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Reducing the Effect of Respiratory Motion On the Delivered Dose in Proton Therapy Through Proper Field Angle Selection

J Matney

J Matney1*, P Park2 , L Court3 , X Zhu3 , H Li3 , W Liu4 , L Dong5 , R Mohan3 , (1) UNC Health Care, Chapell Hill, NC, (2)Emory University, Atlanta, GA, (3) UT MD Anderson Cancer Center, Houston, TX, (4) Mayo Clinic Arizona, Phoenix, AZ, (5) Scripps Proton Therapy Center, San Diego, CA


TH-C-BRD-8 Thursday 10:15AM - 12:15PM Room: Ballroom D

Purpose: This work investigated a novel planning strategy of selecting radiotherapy beam angles that minimizes the change in water equivalent thickness (dWET) during respiration in order to reduce the effects of respiratory motion in passively scattered proton therapy (PSPT).

Methods: In a clinical trial treating locally-advanced lung cancer with proton therapy, 2-4 co-planar beams were previously selected by dosimetrists in the design of physician-approved PSPT treatment plans. The authors identified a cohort of patients in which respiratory motion affected the planned PSPT dose delivery. For this cohort, this work analyzed dWET during respiration over a 360 degree arc of potential treatment angles around the patient: the dWET was defined as the difference in WET between the full-exhale (T50) and full-inhale (T0) phases of the simulation 4DCT. New PSPT plans were redesigned by selecting new beam angles that demonstrated significant reduction in the value of dWET. Between the T50 and T0 phases, the root-mean-square deviation of dose and the change in dose-volume histogram curves (dAUC) for anatomical structures were calculated to compare the original to dWET reduction plans.

Results: To date, three plans were retrospectively redesigned based on dWET analysis. In the dWET reduction plan, the root mean square dose (T50-T0) was reduced by 15-35% and the DVH dAUC values were reduced by more than 60%.The PSPT plans redesigned by selecting appropriate field angles to minimize dWET demonstrated less dosimetric variation due to respiration.

Conclusions: We have introduced the use of a new metric to quantify respiratory motion in proton therapy: dWET. The use of dWET allows us to minimize the impact of respiratory motion of the entire anatomy in the beam path. This work is a proof of principle that dWET could suggest field angles in proton therapy that are more robust to the effects of respiratory motion.

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