Optimization of a Multi-Isocentric VMAT TBI Technique Incorporating 10FFF and Evaluation of Target and OAR Dose Constraints
Abstract
Purpose
To improve lung-sparing for multi-isocentric total body irradiation (TBI) in adult-sized patients while preserving target coverage, and to determine mean lung dose (MLD) and target maximum dose (PTV D1cc) as a function of body dimensions.
Methods
Multiple planning parameters—arc number, collimator angle, and beam energy—were investigated to improve lung sparing while maintaining coverage. After establishing an optimal technique, 11 TBI CT datasets were retrospectively analyzed. For each patient, a chest-isocenter plan was generated in Eclipse using the developed planning and optimization templates. Once plan optimization plateaued, normalization was employed to achieve 100% prescription coverage of 90% of the target volume. Correlations of MLD with maximum body width, lung volume, and lung-to-body-width ratio were analyzed. PTV D1cc with bodily width was also investigated.
Results
The optimal plan template included two arcs at ~0° collimator angle (6 MV and 10 FFF) and two arcs at ~90° collimator angle (6 MV and 10 FFF). The optimization template included a 2.5 cm flash structure, lung ring structures, the PTV, and heating and cooling structures. Patient maximum body widths ranged from 29.3-63.5 cm; lung volume to maximum body width ratios ranged from 21.0-95.9. MLD was not correlated with maximum body width, lung volume, or lung-to-body-width ratio; however, MLD increased over 67% for maximum body width >43 cm. PTV D1cc ranged from 117-144% and correlated with bodily width (R2=0.8).
Conclusion
A planning and optimization template was developed to enhance lung sparing for a given target coverage in multi-isocentric TBI for adult-sized patients. Incorporating 10FFF energy arcs improved both MLD and PTV D1cc metrics. Some bodily widths >43 cm exhibited MLD >67%. Furthermore, PTV D1cc increased linearly with body width. The results suggest that the current dosimetric goals in the literature (MLD < 67%, PTV D1cc < 120%) may not be realistic for larger patients.