Clinical outcomes of patients diagnosed with cancer of unknown primary or malignancy of undefined primary origin who were referred to a regional cancer center

BACKGROUND: A regional cancer hospital has been identified to be crucial in the management of malignancies of undefined primary origin (MUO) and cancer of unknown primary (CUP). This hospital primarily consists of oncologists with expertise in CUP, pathologists, and interventional radiologists. Early consultation or referral of MUO and CUP to a cancer hospital is deemed important. METHODS: This study retrospectively collected and analyzed the clinical, pathological, and outcome data of all patients (n = 407) referred to the Aichi Cancer Center Hospital (ACCH) in Japan over an 8-year period. RESULTS: In total, 30% of patients were referred for a second opinion. Among 285 patients, 13% had non-neoplastic disease or confirmed primary site and 76% had confirmed CUP (cCUP), with 29% of cCUP being identified as favorable risk. In 155 patients with unfavorable-risk CUP, 73% had primary sites predicted by immunohistochemistry (IHC) and distribution of metastatic sites, whereas 66% of them received site-specific therapies based on the predicted primary sites. The median overall survival (OS) was found to be poor in patients with MUO (1 month) and provisional CUP (6 months). In addition, the median OS of 206 patients with cCUP treated at the ACCH was 16 months (favorable risk, 27 months; unfavorable risk, 12 months). No significant difference was noted in OS between patients with non-predictable and predictable primary-sites (13 vs 12 months, p = 0.411). CONCLUSION: The outcome of patients with unfavorable-risk CUP remains to be poor. Site-specific therapy based on IHC is not recommended for all patients with unfavorable-risk CUP. SUPPLEMENTARY INFORMATION: The online version contains supplementary material available at 10.1007/s10147-023-02316-y.