(Sunday, 7/12/2020) [Eastern Time (GMT-4)]
Room: AAPM ePoster Library
Purpose: The Mevion S250i pencil beam scanning proton therapy system collimates proton beams by adaptive aperture using two different techniques: collimation of outermost target contour only (static) and collimation of each energy layer (dynamic). This study compares dosimetric impact of the static and dynamic apertures for various treatment sites.
Methods: Treatment plans were generated with static and dynamic apertures for four treatment sites (head and neck (H&N), accelerated partial breast irradiation (APBI), Lung, and Prostate) using the same plan and optimization parameters. Five plans were selected for each site. For H&N, multi-field optimization (MFO) was used while single-field optimization (SFO) was used for the other sites. Robust planning (setup uncertainty of 5 mm (3 mm for H&N) and range uncertainty of 3.5%) was employed and the plans were normalized to cover 99% CTV by the prescription. Mean (Dmean) and maximum (Dmax) dose of CTV and critical organs (lung, breast, heart, extended cord, bladder, rectum, and larynx) were compared.
Results: Mean differences in Dmean and Dmax of CTV (dynamic-static; % in prescription) were 2.2% and 3.6% (H&N), 2.8% and 5.1% (APBI), 3.0% and 3.6% (Lung), and -0.1% and 1.4% (Prostate), respectively. The dynamic aperture substantially decreased Dmax of lung for APBI (16.7%), Dmax of extended cord (13.6%) for H&N, and Dmean of larynx (10.5%) for H&N. The dynamic aperture also showed better extended cord saving for Lung (Dmax reduced by 3.2%). There was no substantial dose difference in bladder and rectum for Prostate.
Conclusion: In general, the dynamic aperture saved the surrounding normal tissue better, but it made target dose homogeneity worse. The dose reduction by the dynamic aperture was pronounced in H&N and APBI; however, the target dose escalation and the organ saving were limited in Prostate.
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