PURPOSE: Cardiac implantable electronic devices (CIEDs) were believed to possess a tolerance dose to malfunction during radiotherapy. Although recent studies have qualitatively suggested neutrons as a cause of malfunction, numerical understanding has not been reached. The purpose of this work is to quantitatively clarify the contribution of secondary neutrons from out-of-field irradiation to the malfunction of CIEDs as well as to deduce the frequency of malfunctions until completion of prostate cancer treatment as a typical case. MATERIALS AND METHODS: Measured data were gathered from the literature and were re-analyzed. Firstly, linear relationship for a number of malfunctions to the neutron dose was suggested by theoretical consideration. Secondly, the accumulated number of malfunctions of CIEDs gathered from the literature was compared with the prescribed dose, scattered photon dose, and secondary neutron dose for analysis of their correlation. Thirdly, the number of malfunctions during a course of prostate treatment with high-energy X-ray, passive proton and passive carbon-ion beams was calculated while assuming the same response to malfunctions, where X-rays consisted of 6-MV, 10-MV, 15-MV, and 18-MV beams. Monte Carlo simulation assuming simple geometry was performed for the distribution of neutron dose from X-ray beams, where normalization factors were applied to the distribution so as to reproduce the empirical values. RESULTS: Linearity between risk and neutron dose was clearly found from the measured data, as suggested by theoretical consideration. The predicted number of malfunctions until treatment completion was 0, 0.02±0.01, 0.30±0.08, 0.65±0.17, 0.88±0.50, and 0.14±0.04 when 6-MV, 10-MV, 15-MV, 18-MV, passive proton, and passive carbon-ion beams, respectively, were employed, where the single model response to a malfunction of 8.6±2.1 Sv-1 was applied. CONCLUSIONS: Numerical understanding of the malfunction of CIEDs has been attained for the first time. It has been clarified that neutron dose is a good scale for the risk of CIEDs in radiotherapy. Prediction of the frequency of malfunction as well as discussion of the risk to CIEDs in radiotherapy among the multiple modalities have become possible. Because the present study quantitatively clarifies the neutron contribution to malfunction, revision of clinical guidelines is suggested.