Secondary cancer risk after radiotherapy of seminoma stage one

Background: In radiation treatment of stage one seminoma (SOS) induced secondary cancer in organs at risk (OARs), located in or out of the radiation fields, is a late toxicity of major concern. This study aimed to compare the secondary cancer risk in radiotherapy of SOS in two-dimensional conventional radiation therapy (2D) and three-dimensional conformal radiation therapy (3DCRT). Materials and Methods: CT scan images of 10 patients with SOS were used to design 2D conventional and 3D conformal treatment plans using 25 Gy in 20 sessions. The life attributable risk (LAR) of liver, stomach, and colon were calculated using the organ equivalent dose (OED) model for organs in radiation field and Biologic Effects of Ionizing Radiation VII (BEIR VII) model for organs out of field. Results: LAR of OARs in radiation field such as, liver and stomach was obtained 40% higher in the 2D treatment than in the 3D treatment, while as for the colon, it was 17% lower in the 2D treatment than in the 3D treatment. The LAR values of kidneys located outside the radiation field in the 2D treatment were calculated 0.04%. Conclusion: Increasing the prescribed dose (25 vs. 20) as well as the number of treatment sessions (20 vs. 10) resulted in increasing in the LAR of at-risk organs such as liver, stomach, colon. Therefore, estimating cancer risk of critical organs exposed to radiation through examining the effects of dose fractionation and prescribed doses can be used in optimizing of treatment plan for seminoma, selection of a better treatment method by oncologists, and patient follow-up.