Background and objective To assess the association between frailty/vulnerability and the development of radiotoxicity in older adults at a hospital in Peru. Material and methods Cohort study that included patients with cancer with medical recommendation of radiotherapy, who attended the outpatient department of geriatrics at Naval Medical Center (Lima, Peru) between 2013 and 2015. Before receiving radiation therapy sessions, we measured frailty using Fried phenotype, and vulnerability using VES-13 and G-8 scales. During radiotherapy, we determined the development of radiotoxicity. ROC curves and crude/adjusted Cox regression models were prepared to evaluate the association between the frailty/vulnerability scales and the development of radiotoxicity respectively. Results From a total of 181 patients (mean age: 78.1 years, all males), 33 (18.2%) developed some type of radiotoxicity. The appearance of radiotoxicity was higher in patients with frailty or vulnerability compared with those without these conditions, according to the usual cutoff points of Fried phenotype (10.9% versus 41.9%), VES-13 (5.0% versus 63.4%) and G-8 (5.6% versus 67.6%). The area under the curve was 0.61 (0.55-0.65) for the Fried phenotype, 0.79 (0.69-0.86) for the VES-13, and 0.86 (0.58-0.89) for the G-8. Conclusions We found an association between frailty/vulnerability and the development of radiotoxicity. The G-8 and the VES-13 scales had a better area under the curve than Fried phenotype, suggesting that those could be useful tools when deciding the dose of radiotherapy in geriatric patients.