Testicular mesothelioma is an exceedingly rare malignancy, with most cases originating in the pleura, peritoneum, or pericardium. Primary testicular mesothelioma is an unusual occurrence, especially in elderly patients without a known history of risk factors. This case report presents a 91-year-old male who developed a large, enlarging testicular mass, which was initially suspected to be a neoplastic lesion. Subsequent histopathological examination confirmed biphasic mesothelioma. Despite no significant history of other mass lesions or risk factors, staging revealed multiple hepatic metastases, leading to further treatment considerations.

A 91-year-old male was referred for evaluation of an enlarging right testicular mass, initially 25 x 20 x 18 mm, which had increased to 44 x 28 x 32 mm at the time of presentation. Ultrasound revealed a solid, heterogeneous mass located in the right epididymal body and tail. Additionally, the patient had a large right-sided hydrocele (527ml). The patient underwent radical orchiectomy, and histopathological analysis of the excised tissue revealed biphasic mesothelioma. Tumor markers (AFP 3, bHCG <1) were not suggestive of testicular germ cell tumors. Staging CT scan revealed multiple poorly enhancing lesions in the liver, suspicious for metastases. Further characterization of the hepatic lesions was planned with MRI.

Histopathology confirmed biphasic mesothelioma in the excised testicular tissue. Staging CT scan showed at least six lesions in most liver segments, consistent with hepatic metastases. There were no other primary masses identified. A liver MRI was recommended to guide treatment decisions. The patient was referred to medical oncology for further discussions regarding chemotherapy options, given the advanced stage of the disease.

Testicular mesothelioma, particularly in elderly patients without typical risk factors, is an exceedingly rare and aggressive malignancy. Early recognition and staging are critical for planning appropriate treatment. In this case, metastatic spread to the liver highlights the poor prognosis of testicular mesothelioma. While treatment options may offer some benefit, the prognosis remains guarded due to the advanced stage of the disease. This case underscores the need for consideration of rare diagnoses when evaluating enlarging testicular masses, particularly in atypical presentations.