Introduction: The carcinoma of unknown primary (CUP) is uncommon in the inguinal region, accounting for 1–3% of all CUPs. The diagnostic workup for CUP includes histopathological examination, imaging and, more recently, molecular testing. Nevertheless, the primary site frequently remains unknown.
Case Report: Herein, we report a case of 78-year-old male who was referred for evaluation of a painless swelling in the right inguinal region. Histopathological examination of the biopsy and resection specimens revealed transitional cell carcinoma (TCC) but no primary site was identified despite thorough clinical and radiological evaluation.
Conclusion: This is the first reported case of pure TCC of the inguinal lymph nodes presenting as a CUP. This may have occurred either from an unidentifiable primary lesion or the primary may have since regressed. Although there are no clear guidelines for the management of such patients, treatment must be multimodal and might include surgical resection and perhaps postoperative adjuvant radiotherapy to achieve optimal outcomes. Investigation with PET would have been useful in this case.

Keywords: Carcinoma of unknown primary, Inguinal lymph node, Transitional cell carcinoma