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The Accuracy of the Photon Beam Energy Spectrum Modeled by the Pinnacle Treatment Planning System and Its Effects On SRS and SBRT Treatment Planning

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N Staley

N Staley*, N Sperling , D Shvydka , University of Toledo Health Science Campus, Toledo, OH


SU-J-CAMPUS-TT-4 (Sunday, July 30, 2017) 4:00 PM - 5:00 PM Room: Therapy ePoster Theater

Purpose: To investigate the accuracy of the Pinnacle³ treatment planning system’s (TPS) beam spectra models and to quantify how inaccuracies may affect treatment planning of conformal arc SRS and SBRT patients.

Methods: In beam models commissioned in Pinnacle³ auto-modeled photon spectra are derived based on measured PDD data. Auto-modeled spectra for 6 and 10 MV FFF energies of a Varian Edge linac models were substituted with those obtained from Varian phase space files (PSFs). Using the two sets of beam spectra, verification PDD curves were produced in Pinnacle³, two other TPSs, and through Monte Carlo (MC) simulations, and then compared with commissioning data. Patient conformal arc SRS/SBRT plans (11 brain and 19 lung plans) were recalculated using the PSF spectra models and compared to original plan calculations done with the Pinnacle³ auto-modeled spectra models. Gradient and conformity index values were calculated for each plan and maximum dose values were found for PTVs and surrounding critical structures.

Results: Energy distributions for spectra obtained from PSF and other TPSs were shifted to lower photon energies than Pinnacle³ auto-modeled spectra. The Pinnacle³ calculated PDD curves using the Pinnacle³ modeled spectrum best fit measured commissioning data. Modeled PDD curves created in both Pinnacle³ and MC for the PSF spectra had shallower depth of maximum dose values than those created with Pinnacle³ spectra. Plan dose distributions varied little except for small targets where gradient index values increased for PSF spectra MC calculated plans.

Conclusion: Despite notable variations in spectra, these differences only become clinically significant for SRS/SBRT plans with small targets, particularly ones that are near critical structures. These plans may require fine-tuning of the commissioned model to ensure accuracy. It is recommended that PSF spectra be used as a starting point in auto-modeling rather than a replacement for that process.

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