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Combining Collimation with Spot-Scanning Proton Therapy to Improve Brain Treatments

E Gelover

E Gelover1*, A Moignier1 , B Smith1 , D Wang1 , R Flynn1 , L Lin2 , M Kirk2 , T Solberg2 , D Hyer1 , (1) University of Iowa, Iowa City, Iowa, (2) University of Pennsylvania, Philadelphia, PA


MO-B-BRD-7 (Monday, March 9, 2015) 10:00 AM - 12:00 PM Room: Ballroom D

Purpose: This work investigates the benefits of using a Dynamic Collimation System (DCS) for penumbra reduction during the treatment of brain tumors with proton spot scanning. The DCS consist of two pairs of orthogonal trimmer blades driven by linear motors; by intercepting the proton pencil beam near the lateral boundary of the target in the beam’s eye view, the DCS yields beam spots with a reduced size. To quantify the effect of these smaller spots near the boundary of the target, a comparison of un-collimated and collimated treatment plans has been performed.

Methods: Spot scanning treatment plans for five brain patients previously treated with proton therapy were created in RDX, an in-house treatment planning system that utilizes the Hong pencil beam algorithm. To insure an accurate starting point for the comparison, the characteristics of the IBA Universal Nozzle were modeled in RDX and used to reproduce the clinical un-collimated treatment plans. Collimated treatment plans were then created by taking into account the effects of the DCS on each beam spot. The plans were re-optimized with the goal of maintaining target coverage and improving conformity. All other parameters (spot locations, energy layers, air gap, etc) remained unchanged.

Results: The collimated plans demonstrated almost identical PTV coverage, but a significant reduction in the mean dose to heathy tissue surrounding the target. The average reduction of the mean dose to a 10-mm ring surrounding PTV is 13.77% (95% Confidence Interval: 11.65%-15.89%, P<=0.0001). The conformity index for the 100% isodose line of the collimated plans yielded an improvement of 7.6% in comparison with the un-collimated plans.

Conclusion: The improvement in lateral penumbra obtained from the use of the DCS in combination with spot scanning proton therapy can reduce the dose to healthy tissue surrounding the target while preserving target coverage.

Funding Support, Disclosures, and Conflict of Interest: This work is supported by IBA (Ion Beam Applications S.A.)

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