Evaluation of set-up errors and determination of set-up margin in pelvic radiotherapy by electronic portal imaging device (EPID)

Introduction and purpose: The error in set-up of patients is an inherent part of treatment processes. The positioning errors can be used to determine the margins of the planning target volume (PTV) to cover the target volume, while minimising the radiation dose delivered to normal tissues. This study aimed to evaluate random and systematic errors occurring in inter-fraction set-ups of pelvic radiotherapy measured by electronic portal imaging device (EPID) and then to propose the optimum clinical target volume (CTV) to PTV margin in pelvic cancer patients. Materials and methods: This study examined 22 patients treated with pelvic radiotherapy. A total of 182 portal images were evaluated. Population random (sigma) and systematic (sigma) errors were determined based on the portal images in three directions (X, Y and Z). The set-up margin for CTV to PTV was calculated by published margin formulae of International Commission on Radiation Units and measurements (ICRU) report No. 62 recommendation and formulas presented by Stroom and Heijmen and Van Herk et al. Results: Systematic set-up errors for radiotherapy to patients ranged between 2 center dot 36 and 4 center dot 99 mm, and random errors ranged between 1 center dot 51 and 2 center dot 74 mm. The margin required to cover the target volume retrospectively was calculated based on ICRU 62 and formulas presented by Stroom and Heijmen and Van Herk et al. were used to calculate the range 2 center dot 8-5 center dot 7 mm, 5 center dot 7-11 center dot 9 mm and 6 center dot 9-14 center dot 4 mm, respectively. Conclusion: According to our findings, it can be concluded that by extending the CTV margin by 6 center dot 9-14 center dot 4 mm, we can ensure that 90% of the pelvic cancer patients will receive >= 95% of the prescribed dose in the CTV area.