2507 - Characterization of Clinical Feasibility, Reproducibility and Dosimetric Impact of Palliative Immobilization Devices to Optimize Patient Comfort

V. L. Doss, D. Urie, G. Atkins, L. Bell, M. Hanna, E. Huang, A. N. Souranis, S. Han-Oh, and A. W. LaVigne; Department of Radiation Oncology and Molecular Radiation Sciences, Johns Hopkins University School of Medicine, Baltimore, MD

Purpose/Objective(s): Roughly 40% of patients undergoing radiation annually in the United States do so with palliative intent. Patients presenting with cancer-related pain are at increased risk of treatment-related exacerbation due to immobilization on a carbon fiber table, often without additional cushioning or supportive material for comfort. Acute, positional discomfort can heighten patient distress while leading to delayed or unsuccessful treatment delivery. Using commonly available cushioning materials, we evaluated the quality and reproducibility of palliative radiotherapy plans to identify clinically acceptable immobilization modifications. Materials/

Methods: Nine immobilization variations were used for phantom-based CT-simulation—standard memory and egg crate foam, three different thicknesses of mattress pads, an alpha cradle, a vac loc bag (with and without standard memory foam) and a hover mattress. Two beam arrangements (AP/PA and AP/RPO/LPO) were created for a thoracic spine target, and treatment plans were generated with three different beam energies (6MV, 10MV, 15MV). Clinical goals for target coverage and organs at risk were evaluated for each plan, with variations computed from an established standard—the vac loc bag. Plans were delivered to the phantom using the vac loc bag with and without memory foam immobilizations to test set up reproducibility by investigating the accuracy and precision of repeated alignment with cone beam CT (CBCT).
Results: Clinically acceptable plans, as defined by 95% CTV coverage at prescription dose, were achieved with AP/PA and AP/RPO/LPO beam arrangements for each immobilization material with variation ranging from -5.4% to 2.4%. Choice of immobilization device had minimal impact on relevant organs at risk, such as maximum skin dose, with a percent difference range of -1.9% to 1.4%. Beam energy proved to have more notable impact on dosimetric variation for target coverage, particularly with the AP/RPO/LPO beam arrangement. Average shifts in the XYZ planes were 1.7, 1.0 and 1.8mm (standard deviation (SD) 1.1-1.2mm) for the vac loc bag and 0.6, 0.7 and 0.9mm (SD 0.6-0.9mm) for the vac loc bag-memory foam combination respectively. Average rotational shifts for pitch, roll and yaw were 0.5, 0.4 and 0.3mm (SD 0.1-0.4mm) for the standard and 0.7, 0.5 and 0.4mm (SD 0.1-0.2mm) for the combination respectively.
Conclusion: Preliminary analysis suggests that clinically acceptable palliative radiation plans can be feasibly and reproducibly achieved with commonly available cushioning materials, without meaningful decrease in target coverage, increase in dose to organs at risk, or compromise in set up accuracy and precision. Future directions include evaluation of these metrics for all immobilization variations, in addition to dose delivery with optically stimulated luminescent dosimeters.