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An Investigation into the Accuracy of the Photon Beam Energy Spectrum Modeled by the Pinnacle Treatment Planning System and Its Effects on Treatment Planning

Staley, Noah D
http://rave.ohiolink.edu/etdc/view?acc_num=mco1481308003486075

Abstract Details

2016, Master of Science in Biomedical Sciences (MSBS), University of Toledo, Biomedical Sciences (Medical Physics: Diagnostic Radiology).
The convolution-superposition dose calculation algorithm of the Pinnacle3 Treatment Planning System (TPS) necessitates that a model of the beam be created for every licensed energy during the TPS commissioning process. To model photon beams in Pinnacle3, the user is required to measure depth dose curves and beam profiles for a variety of geometries (combinations of various source-to-surface distances (SSD), beam modifiers, and field sizes) for each energy and then import the gathered data into the TPS. Achieving the best fit of the measured profiles is the main objective of this modeling process, with different modeling parameters derived from different profiles. Specifically, Pinnacle3 models beam spectrum and electron contamination to create curves that fit the measured depth dose curves. Although the curves produced by the auto-modeling process may closely fit the measured data, they depend on many variables and a range of acceptable solutions are possible. Due to the uncertainties inherent to the modeling process, the derived photon beam spectrum and electron contamination may not accurately represent the true physical nature of their real world counterparts. A study of the accuracy of Pinnacle3's models was performed for 6 and 10 MV flattening-filter free (FFF) beams of a Varian Edge linear accelerator using two distinct routes of investigation: first, direct comparison of the Pinnacle3 spectra with those previously published or derived with alternative models, and second, indirect comparison through the inspection of resulting percent depth dose (PDD) curves in water phantoms. For the former approach, the modeled spectra produced by Pinnacle3 were compared with phase-space file (PSF) Monte Carlo modeled spectra for TrueBeam (and Edge) linear accelerators, as well as spectra for the same energy found in models from other TPSs. As for the latter, a virtual model of the TrueBeam photon source with a water phantom set up directly beneath the accelerator head was created in the Monte Carlo N-Particle (MCNP) radiation transport package and PDD curves were collected for both Pinnacle3 and PSF-MC modeled spectra. The direct effect of beam spectra on dose calculations in realistic patient plans was evaluated as the final step. New photon beam models were created in the Pinnacle3 TPS by replacing the originally modeled spectra with spectra obtained from the Varian PSF and an alternate version of the Edge machine was commissioned using the new photon beam models. A number of previously treated lung and brain patient plans that used 6 and 10 MV FFF beams and the conformal arc modality were identified. These plans were recalculated using the newly commissioned machine and were renormalized before comparing their dose distributions to those found in the original plans. Plans were selected so that tumors were located in areas with high heterogeneities and high dose gradients, increasing the effect that a change in photon spectrum had on the dose distribution. Differences between plan sets were evaluated by comparing maximum doses to targets, dose gradient, target conformity, and maximum doses to local critical structures. The spectral distributions for the PSF MC-modeled spectra and spectra modeled by other TPSs were found to be shifted so that lower energy photons had greater intensities compared to the Pinnacle3 modeled spectrum for both 6 and 10 MV FFF beams. The average photon energies for spectra modeled outside the Pinnacle3 TPS were likewise lower: average photon energy of the 6 MV FFF PSF MC spectrum was EAVG=0.537 MV, down from the Pinnacle3 spectrum value of EAVG=1.224 MV, and for the 10 MV FFF PSF MC spectrum EAVG=0.750 MV, down from the Pinnacle3 spectrum value of EAVG=1.545 MV. The Pinnacle3 calculated PDD curve using the Pinnacle3 modeled spectrum best fit measured commissioning data, although the measured data fell in between PDD curves modeled in MCNP using Pinnacle3 and PSF MC modeled spectra. Modeled PDD curves created in both Pinnacle and MCNP for the PSF MC spectra had shallower depth of maximum dose values than PDD curves created using the Pinnacle3 spectra for both energies. After evaluating multiple treatment plans, it was found that replacing the spectra used in Pinnacle3's calculations typically resulted in minor variations in dose distribution inside target volumes, with the exception of very small targets. The latter finding is particularly clinically significant for SRS/SBRT plans, where very high doses delivered to tumors require the highest possible accuracy in dose calculations and more extensive fine-tuning of the commissioned model may be warranted.
Diana Shvydka (Committee Chair)
E. Ishmael Parsai (Committee Member)
Nicholas Sperling (Committee Member)
86 p.
Physics; Radiation
Medical Physics; Pinnacle; Spectrum; TrueBeam; Radiation Therapy; Treatment Planning

Recommended Citations

Citations

  • Staley, N. D. (2016). An Investigation into the Accuracy of the Photon Beam Energy Spectrum Modeled by the Pinnacle Treatment Planning System and Its Effects on Treatment Planning [Master's thesis, University of Toledo]. OhioLINK Electronic Theses and Dissertations Center. http://rave.ohiolink.edu/etdc/view?acc_num=mco1481308003486075

    APA Style (7th edition)

  • Staley, Noah. An Investigation into the Accuracy of the Photon Beam Energy Spectrum Modeled by the Pinnacle Treatment Planning System and Its Effects on Treatment Planning. 2016. University of Toledo, Master's thesis. OhioLINK Electronic Theses and Dissertations Center, http://rave.ohiolink.edu/etdc/view?acc_num=mco1481308003486075.

    MLA Style (8th edition)

  • Staley, Noah. "An Investigation into the Accuracy of the Photon Beam Energy Spectrum Modeled by the Pinnacle Treatment Planning System and Its Effects on Treatment Planning." Master's thesis, University of Toledo, 2016. http://rave.ohiolink.edu/etdc/view?acc_num=mco1481308003486075

    Chicago Manual of Style (17th edition)

mco1481308003486075
912
© 2016, some rights reserved.Creative Commons License An Investigation into the Accuracy of the Photon Beam Energy Spectrum Modeled by the Pinnacle Treatment Planning System and Its Effects on Treatment Planning by Noah D Staley is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 3.0 Unported License. Based on a work at etd.ohiolink.edu.
This open access ETD is published by University of Toledo Health Science Campus and OhioLINK.