Introduction and purpose: In radiation therapy, accurate dose distribution to the target volume requires precision in treatment setup. Patient setup errors are unavoidable and always present during the treatment process. By identifying these errors, it is possible to determine the setup margin from the Clinical Target Volume (CTV) to the Planning Target Volume (PTV) to cover the target volume while minimizing the dose to adjacent healthy tissues. This study aims to evaluate the random and systematic errors occurring during the setup of pelvic radiation therapy using the On-Board Imaging (OBI) system, and subsequently propose an optimal setup margin distance from CTV to PTV for patients with pelvic cancer.

Materials and methods: A total of 40 patients evenly divided between rectal and cervical cancer for VMAT treatment were given radiation therapy at Vinmec Times City International Hospital from 01/2023 to 8/2024. The patients were scanned by CT simulation on CT Optimal 580 (GE Medical System, Milwaukee, Wisconsin USA), planning on the Eclipse software (ver 13.0) of varian firm (USA), The patients were taken photo verification 2D-KV images (daily) and 3D-CBCT (Monday and Thursday) are analyzed online on OBI software and compared with DRR images from the plan. , thereby evaluating and recording errors in 3 directions front after (AP Anterior-Posterior); above and below (SI Superior-Inferior); left right (LR Lateral). Random and systematic errors are identified based on the collected error data, The set-up margin for CTV to PTV was calculated by published margin formulae of Van Herk et al.

Results: A total of 534 pairs of 2D-KV cervical images and 448 pairs of 2D-KV rectal images were performed and evaluated. For cervical cancer, the systematic error and random error in the AP, SI, and LR directions were (1.28 ± 1.92 mm), (2 ± 3.5 mm), and (1.41 ± 1.78 mm), respectively. For rectal cancer, the systematic error and random error in the AP, SI, and LR directions were (1.18 ± 1.57 mm), (2.01 ± 3.06 mm), and (0.89 ± 1.89 mm), respectively. The setup margin required to cover the target volume retrospectively was calculated based on Van Herk et al were: 5mm (AP), 7mm (SI), 5mm (LR) for the cervix uteri and 5mm (AP), 7.5mm (SI), 5mm (LR) for the rectum.

Conclusion: The assessment and verification of images before treatment with OBI system helps reduce setting errors, increase dose in tumors, reduce unwanted side effects and allows estimating the setup of the planning target volume (PTV) expansion according to our configuration. Based on the results of the study, we propose that setup margin with radiotherapy for the pelvis at our hospital are: 5mm (AP), 7mm (SI), 5mm (LR) for the cervix uteri and 5mm (AP), 7.5mm (SI), 5mm (LR) for the rectum.

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