Abstract
Metaplasia is a transformation of one mature cell type to other kinds of mature cells. Metaplasia is hardly detected in benign tumors, whereas it is frequently observed in malignant tumors. In this study, we report the first case of osseous metaplasia in canine leiomyoma. The region of osseous metaplasia was highly eosinophilic and had various sizes of distinct lacunae. The osseoid material was confirmed by Von Kossa staining. Except for the osseous metaplasia, the mass presented typical features of leiomyoma without any histopathological features of malignancy. The characteristics of malignant tumors, including pleomorphism, mitotic figures, and lymphatic metastasis were not identified and the mass was proven to be a benign tumor. The osseous metaplasia in leiomyoma without dysplastic changes might be due to plasticity which is a unique feature of smooth muscle cells, whether the tumor is malignant or not. This case suggests the possible occurrence of osseous metaplasia in leiomyoma, which has been overlooked due to the lack of reports. Also, it is recommended to pay attention to making a diagnosis of smooth muscle tumor with metaplasia so as not to be confused with leiomyosarcoma and leiomyoma variants.