Penile Metastasis as the Presenting Symptom of Colorectal Carcinoma: A Rare Case Report

<b>Background:</b> Secondary penile cancer, despite the region's rich vascularization, is a rare phenomenon with only around 500 cases ever reported, typically of genitourinary origin and in even rarer cases, of colorectal adenocarcinoma. Unfortunately, the underlying mechanisms are not well elucidated, and prognosis remains poor with a late onset and median overall survival of 9 months for colon adenocarcinoma. Secondary penile cancer often presents alongside concurrent metastases months or years following successful treatment of the primary tumor. However, we report a case of an isolated penile metastasis as the presenting symptom of colon adenocarcinoma with no identifiable primary lesion or history of malignancy. <b>Case Summary:</b> A 67-year-old African-American male presented with a 1-month history of voiding symptoms, whereupon follow-up revealed a penile mass near the left base of the penis. Postoperative histopathological, immunohistochemical, and genomic analyses revealed characteristics of invasive and metastatic colorectal adenocarcinoma. Initial diagnostic testing revealed elevations in serum tumor markers CA 19-9 and carcinoembryonic antigen, while whole body PET/CT scan and colonoscopy failed to identify any tumorigenic lesions or primary colorectal malignancy. Although hypermetabolic activity was noted near the base of the penis and bilateral inguinal lymph nodes, the patient is currently on chemotherapy with a modified FOLFOX-6 regimen with active surveillance and no adverse effects. <b>Conclusion:</b> Here, we report a rare case of isolated penile metastasis as the first presentation of colon adenocarcinoma with no primary lesion. Regardless of origin, secondary penile cancer is a rare phenomenon with a poor prognosis. While the exact mechanism of spread is uncertain, the most probable mode of dissemination is through venous networks. There is also no standard of treatment relying on surgical, therapeutic, and palliative management. Although unclear, our unique presentation may portend a more favorable prognosis with continued treatment and observation.