Comparison of thermoplastic masks and knee wedge as immobilization devices for image-guided pelvic radiation therapy using Cone Beam Computed Tomography

Introduction: Pelvic radiotherapy is generally performed with the use of an immobilization and positioning device.

Aim and objective: The objective of the study was to ascertain and compare setup errors between the two positioning devices.

Materials and methods: A total of 35 patients of stage II and III cervical cancers were enrolled in the study and divided into two groups, one using knee wedge and the other using thermoplastic pelvic mask as an immobilization device. Radiation was planned by four field box conformal technique. The random and systematic setup errors were then calculated for each patient in both the groups in the mediolateral (ML), superoinferior (SI), and anteroposterior (AP) directions.

Results: The translational mean setup variation in the lateral, longitudinal, and vertical direction is 0.17 ± 0.24, -0.12 ± 0.48, and -0.18 ± 0.27 cm for thermoplastic pelvic mask and -0.03 ± 0.26, -0.04 ± 0.48, and -0.09 ± 0.37 cm for knee wedge, respectively. The systematic setup error and random errors were 0.24, 0.48, 0.27 cm and 0.31, 0.60, and 0.40 cm for thermoplastic mask and 0.26, 0.48, and 0.37 cm and 0.38, 0.37, and 0.45 cm for knee wedge in ML, SI, and AP axis, respectively. The one way analysis of variance test was applied to compare the setup errors in between the three axes for both the immobilization devices. To compare the positioning accuracy of thermoplastic mask and knee wedge, Student's t-test was applied. Both the tests were found to be insignificant (P value > 0.05).

Conclusion: Thermoplastic mask and knee wedge are equally effective as immobilization devices for treating cervical cancers with conformal techniques.