Testicular germ cell tumour regression is a rare phenomenon, where the primary testicular tumour spontaneously regresses, typically with metastatic disease at presentation. We present a case of a regressed germ cell tumour (GCT) in a 44-year-old post-pubertal male. Initially treated for suspected infection, the patient\'s testicular swelling prompted further investigation, leading to a radical orchidectomy that revealed the unusual histomorphologic findings of an entirely necrotic, non-seminomatous GCT consistent with a pure embryonal carcinoma.

Testicular neoplasms, or testicular cancer, are not typically seen in the emergency department (ED) since their presentation involves a painless hard mass that emerges slowly over time. Uncommon presentation of testicular neoplasm to the ED with acute onset of scrotal pain may present challenges as an incomplete physical examination without supplemental imaging and laboratory workup may overlook the diagnosis of testicular neoplasm. As a result, a delay in proper treatment may occur. Early recognition of testicular neoplasm can decrease morbidity and mortality and improve overall patient survival. Here, we present a case of a 32-year-old male who presented in the ED with an acute onset of testicular pain localized on the posterior right side of the scrotum. Despite the unusual presentation, a complete physical examination, including a complete genitourinary system exam, was performed. During the physical examination, a high index of suspicion for testicular neoplasm was present. Necessary imaging and laboratory workup were ordered. Based on the findings, testicular neoplasm was highly suspected. Thus, surgical intervention was pursued to remove the suspicious mass and pathology revealed a mixed germ cell tumor. Further imaging and laboratory workup showed metastasis into other organ systems, and medical management was chosen to treat the metastatic neoplasm systemically.

A 25-year-old male presented to our hospital with two months of progressively worsening left arm swelling, intermittent left-sided chest pressure, and a painless right testicular mass. CT of the chest, abdomen, and pelvis revealed a large mediastinal mass, multiple lung nodules, and several large right testicular nodules. The patient underwent a CT-guided biopsy of his right lung nodule, followed by a radical right inguinal orchiectomy. The testicular biopsy revealed a mixed germ cell tumor (GCT) consisting of 97% seminoma and 3% teratoma, while the lung biopsy revealed metastatic choriocarcinoma. The patient was treated with four cycles of bleomycin, etoposide, and platinum (BEP) and showed a great clinical response, with only residual disease in his retroperitoneal lymph nodes. He was referred for retroperitoneal lymph node dissection (RPLND); however, there was a delay of several months, which led to the recurrence of his disease. He received four cycles of paclitaxel, ifosfamide, and cisplatin and showed a moderate response. He later received salvage chemotherapy with high-dose carboplatin and etoposide and underwent bone-marrow transplant, leading to complete clinical response and eradication of his disease. There are different subtypes of testicular GCTs, each with distinct pathogenesis, treatment modality, and prognosis. In this report, we discuss the case of a patient who presented with a mixed GCT consisting of seminoma and teratoma in his testicle, which had metastasized as choriocarcinoma to his lung and mediastinum. This report elucidates the potential for testicular GCTs to metastasize as a pathologically different cancer compared to the primary tumor. This phenomenon has significant clinical ramifications, as it can considerably alter a patient\'s treatment and prognostic outcomes.