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Monte Carlo Shows Large Improvement Over Analytic Algorithms in Calculating Proton Therapy Dose to Thoracic Tumors in IROC Lung Phantom

P Taylor

P Taylor*, S Kry , D Followill , UT MD Anderson Cancer Center, Houston, TX


TH-AB-605-12 (Thursday, August 3, 2017) 7:30 AM - 9:30 AM Room: 605

Purpose: To explore the poor agreement between the calculated and measured doses in the IROC Houston proton lung phantom.

Methods: IROC’s anthropomorphic lung phantom provides a consistent QA tool that assesses comparability in dose delivery between proton centers participating in NCI-funded trials. The phantom contains TLD and film to measure the delivered dose. A motor moves the target insert within the thorax, mimicking a human breathing cycle. The pass rate for this phantom is only 63%, and irradiations that do pass, do so marginally, despite the generous 7%/4mm acceptance criteria. Five institutions that previously irradiated the phantom with their clinical analytic dose algorithms were asked to recalculate dose to the phantom with a Monte Carlo algorithm. These recalculations were collected and compared to measurements.

Results: Monte Carlo recalculations showed a striking improvement in agreement with the physical measurements, compared to the clinical analytic algorithm calculations. The analytic algorithms calculate, on average, the dose to the center of the target to be 7% higher than TLD measurements, whereas Monte Carlo algorithms are, on average, only 1.5% higher. The difference is even greater at the edge of the GTV, where analytic algorithms overpredict dose by up to 27% as compared to physical measurements. Monte Carlo recalculations agreed within 5% of the measurements. All comparisons showed a region of at least 10% dose discrepancy between the analytic and Monte Carlo algorithms inside the target volume.

Conclusion: Analytic algorithms do a poor job predicting proton dose in lung tumors. There are many uncertainties in proton dose delivery, but eliminating this one uncertainty will reduce the overall uncertainty. For the safety of patients and the efficacy of treatment, proton therapy centers should switch to Monte Carlo or other advanced dose calculation algorithms for treatment planning for thoracic tumors.

Funding Support, Disclosures, and Conflict of Interest: This work was supported by Public Health Service grants 2U19CA021239-35, CA081647, CA10953, and CA180803 awarded by the Department of Health and Human Services National Cancer Institute.

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