Ossifying renal tumor of infancy: the first Japanese case with long-term follow-up.

Presented is the first Japanese case of ossifying renal tumor of infancy (ORTI), which is an extremely rare tumor and of which only nine cases have been reported in the literature. The patient was a four-month-old Japanese boy presenting as hematuria. A small nodular tumor (approximately 1 cm in size) protruded into the right pelvis from the superior caliceal region. Histologically the tumor originated from the tip of one medullary papilla and consisted of small rounded cells with a dense arrangement and several foci of minimally calcified osteoid lesions, including fewer but larger cells. Transition from the small to larger cells was suggested. The small cell only expressed vimentin, but the larger coexpressed vimentin, desmin, and epithelial membrane antigen. These findings suggested that the small cells differentiated to the larger cells with an epithelial nature associated somehow with osteoid formation. Embryonal metaplasia was assumed to occur in the collecting ducts entrapped by the tumor. The patient has been well without recurrence for more than 23 years. This is the longest survival period compared with other reported cases and confirms the benign nature of ORTI. It is absolutely crucial to make an accurate diagnosis of ORTI to avoid unnecessary treatment.