Uterine leiomyoma versus leiomyosarcoma: a new attempt at differential diagnosis based on their cellular characteristics.

Aim: To differentiate uterine leiomyoma (LM) with 'pseudosarcomatous' features from leiomyosarcoma (LMS). We hypothesized that because pseudosarcomatous LM is only a benign variant of conventional LM, it may have some characteristics different from LMS.

Methods And Results: Fourteen uterine smooth muscle tumours, which had been diagnosed as LMS previously, were reviewed and divided into adverse outcome (true LMS; seven cases) and excellent outcome groups (presumed pseudosarcomatous LM; seven cases) based on their clinical course. The tumours of the adverse outcome group were composed of rather uniform long spindle cells similar to those of leiomyosarcoma arising from the soft tissue. All tumours in this group had coagulative necrosis and all but one case were diagnosed preoperatively as malignant. In contrast, the tumours of the excellent outcome group showed significant histological diversity both between tumours and also in the same tumour and areas indistinguishable from conventional LM were often seen. All the tumours of this group were diagnosed preoperatively as benign. Immunohistochemically, all tumours of the excellent outcome group expressed progesterone receptor, often intensely, and also frequently expressed oestrogen receptor. In contrast, all the tumours of the adverse outcome group lacked them completely.

Conclusion: Pseudosarcomatous LM has characteristics useful for differentiating it from LMS, which are common in conventional LM.