leiomyomas of uterus resected 19 years ago showed that they were typical leiomyomas, showing well-circumscribed tumors, composed of well-differentiated smooth muscle cells without nuclear atypia. The presence of radiological and pathological findings of malignancy of the sacral tumor excluded the possibility of metastasizing leiomyoma, suggesting that the sacral tumor was another primary tumor.
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