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Uncertainties in Treatment Planning for IROC-Houston Proton Phantom QA Program Due to Variable CT Technique and Proton Energy

M McInnis

M McInnis*, P Taylor , F Poenisch , L Court , M Guindani , D Followill , UT MD Anderson Cancer Center, Houston, TX


SU-E-T-772 (Sunday, July 12, 2015) 3:00 PM - 6:00 PM Room: Exhibit Hall

To understand the uncertainties in proton therapy treatment planning for the IROC- Houston proton phantom QA program due to variations in CT technique and proton energy.

A CT phantom used by IROC-H during therapy site visits was scanned using three CT techniques (80, 120, 140kV) with a CT scanner used for proton therapy simulations and irradiated with a passively scattered beam at three energies (140, 200, 250 MeV) to measure, respectively, HU and Relative Linear Stopping Power (RLSP) in order to create HU to RLSP calibration curves for comparison with reference curves used by current proton treatment planning systems. The phantom has proton equivalent materials with a wide variety of HU and RLSPs to allow for the creation of a calibration curve for common tissue equivalent materials. Treatment plans were created for a lung phantom using the various CT technique/ beam energy calibration curves to determine the differences in the dose distributions by performing a gamma analysis.

Comparison of the calibration curves created using the phantom materials showed a maximum difference of 12% for a given material between the custom curve and the reference curve currently used by the treatment planning system. The highest differences were a result of using an 80 kV CT technique and a 250 MeV high proton energy. A comparison of the completed treatment plans will be presented.

These results indicate the possibility of differences in proton HU to RLSP calibration curves caused by varying CT technique and proton energy that could manifest as differences in planned and delivered dose distributions, particularly at high proton energies and low kV CT techniques. These differences could result in discrepancies not accounted for by IROC-Houston and could possibly affect proton institutions’ pass rate when irradiating the proton phantoms.

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