How Sensitive is Mass-Based Inverse Optimization to IMRT Delivery Parameters?

Purpose: To determine the sensitivity of changes to IMRT delivery parameters for mass-based optimization schemes: Dose-Mass- (DM) and Energy-based (Energy), compared to Dose-Volume-based (DV) optimization. Methods: Twelve Head-and-Neck (HN) and twelve lung cases were retrospectively optimized using DM and Energy optimization. In both optimization approaches nine equidistant, split beams were used for step-and-shoot deliverable IMRT. Changes to two parameters were investigated: the number of IMRT segments (5 and 10 per beam) and the minimum allowed segment area (2 and 6 cm²). Plans were normalized such that 95% of the PTV received the same dose. Dose Indices (DIs) were used for evaluation. For the lung cases, DIs included: 1%_cord, 33%_heart, 20% and 30%_both-lungs, and 50%_ esophagus. In the HN cases: 1%_cord, 1%_brainstem, left/right parotids_50%, 50%_larynx, and 50%_esophagus. Results: The lung cases demonstrated that the Energy plans were more sensitive to segment area; changing the segment area resulted in a statistically significant dose increase for 1%_cord, 30%_both-lungs and 50%_esophagus. Changes to the number of segments yielded on average statistically significant differences in dose to 1%_cord in Energy plans, 50%_esophagus in DM plans, and 20%_both-lungs in DV plans. When the segment area was changed, the HN cases yielded statistically significant differences in doses to 1%_cord, 1%_ brainstem, 50%_left and right parotids, and 50%_larynx for the Energy plans and 50%_larynx for DM plans. Moreover, changing the number of segments resulted in significant dose decrease for 50%_parotids and 50%_esophagus for the Energy plans and 50%_larynx for DV plans. Conclusions: This study showed that both lung and HN Energy plans exhibit larger sensitivity than DV and DM plans to changing IMRT delivery parameters, especially when increasing the minimum segment area rather than with varying the number of segments.