A comparison between two different immobilization devices for radiation therapy treatment of pelvic cancer using VMAT.

Objectives: To assess setup reproducibility of low kneefix with feetfix (LKF-FF) system and its operator-reported convenience by reference to low dual leg positioner (LDLP), among patients treated with pelvic radiotherapy.

Methods: A retrospective controlled trial was carried out at the radiotherapy unit. It included patients who underwent radical radiotherapy to the pelvis using VMAT, and who benefitted from LDLP (N = 30) or LKF-FF (N = 30) immobilization system. Average absolute shifts (AAS) and total vector errors (TVE) were computed and compared between the two systems, using translational (lateral, longitudinal and vertical) and rotational (X, Y and Z planes) directions. Accuracy rates were computed on pooled data including 1529 VMAT images, 819 in LDLP and 710 in LKF-FF groups, using different cutoffs. Radiotherapists' subjective assessment of the device's ease of setup, handling, cleaning, and storage, and patient comfort was carried out comparatively between the two devices.

Results: No statistically significant difference was observed between the two systems in systematic settings, while LKF-FF outperformed LDLP in random settings; notably in vertical translation and X and Z rotational shifts. Analysis of TVEs showed significant decrease in rotational TVE in LKF-FF group (mean=1.38° versus 2.38, p = 0.003) by reference to LDLP, respectively; however, both systems had comparable translational TVE (p = 0.590). In pooled analysis, LKF-FF enabled an overall increase in setup accuracy rates in rotational directions by up to 15% and 19% at ±1° and ±2° accuracy levels, respectively (p<0.05). Subjective assessments showed that the two immobilization systems were comparable regarding all investigated dimensions; however, the overall radiotherapists' preference leaned toward LDLP.

Conclusion: The newly implemented LKF-FF system outperformed LDLP in terms of setup reproducibility, notably in rotational directions, where it enhanced setup accuracy rates by up to 19%. Long-term use of LKF-FF may improve the users' satisfaction.