To estimate the risk of developing a second primary cancer (SPC) among women diagnosed with breast cancer in Granada, Spain.

This population-based observational cohort study utilized data from the Granada Cancer Registry. We included all women aged 15 and above diagnosed with malignant breast cancer from 1985 to 2019.

The analysis used the standardized incidence ratio to compare breast cancer patients’ risk of developing a SPC with the risk in the general population.

Out of 12,260 women diagnosed with breast cancer, 1032 (8.4 %) developed a SPC. The median time to SPC diagnosis was 7.2 years. The most common types of SPC were non-melanoma skin, colorectal, endometrial and haematological malignancies. The overall excess risk of a SPC was 33 % (95 % CI = 25 %–41 %), with the highest increased risks observed for non-melanoma skin, digestive, and genitourinary malignancies. The risk of a SPC was 2.2 times greater than expected in women under 50, and 1.2 times in women over 50. For those under 50, the risk was nearly 6 times higher for digestive sites, and 3 to 5 times higher for lung, non-melanoma skin, genitourinary, and haematological cancers. In women over 50, excess risks were also observed in those sites, but to a lesser extent.

Women with breast cancer have a significantly increased risk of developing a SPC, especially non-melanoma skin, digestive, and genitourinary cancers. Personalized prevention and follow-up strategies are needed to reduce the risk in women diagnosed with breast cancer, particularly younger women.