Program Information
An Analytical Model to Correct the Aperture Scattered Dose in Clinical Proton Beams
B Sun1*, S Liu2 , T Zhang3 , T Zhao4 , D Yang5 , K Grantham6 , S Goddu7 , J Bradley8 , S Mutic9 , (1) Washington University in St. Louis, St. Louis, MO, (2) Washington University in St. Louis, St. Louis, MO, (3) Washington University School of Medicine, St. Louis, MO, (4) Washington University School of Medicine, St. Louis, MO, (5) Washington University in St Louis, St Louis, MO, (6) ,,,(7) Washington University, St. Louis, MO, (8) Washington University School of Medicine, Saint Louis, Missouri, (9) Washington University School of Medicine, Saint Louis, MO
Presentations
SU-F-T-142 (Sunday, July 31, 2016) 3:00 PM - 6:00 PM Room: Exhibit Hall
Purpose:Apertures or collimators are used to laterally shape proton beams in double scattering (DS) delivery and to sharpen the penumbra in pencil beam (PB) delivery. However, aperture-scattered dose is not included in the current dose calculations of treatment planning system (TPS). The purpose of this study is to provide a method to correct the aperture-scattered dose based on an analytical model.
Methods: A DS beam with a non-divergent aperture was delivered using a single-room proton machine. Dose profiles were measured with an ion-chamber scanning in water and a 2-D ion chamber matrix with solid-water buildup at various depths. The measured doses were considered as the sum of the non-contaminated dose and the aperture-scattered dose. The non-contaminated dose was calculated by TPS and subtracted from the measured dose. Aperture scattered-dose was modeled as a 1D Gaussian distribution. For 2-D fields, to calculate the scatter-dose from all the edges of aperture, a sum of weighted distance was used in the model based on the distance from calculation point to aperture edge. The gamma index was calculated between the measured and calculated dose with and without scatter correction.
Results: For a beam with range of 23 cm and aperture size of 20 cm, the contribution of the scatter horn was ~8% of the total dose at 4 cm depth and diminished to 0 at 15 cm depth. The amplitude of scatter-dose decreased linearly with the depth increase. The 1D gamma index (2%/2 mm) between the calculated and measured profiles increased from 63% to 98% for 4 cm depth and from 83% to 98% at 13 cm depth. The 2D gamma index (2%/2 mm) at 4 cm depth has improved from 78% to 94%.
Conclusion: Using the simple analytical method the discrepancy between the measured and calculated dose has significantly improved.
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