Metastasizing pleomorphic adenoma presenting as an asymptomatic kidney tumor twenty-nine years after parotidectomy – urological viewpoint and overview of the literature to date

Pleomorphic adenomas (benign mixed tumors) are the most common tumors of glandular origin in the head and neck and are one of the few benign neoplasms that can undergo malignant transformation.(1) Mixed tumors that are seemingly benign at the microscopic level but metastasize have been termed metastasizing mixed tumors (MZMTs). The entity of metastasizing benign mixed tumors has been reported since the early 1940s, with up to approximately 50 cases described in the literature to date. Despite their bland morphologic appearance, MZMTs have been associated with an overall mortality rate of about 20-40%. We report the case of a MZMT of the kidney almost 30 years after lateral parotidectomy owing to the same tumor entity. For benign mixed tumors, we are unaware of more than two other cases of metastasis to the kidney that have been published, whereas metastases to the bone, lung, and lymph nodes are more common. Parotidectomy is widely accepted as the first choice of treatment,(13) but once metastases have occurred the therapeutic strategy is uncertain with surgery being the only curative option in cases with resectable disease. This case report provides information about the rare event of metastatic disease to the kidney and points out therapeutic strategies. However, in view of the general lack of adequate information in the literature, the best therapy for systemic disease still remains unresolved.