Clinical History:

A 52-year-old woman had irregular menstrual bleeding that persisted for months. On physical examination, the cervix appeared normal, but the uterus was enlarged and irregular. No adnexal masses were palpable. An endometrial biopsy had not shown evidence for a malignancy. She underwent a hysterectomy, and at surgery, the uterus appeared nodular. In surgical pathology on dissection, the nodules were firm, white, discrete, and appeared to arise in the myometrium.

The leiomyoma is composed of whorls of smooth muscle cells. The nuclei are regular and uniform. Mitotic figures (if present) are very rare.

Questions:

1. If multiple tumor masses are present, then why isn't this metastatic?

This is an example of multicentric origin of a neoplasm within an organ. The benign nature of these neoplasms is confirmed histologically. Each individual tumor is benign. Malignant tumors can also sometimes be multicentric in origin, but in general there is usually a single primary tumor from which metastases originate.

2. What is the malignant counterpart of this lesion and does it often arise from leiomyoma?

Leiomyosarcoma. Such malignant smooth muscle tumors usually arise de novo. Although about one out of every five women has a leiomyoma, leiomyosarcomas are quite rare.