Second cancers after radiotherapy: update and recommandations

While radio-induced cancers are well known since the first years of the xxth century, they did not represent a major concern for radiation oncologists for many decades. With better and better results of modern radiotherapy and prolonged follow-up of patients, secondary radio-induced cancers should now be systematically taken into account when irradiating patients. The analysis of the available literature allows to stress a few main points; (1) cancer patients are at a higher risk for developing secondary cancers than the general population, radiotherapy being only responsible for a (small) proportion of those second malignancies; (2) the clinical data emphasize the role of age, with children being much more susceptible to the carcinogenic effect of ionizing radiation than adults; (3) most radio-induced cancers occur in or close to the high-dose treatment volume; (4) the relative risk of radio-induced cancer appears to be different for different organs; (5) the relative risks of radio-induced cancers tend to be lower in the medical cohort studies than in the Japanese A-Bomb survivor studies; (6) several cofactors (genetic, lifestyle...) account for the risk of secondary and radiation-induced cancer. The exact shape of the dose/effect (carcinogenesis) curve is still debated, particularly for the high doses of radiotherapy, with a direct impact on risk calculations, which can be very different if using different radiobiological models. In spite of some uncertainties, a few main recommendations could be proposed to reduce as much as possible the risk of radio-induced cancer after radiotherapy: (1) adapting the irradiation technique; (2) reducing the target volumes; (3) adapting to patient's age; (4) adapting to specific organs; (5) and optimizing the imaging dose. In conclusion, even if radio-induced cancers are rare, they must be kept in mind each time a radiotherapy is proposed in 2018.