SU-E-T-722: Investigation of Range and Dosimetric Calculation Uncertainty for a Proton Treatment Planning System Using an Animal Tissue Phantom

Abstract

Purpose: Accurate determination of proton dose and penetration range is critical in proton therapy. The purpose of this study is to investigate the range and dosimetric calculation accuracy for a commercial proton therapytreatment planning system (TPS). Methods: A lamb leg and solid water plates were used as phantoms in this study. Radiochromic films were inserted in the phantom plates, at depths roughly at the center of spread out Bragg peak (SOBP), the 80%, 50% and 20% distal falloff planes according to TPS calculations. The phantom was scanned by a CT scanner with 1.25 mm slice thickness. A treatment plan was created, using the XiO TPS (CMS, St. Louis, MO). Two beams were used in the plan, one with a compensator and the other without it. Image guidance was used to align the phantom before proton beams were delivered according to the treatment plan. The film‐measured doses were compared to TPS calculations. Results: At the center of SOBP plane, the measured dose agrees well with TPS calculation. The difference for field size is within 2 mm, and the penumbra difference is about 1 mm. At the distal falloff planes, the dose difference became large, up to 40% or more. The measured doses at various distal planes were used to reconstruct the proton ranges, which were about 2–3% deeper than TPS calculations. The detailed causes of the range uncertainty and their relative contributions to the uncertainty are being investigated. Conclusions: The XiO TPS calculates dose relatively accurately at the center of SOBP but with a large uncertainty in distal falloff region. The degree of accuracy seems to depend on tissue heterogeneity and compensator complexity. Using an animal tissue phantom, the study provides quantitative and relevant data to determine planning parameters, such as the range uncertainty, in protontreatment planning.