## 4 Discussion  
This treatment planning study deals with the radiogenic risks attributable to radiotherapy with photon beams for thymoma. The risk assessment is of great importance for patients with thymoma who are often diagnosed between 40 and 60 years of age [2] and have a long-term survival [4]. Three different treatment plans with the three most common photon-beam radiotherapy techniques used for the management of this disease were generated for each study participant. The dosimetric data extracted from the 3D-CRT, IMRT, and VMAT plans were employed to estimate the risks related to the development of secondary malignancies in the surrounding heavily irradiated organs and to the presence of cardiac diseases in cancer survivors. The LAR range for the appearance of second primary cancers, as derived from all study participants, was 0.05% to 6.85% by the OAR under investigation and the applied radiotherapy technique. The highest mean LAR values were observed for lung cancer induction irrespective of the treatment used. The mean organ-specific LAR values associated with the three radiotherapy techniques were similar. The maximum absolute difference was found for the mean LAR value for right breast cancer induction as estimated by 3D-CRT and IMRT plans. This maximum risk difference was equal to 0.3%. Despite the small absolute differences, some statistical comparisons of the organ- and patient-specific cancer risks derived from 3D-CRT, IMRT, and VMAT plans led to significant results. The IMRT significantly reduced the breast, esophageal, and thyroid cancer risk compared to the 3D-CRT. VMAT resulted in considerably smaller probabilities of esophageal and thyroid malignancies than conformal treatment. No statistical difference was observed for the lung cancer risk estimated by the three radiotherapy techniques. The IMRT was found to lead to significantly lower left and right breast cancer risks with respect to VMAT. The radiotherapy-induced cancer risk values need to be compared with the baseline risks of healthy females. This comparison may accent the true effect of radiotherapy in the process of carcinogenesis. For example, the mean LAR for developing lung, breast, esophageal, and thyroid malignancies following the present thymoma IMRT was estimated to be 4.49%, 2.06%, 0.30%, and 0.05%, respectively (Table 3). The corresponding baseline risks for a healthy 50-year-old female in the USA were 6.15%, 11.51%, 0.24%, and 1.05% according to the SEER Cancer Statistics Review [29]. The ratio of the baseline risk to the lifetime radiation-induced lung, breast, esophageal, and thyroid cancer risk is 1.4, 5.6, 0.8, and 21.0, respectively. These ratios show that the probabilities for lung and esophageal cancer induction after IMRT for thymoma are comparable with those for the unexposed population, and, therefore, they should not be ignored in the follow-up of the irradiated cancer survivors. The probabilities for breast and thyroid cancer induction due to radiotherapy for thymoma are much smaller than the nominal probabilities. To our knowledge, this is the first study providing the lifetime risk for second cancer induction following thymoma irradiation with photons. Previous studies gave the excess absolute risk (EAR) of carcinogenesis, making the comparison with our results to be rather difficult [8; 9; 10; 11]. Moreover, most of the above reports [9; 10; 11] used different approaches for cancer risk assessments from the model employed in our work. They selected the plateau or the linear-exponential models, which do not take into account the tissue-specific ability for cell repopulation between dose fractions. No comparison has been made between the EAR estimates and the nominal risks for cancer appearance in the published reports. [PERSON] et al. [9] reported that the IMRT compared to 3D-CRT significantly reduces the incidence rate to bilateral breast cancer risk. They also reported an insignificant difference in the lung, thyroid, and esophageal cancer risk estimates derived from 3D-CRT and IMRT. [PERSON] et al.