Abstract P3-12-14: Incidence of radiation induced sarcoma attributable to radiotherapy in adults: A population-based study of the SEER cancer registry across 17 primary tumor sites
Authors: Snow AE , Struycken LC , Koc M , Greco JP , Zhu Y , In GK , Dorff TB , Lang JE 
Published: May 2019

Abstract
Background: More than half of cancer patients will receive radiotherapy during the treatment of their cancer; however, a rare and fatal treatment complication is radiation induced sarcoma (RIS). Notably, a large percentage of RIS patients are breast cancer survivors. Previous studies have investigated the incidence of all secondary cancers due to radiotherapy but have not noted the incidence of RIS in a contemporary population-based cohort in the US. To evaluate the relative risk of RIS, we examined data from the Surveillance, Epidemiology, and End Results (SEER) database. We hypothesized that breast cancer would have a higher incidence of RIS compared to 17 other primary cancer sites.
Methods: This was a retrospective cohort study that examined SEER registries from 1973 and 2013. We included patients aged 18 years or older who were diagnosed with a primary cancer. We excluded patients with missing information on initial radiotherapy treatment or stage and those who died less than two years from the date of their primary cancer diagnosis. RIS was defined as those who developed a secondary sarcoma near the site of their original malignancy and after a 24-month latency period.
Results: Our study included 1,745,867 patients with an average age of 60 years and mean follow up time of 9.2 years. Breast cancer comprised the largest number with 693,701 patients of which 161 (0.02%) had a secondary sarcoma. Prostate cancer comprised the second largest group with 594,271 patients of which 75 (0.01%) had a secondary sarcoma. Of the 359 patients with secondary sarcomas, 242 (67.4%) had RIS. Breast cancer had the highest number of RIS patients at 126 (52.1%) compared to all combined non-breast cancer sites at 116. Interestingly, 172 (71.1%) RIS patients were females, which correlated with the attributable RIS cases from breast and uterine cancer. The relative risk of RIS in breast cancer versus 17 other primary cancer sites was 1.21 (CI: 1.01-1.45, p &lt;0.03, adjusted for age at primary diagnosis, gender, and latency).
Conclusions: This study confirms the low incidence of RIS in breast cancer, which has been demonstrated to be around 0.03% - 0.2% in the literature. Our results suggest that breast cancer portends a somewhat higher risk for RIS compared to 17 primary solid malignancies. Although some of the risk is attributable to the high incidence rate of breast cancer, breast cancer patients had a higher relative risk of RIS than other solid tumors after adjusting for covariates. Female gender may be associated with an increased risk for RIS compared to males. Further investigations should be performed to confirm these epidemiological findings given that radiation fields, doses, intention to cure, margin status and systemic therapy are not recorded in SEER. Nonetheless, this study is hypothesis generating in that the majority of RIS cases in solid tumors are attributable to breast cancer in this large population-based series.
Citation Format: Snow AE, Struycken LC, Koc M, Greco JP, Zhu Y, In GK, Dorff TB, Lang JE. Incidence of radiation induced sarcoma attributable to radiotherapy in adults: A population-based study of the SEER cancer registry across 17 primary tumor sites [abstract]. In: Proceedings of the 2018 San Antonio Breast Cancer Symposium; 2018 Dec 4-8; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2019;79(4 Suppl):Abstract nr P3-12-14.