Second primary cancers in people who had cancer as children: an Australian Childhood Cancer Registry population-based study.

Objective: To investigate the incidence of second primary cancers in people diagnosed with cancer during childhood.

Design, Setting: Retrospective, population-based study; analysis of Australian Childhood Cancer Registry data.

Participants: People alive at least two months after being diagnosed before the age of 15 years with a primary cancer, 1983-2013, followed until 31 December 2015 (2-33 years' follow-up).

Main Outcome Measures: Risks of second primary cancer compared with the general population, expressed as standardised incidence ratios (SIRs).

Results: Among 18 230 people diagnosed with cancer during childhood, 388 (2%) were later diagnosed with second primary cancers; the estimated 30-year cumulative incidence of second cancers was 4.4% (95% CI, 3.8-5.0%). The risk of a new primary cancer was five times as high as for the general population (SIR, 5.13; 95% CI, 4.65-5.67). Relative risk of a second primary cancer was greatest for people who had childhood rhabdomyosarcoma (SIR, 19.9; 95% CI, 14.4-27.6). Relative risk was particularly high for children who had undergone both chemotherapy and radiotherapy (SIR, 9.80; 95% CI, 8.35-11.5). Relative risk peaked during the 5 years following the first diagnosis (2 to less than 5 years: SIR, 10.3; 95% CI, 8.20-13.0), but was still significant at 20-33 years (SIR, 2.58; 95% CI, 2.02-3.30). The most frequent second primary cancers were thyroid carcinomas (65 of 388, 17%) and acute myeloid leukaemias (57, 15%).

Conclusions: Survivors of childhood cancer remain at increased risk of a second primary cancer well into adulthood. As the late effects of cancer treatment probably contribute to this risk, treatments need to be refined and their toxicity reduced, without reducing their benefit for survival.