Risk of secondary sarcoma after abdominopelvic cancer treatment: Results from a contemporary cohort.

Abstract

317 Background: Our objective was to explore the risk of sarcoma in a large cohort of patients with localized abdominopelvic cancer treated with radiotherapy (RT) compared to surgery. The risk of other secondary non-sarcoma solid organ tumors was also explored. Methods: This is a population-based retrospective cohort study examining the risk of sarcoma in patients with localized prostate, bladder, colorectal, cervical, uterine, and testis cancer between January 1, 2002-December 31, 2016. Multivariable Cox proportional hazard analysis was used to compare time from primary treatment (radical surgery or RT) to second cancer, adjusting for age, comorbidity, income quintile, and rurality, accounting for death as a competing risk. The standardized incidence rate (SIR) was calculated as the ratio of the observed divided by the age- and sex-stratified expected number of sarcoma cases from the Ontario population. Results: 173,580 patients were included (79,662 underwent surgery and 93,918 underwent RT). Most patients had genitourinary (51.4%) or colorectal cancer (39.9%) and 24.4% received chemotherapy. Overall, 332 sarcomas developed over a median 5.7 years (IQR: 2.2-8.9) and were more common in the RT group (239/93918 [0.3%]) compared to the surgery group (93/79662 [0.1%], p<0.001). RT exposure (unadjusted HR=2.67, 95%CI:2.10-3.40, p<0.001, adjusted HR=2.59, 95%CI:2.03-3.31, p<0.001) and perioperative chemotherapy use (HR=1.31, 95%CI:1.03-1.69, p=0.038) were associated with an increased relative rate of sarcoma. Patients who received RT had an increased risk of sarcoma compared to the general population (SIR=1.46, 95%CI:1.12-1.90, p=0.005). For our secondary outcome, patients who received RT had an increased hazard of developing a second non-sarcoma solid malignancy compared to patients who underwent surgery (HR=1.16, 95%CI:1.10-1.22, p<0.001). Conclusions: This is the largest cohort investigating sarcoma in patients with abdominopelvic cancer. Patients treated with RT have a markedly increased risk of sarcoma compared to the general population. RT was an independent risk factor for sarcoma and non-sarcoma second malignancy.