Mesectodermal Leiomyoma of the Ciliary Body: A Unique Variant of Leiomyoma with Myogenic and Neurogenic Histological Features.

Mesectodermal leiomyoma of the ciliary body is a rare benign tumor, showing both neurogenic and myogenic characteristics. This tumor typically shows predilection for women of reproductive age. Because it is almost impossible to clinically distinguish this tumor from malignant melanoma, unnecessary eye enucleations have been unfortunately performed. Herein, we report a case of mesectodermal leiomyoma of the ciliary body in a young Japanese woman. She was referred to our hospital due to a slow-growing mass in her left iris. A malignant tumor could not be clinically ruled out and surgery with intraoperative pathology consultation was performed. Intraoperative frozen section diagnosis was a benign tumor with neurogenic features, and a simple excision of the tumor was performed. Histologically, the tumor was composed of diffuse growth of spindle cells with fibrillary indistinctive cytoplasm. Immunohistochemical examination showed diffuse positive staining of α-smooth muscle actin, h-caldesmon, calponin, and CD56. Scattered tumor cells were weakly positive for desmin. Neither melanocytic markers nor neural markers except for CD56 were positive. We diagnosed this tumor as mesectodermal leiomyoma. Mesectodermal leiomyoma is rare and often misdiagnosed as malignant melanoma. To avoid overtreatment, a correct preoperative diagnosis is essential.