Second primary cancers among males with a first primary prostate cancer: a population-based study in Northern Portugal.

Increased survival and life expectancy among patients with prostate cancer results in an increased risk of developing a second primary cancer (SPC). We aimed to describe the occurrence of SPCs in a population-based cohort with a prostate first primary cancer (FPC), in Northern Portugal. A cohort of 13,222 patients with a prostate FPC from the North Region Cancer Registry of Portugal, diagnosed between 2000 and 2009, was followed until 31 December 2021, for synchronous (within six months of FPC diagnosis) and metachronous SPCs (all others). We describe absolute and relative frequencies of SPCs, incidence rates and standardized incidence ratios of SPCs (compared to the male general population), and cumulative incidence of metachronous SPCs. A total of 1953 (14.8%) patients with a prostate FPC developed an SPC, mostly of the colon, lung and bladder; synchronous SPCs occurred mainly in the bladder. Compared to the general male population, patients with a prostate FPC had a globally lower incidence of all cancers, and lung and oesophagus cancers, but a higher incidence of bladder and pancreas cancers. Overall, the incidence of synchronous SPCs was also significantly higher, likely reflecting the incidental diagnosis of SPCs. The 20-year cumulative incidence of metachronous SPCs was 15.4%. Patients with a prostate FPC had a lower overall incidence of SPCs than the general male population, despite a higher incidence in the first six-months after the SPC diagnosis. After that period, one out of seven may be expected to develop an SPC within two decades. Continued cancer surveillance among survivors is needed.