Second Primary Malignancies in Patients with Colorectal Cancer: The Frequency, Types, and Timeline.

: Colorectal cancer (CRC) is the third most common cancer worldwide. Accordingly, we aimed to investigate the frequency, types, and timelines of other cancers in patients diagnosed with CRC. : Patients diagnosed with CRC between 1999 and 2023 were retrospectively included. We collected data on the demographics and diagnosis of any additional cancer, either prior to or following the CRC diagnosis. Data were retrieved from the Clalit database using the MDClone platform. : A total of 65,774 CRC patients were identified between 1999 and 2023. A second primary malignancy (SPM) was diagnosed in 13,679 patients (21%). Being female, smoking, and having chronic obstructive pulmonary disease were more frequent among patients with a SPM. Breast, prostate, and lung cancers were the most prevalent SPMs. The risk of developing SPM was highest during the first three years after CRC diagnosis, especially with respect to lung cancer, whereas the majority of breast and prostate cancers were diagnosed during the three years before CRC; the cumulative standardized incidence ratio was calculated to be 4.1 for any cancer. : CRC patients have an increased second primary cancer. Patients diagnosed with CRC should be encouraged to undergo screening for other cancers, and those with Lynch-related cancers should be investigated for the syndrome. Patients with a diagnosis of breast or prostate cancer should be considered for colonoscopy screening.