Background: Cancers of unknown primary (CUP) are defined as histologically confirmed metastatic cancers that do not have an identified primary site of origin despite an appropriate diagnostic work-up. It is known that accessibility to and quality of medical care are associated with cancer diagnosis. This study aimed to describe the demographic, histologic, and temporal trend characteristics of CUP patients in the Military Health System (MHS), which provides universal healthcare access.

Methods: The data were obtained from the Department of Defense (DoD)'s Automated Tumor Registry (ACTUR) which collects cancer data from beneficiaries who were diagnosed or received treatment in the (MHS). We described the demographic and histologic distributions in CUP patients aged 18 or older diagnosed from 1987-2013. We calculated the proportion of CUP patients among all metastatic cancers and the most common histologic categories of those tumors. We then evaluated whether the proportion of histologic types changed over time.

Results: CUP comprised 15.1% of all metastatic cancers in ACTUR during the study period. The majority of CUP within ACTUR was well and moderately differentiated adenocarcinoma (51.3%) and poorly differentiated carcinomas (23.2%) followed by squamous cell carcinomas (12.5%). The percentages of CUP among metastasized cancers of the same histologic category ranged 12-15% for well and moderately differentiated adenocarcinomas, squamous cell, and poorly differentiated carcinomas, and 41-46% for malignant neuroendocrine carcinomas and undifferentiated neoplasms. However, the percentages varied by sex, race, and age for certain pathologies. The proportion of CUP patients among all metastatic cancer patients has steadily declined from 22.4% to 8.3% from 1987 to 2013.

Conclusion: The proportion and trends of CUP in the MHS were generally consistent with other descriptive CUP studies. This study provides a description of CUP in a healthcare system with universal access and provides a foundation for future studies on CUP.

Citation Format: Julie A. Bytnar, Jie Lin, Joel T. Moncur, Craig D. Shriver, Kangmin Zhu. Cancers of unknown primary: A descriptive study in the U.S. military health system [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2021; 2021 Apr 10-15 and May 17-21. Philadelphia (PA): AACR; Cancer Res 2021;81(13_Suppl):Abstract nr 797.