(Monday, 7/30/2018) 1:15 PM - 1:45 PM
Room: Exhibit Hall | Forum 4
Purpose: To evaluate how using models of proton therapy that incorporate variable relative biological effectiveness (RBE) versus the current practice of using a fixed RBE of 1.1 affect dosimetric indices on treatment plans for patients treated with intensity-modulated proton therapy (IMPT).
Methods: Treatment plans for 4 groups of patients who received IMPT for brain, head-and-neck, thoracic, or prostate cancer (100+ patients/group) were selected. Dose distributions were recalculated in five ways: one with a fast-dose Monte Carlo calculator with fixed RBE, and four with RBE calculated as a function of linear energy transfer (LET) according to four different models: Wilkens-Oelfke, McNamara, Wedenberg, and repair-misrepair-fixation (RMF). Differences between dosimetric indices (D02, D05, D20, D50, D95, D98, and mean dose) for target volumes and organs at risk (OARs) on each plan were compared between the fixed-RBE and variable-RBE calculations.
Results: In analyses of all target volumes, at least one model showed decreases in D98 of 8% in 2% of the plans for brain and thorax cancer. For brain plans, two of the variable-RBE models predicted an increase in D02 and D05 of 10 Gy for three OARs in 25% of plans. For head-and-neck plans, three models predicted an increase in D05 of at least 10 Gy in 25% of the patients. For thorax plans, three models showed an increase in D05 of at least 8 Gy to the heart in 25% of the plans; a similar D05 increase of at least 8 Gy to the esophagus in 25% of patients was noted in two models. For prostate plans, two models showed an increase of at least 10 Gy to three OARs.
Conclusion: The variable-RBE models predicted significantly increased doses to various OARs, suggesting that strategies to reduce high-LET dose in critical structures should be developed to minimize possible toxicity associated with IMPT.
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