Immunohistochemistry-guided management of colon-like CUP in a 20-year-old female: a case report

Cancer of unknown primary site (CUP) is characterized by metastatic disease without an identifiable primary tumor, even after extensive evaluation. It accounts for approximately 2% of all cancers and is most commonly diagnosed in older adults, with the highest incidence in patients aged 60-75 years. Diagnosis involves clinical, pathological, and imaging evaluations, including PET and immunohistochemical (IHC) staining, to determine tumor lineage and guide treatment. We present a case of a 20-year-old female with a right inguinal mass. Initial evaluation revealed metastatic adenocarcinoma with no primary tumor identified. The patient’s history included a prior breast fibroadenoma excision, and comprehensive diagnostic work-up, including PET scan, was unremarkable. Immunohistochemistry results were consistent with colon-like CUP, and the patient underwent excision of the inguinal node followed by adjuvant chemotherapy with the FOLFOX regimen. CUP treatment and prognosis vary by tumor subtype, with colon-like CUP typically managed using colorectal cancer-specific therapies. This case underscores the importance of IHC in guiding treatment for CUP and highlights the role of site-specific therapy in improving patient outcomes. Further studies are needed to validate treatment strategies in younger patients with CUP.