The potential impact of the tension of the pelvic muscles on set-up errors in radiotherapy for pelvic malignancies

Impact of belly board immobilization devices and body mass factor on setup displacement using daily cone-beam CT in rectal cancer radiotherapy

OBJECTIVE

The 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.

METHODS

Twenty-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.

RESULTS

The 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.

CONCLUSION

Compared 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.

A study of the effect on accuracy of the introduction of a bellyboard as an immobilisation device to the radical radiotherapy treatment of prostate cancer patients

Ensuring reproducibility in any radiotherapy technique is vital to ensure that a geometric miss does not occur. Patient position and comfort play a major part in the reproducibility of setups.

Purpose: To study the effect on accuracy of the introduction of a bellyboard to the existing prone technique used for radical radiotherapy treatment of prostate cancer.

Materials and Methods: Thirty patients were treated using the bellyboard in the standard method. Portal images were compared with those of a control group of patients who had treatment with no rigid immobilisation. Patients in both groups were males with prostate cancer, who underwent radical radiotherapy treatment. Images were analysed using anatomy matching, and deviations from the isocentre were noted.

Results: Statistical analysis of the results showed no statistical significance between the groups, though within the control group there were more deviations over 0.5 cm from the isocentre position than in the bellyboard group.

Conclusions: Introduction of the bellyboard was seen to be equivalent to the use of the control technique. When the deviations from isocentre position were evaluated for each group, the control group had more deviations over 0.5 cm than the bellyboard group.