Relative risk of second malignant neoplasms highest among young adult cancer patients - a population-based registry study in Finland.

Background and purpose: The objective of this study was to explore the incidence of second malignant neoplasms (SMNs) among adult cancer patients in Finland diagnosed with their first primary cancer (FPC) in 1992-2021.

Material and methods: The study used data from the population-based Finnish Cancer Registry (FCR). Risk estimates were calculated using the standardised incidence ratio (SIR), the ratio of observed second cancers compared to the expected numbers assuming the same cancer incidence as the corresponding sex-age-calendar year -split of the general population.

Results: A total of 573,379 FPCs were diagnosed during 1992-2021. During the follow-up, 60,464 SMNs were diagnosed. Male cancer patients had neither a decreased nor an increased risk (SIR 1.00 [95% CI, 0.99-1.01]) and female patients had an 8% increased risk (SIR 1.08 [95% CI, 1.06-1.09]) of developing any SMN compared to a FPC in the general population. The highest SIR of any SMN was observed in patients aged 20-39 -years at FPC diagnosis, and the SIR decreased by increasing age at diagnosis. Patients with lymphoid and haematopoietic tissue neoplasms, cancers of the mouth and pharynx, endocrine glands, respiratory and intrathoracic organs, skin, and urinary organs had the highest SIRs, while patients with cancers of the male genital organs and the female breast had the lowest SIRs.

Interpretation: Elevated SIRs were observed in cancer patients diagnosed at an early age and for FPCs known to be in large part attributable to lifestyle factors, which highlights the importance of monitoring and encouraging lifestyle changes.