Primary malignant mesothelioma of the tunica vaginalis presenting as hydrocele: imaging pitfalls.

A man in his late 70s presented with progressive, painless scrotal swelling. The initial ultrasound was reported as a simple hydrocele, and he was scheduled for surgery. At operation, friable nodules on the tunica vaginalis were biopsied, confirming malignant epithelioid mesothelioma. Staging CT showed no pleural or peritoneal disease, supporting a primary tunica vaginalis origin. As the tunica vaginalis is an embryological continuation of the peritoneum, it is exceptionally rare for mesothelioma to arise here as a true primary rather than as a secondary spread from the peritoneum. This case highlights a rare primary malignant mesothelioma masquerading as a hydrocele, the limitations of ultrasound as a sole diagnostic tool and the importance of maintaining suspicion when operative findings are discordant with imaging.