Metastatic Prostate Cancer with a Testicular Mass: Rare Metastasis or Unexpected Primary Germ Cell Tumor?

We describe the case of a 78- year- old man with metastatic hormone- sensitive prostate adenocarcinoma who was diagnosed with a metachronous right testicular seminoma during follow- up. He initially presented with low back pain, weight loss, and a PSA level of 909 ng/mL. Prostate biopsy revealed a Gleason 9 (4+5)(4+5) adenocarcinoma with extensive osseous metastases and pelvic and interatocoval nodal involvement. He initiated androgen deprivation therapy with leuprorelin (preceded by bicalutamide) combined with apalutamide, achieving good biochemical and clinical response. At 12 months, progression of the interatocoval lymphadenopathy was observed, and he underwent palliative radiotherapy. At month 14, evaluation of right testicular enlargement revealed a mass subsequently confirmed as seminoma (pT1b, R0) following radical orchiectomy. Post- radiotherapy and post- orchiectomy computed tomography (CT) demonstrated significant regression of the retroperitoneal lymphadenopathy. This case underscores the importance of considering second primary malignancies during prostate oncologic follow- up, particularly when new clinical findings are not fully explained by the underlying disease.