A seminal vesicle cyst complicated with a tumor like nodular mass of benign proliferating prostatic tissue: a case report with ultrastructural and immunohistochemical studies.

We report a seminal vesicle cyst complicated with a tumor-like nodular mass of benign proliferating prostatic tissue. The patient was a 53-year-old Japanese man. A cyst of approximately 4.5 cm in diameter was discovered at the left seminal vesicle area. In the inner part of the cyst, a papillary nodular mass of 0.7 cm in diameter was seen. Under the clinical diagnosis of a seminal vesicle cyst with a tumorous mural nodule, the patient underwent resection of the seminal vesicle cyst to rule out the possibility that the nodular mass in the cyst was a neoplasm of an especially malignant nature. Microscopic examination of the excised specimen revealed a small dome-like nodular mass on the luminal surface of the cyst consisting of nodular proliferation of benign tubular gland tissue with various configurations. Conventional histologic, immunohistochemical, and ultrastructural analysis showed the proliferating cells in the nodular mass consisted of the benign prostate type. It is extremely important to differentiate between a benign proliferation and a malignant one, when the nodular mass is found in the seminal vesicle cyst.