Background: An increasing number and proportion of cancer patients with apparently localised disease are treated with chemotherapy and radiation therapy in contemporary oncology practice. In a pilot study of radiation-induced sarcoma (RIS) patients, we demonstrated that chemotherapy was associated with a reduced time to development of RIS. We now present a multi-centre collaborative study to validate this association. Methods: This was a retrospective cohort study of RIS cases across five large international sarcoma centres between 1 January 2000 to 31 December 2014. The primary endpoint was time to development of RIS. Results: We identified 419 patients with RIS. Chemotherapy for the first malignancy was associated with a shorter time to RIS development (HR 1.37; 95% CI: 1.08-1.72; P=0.009). In the multi-variable model, older age (HR 2.11; 95% CI 1.83-2.43; P<0.001) and chemotherapy for the first malignancy (HR 1.61; 95% CI 1.26-2.05; P<0·001) were independently associated with a shorter time to RIS. Anthracyclines and alkylating agents significantly contribute to the effect. Conclusions: This study confirms an association between chemotherapy given for the first malignancy and a shorter time to development of RIS.