Incidence and comparative prognosis of cancers with metastasis to noncommon sites: A population-based study

Primary tumors have common sites of metastasis such as lymph nodes, bones, liver, lungs, and brain; however, they can also metastasize to other uncommon sites such as adrenals, bone marrow, and skin among others. Our study aimed to investigate the relationship between uncommon sites of metastasis at the time of diagnosis and median survival in a number of primary tumors using the Surveillance, Epidemiology, and End Results (SEER) database. This retrospective cohort study conducted between September-October 2021 included patient-level SEER data for 2016-2018 using SEER Research Data, 9 Registries, Nov 2020 Sub (1975-2018). Descriptive analysis for complete cohort and median survival for each primary within the cohort was performed using R software. A total of 25,345 patients (females, 51.4%) were diagnosed with primary tumors with metastasis to uncommon sites at the time of diagnosis; the mean age at diagnosis was 68 years. Lung and bronchus primaries constituted the largest proportion of cohort (41.9%) that metastasized to uncommon sites, followed by nonHodgkin lymphoma-nodal (7.4%), pancreas (6.6%), stomach (3.7%), and ovarian (3.4%). The incidence of metastasis to uncommon sites was most common in respiratory cancers in ages 61-80 years (25%) and least in breast primaries in ages 18-40 years (0.1%), and was higher in Whites compared to other races. Regarding median survival, liver cancer with metastasis to uncommon sites had the worst prognosis (1 month), whereas small intestine tumors were associated with a better prognosis, median survival of 13 months. In this cohort study, the lung and bronchus cancers were the most common primaries metastasized to uncommon sites at diagnosis. The liver tumor had the worst survival compared to other tumors. These findings will help redirect the available screening tools to improve survival in patients with primary tumors with metastasis at diagnosis and may also play an essential role in future research and achieve a better prognosis for cancer patients.