Room: AAPM ePoster Library
Purpose: investigate dosimetric effects of applicator rotational errors of the multi-channel cylinder using direct simulation in 3D image registration software.
Methods: consecutive GYN cancer patients who received multi-channel high dose rate brachytheapy had dose distributions re-evaluated to assess the impact of rotational errors. Those errors were simulated by virtually rotating the 3D dose cloud using commercial image registration software. The rotational errors of ±1°, ±3°, ±5°, ±7°, ±15°, ±30°, ±60°, ±90° along the central axis of the applicator were simulated. Tabular DVHs were generated for the target and critical structures (i.e. small bowel, sigmoid, rectum and bladder). The differences in dosimetric parameters between the reference plan and each angle of rotation were analyzed for the target D90% and D2cc for several organs-at-risk (OARs).
Results: dose variation due to device rotation resulted in changes in <5% in D90% for angles of rotation =15°. Rotation of 30°, 60° and 90° angles resulted in differences of 11% , 28%, and 39%, respectively. Likewise, the change in D2cc for all OARs was also under 5% for angles of rotation less than or equal to 15°. Among the studied OARs, the change in rectal D2cc was observed as the most sensitive to a large angle of rotation resulting in 10.7%, 25.7% and 40.3% when the dose cloud was rotated by 30°, 60° and 90°, respectively. In contrast, the small bowel D2cc had a relatively low dosimetric change (i.e. 7.4%) even at a higher angle of rotation (90°).
Conclusion: this study, we report rotation-induced variations in dosimetric parameters that were determined with the help of commercially available software. Limiting the applicator rotation to less than 15° keeps dosimetric deviation under 5% for both the target and the OARs. Before each treatment, the applicator immobilization and QA checks should be employed to prevent multichannel vaginal cylinder rotation.