ObjectiveThe objective of this study is to evaluate the impact of different belly board and daily changes in patient's body‐mass factor (BMF) on setup displacement in radiotherapy for rectal cancer.MethodsTwenty‐five patients were immobilized using the thermoplastic mask with belly board (TM‐BB), and 30 used the vacuum bag cushion with belly board (VBC‐BB), performing daily cone‐beam computed tomography (CBCT) scans 625 times and 750 times, respectively. Daily pretreatment CBCT scans were registered to the planned CT images for BMF change determination and setup displacement measurement. Independent t‐tests compared setup displacement between the two groups in left‐right (LR), superior‐inferior (SI), and anterior‐posterior (AP) directions, as well as the BMF changes. The impact of daily BMF changes on setup displacement was evaluated using multivariate logistic regression and 10‐fold cross‐validation.ResultsThe setup displacement for TM‐BB in the LR, SI, and AP directions were 0.31 ± 0.25, 0.58 ± 0.40, and 0.19 ± 0.18 cm, respectively, while VBC‐BB showed 0.19 ± 0.15, 0.26 ± 0.22, and 0.36 ± 0.29 cm in the corresponding directions, respectively. Margins of planning target volume (PTV) for TM‐BB were 8, 10, and 6 mm in LR, SI, and AP directions, while VBC‐BB showed margins of 5,7, and 8 mm, respectively. The daily BMF changes for both groups were ranked in descending order as follows: sacral rotation angle (RS), hip lateral diameter (HLD), and hip anterior‐posterior diameter (HAPD). HAPD was the main factor affecting setup displacement in both the AP and SI directions in TM‐BB, while RS was the primary factor for setup displacement in the AP direction in VBC‐BB.ConclusionCompared with TM‐BB, VBC‐BB had a larger AP displacement but smaller in LR and SI displacement. Daily changes in BMF have distinct effects on setup displacement in different immobilization devices. Image‐guided radiation therapy (IGRT) is highly recommended and BMF changes should be given consideration during radiotherapy.
