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Title: SU-G-TeP2-14: Universal Function Form for Photon Open Field In-Water Output Ratio

Abstract

Purpose: In-Water-Output-Ratio (IWOR) plays a significant role in linac-based radiotherapy treatment planning, linking MUs to delivered radiation dose. For an open rectangular field, IWOR depends on both its width and length, and changes rapidly when one of them becomes small. In this study, a universal functional form is proposed to fit the open field IWOR tables in Varian TrueBeam representative datasets for all photon energies. Methods: A novel Generalized Mean formula is first used to estimate the Equivalent Square (ES) for a rectangular field. The formula’s weighting factor and power index are determined by collapsing all data points as much as possible onto a single curve in IWOR vs. ES plot. The result is then fitted with a novel universal function IWOR=1+b*Log(ES/10cm)/(ES/10cm)^c via a least-square procedure to determine the optimal values for parameters b and c. The maximum relative residual error in IWOR over the entire two-dimensional measurement table with field sizes between 3cm and 40cm is used to evaluate the quality of fit for the function. Results: The two-step fitting strategy works very well in determining the optimal parameter values for open field IWOR of each photon energies in the Varian data-set. Relative residual error ≤0.71% is achieved formore » all photon energies (including Flattening-Filter-Free modes) with field sizes between 3cm and 40cm. The optimal parameter values change smoothly with regular photon beam quality. Conclusion: The universal functional form fits the Varian TrueBeam open field IWOR measurement tables accurately with small relative residual errors for all photon energies. Therefore, it can be an excellent choice to represent IWOR in absolute dose and MU calculations. The functional form can also be used as a QA/commissioning tool to verify the measured data quality and consistency by checking the IWOR data behavior against the function for new photon energies with arbitrary beam quality.« less

Authors:
; ; ; ; ; ; ; ; ; ;  [1];  [2]
  1. University of Nebraska Medical Center, Omaha, NE (United States)
  2. Duke University Medical Center, Durham, NC (United States)
Publication Date:
OSTI Identifier:
22649394
Resource Type:
Journal Article
Resource Relation:
Journal Name: Medical Physics; Journal Volume: 43; Journal Issue: 6; Other Information: (c) 2016 American Association of Physicists in Medicine; Country of input: International Atomic Energy Agency (IAEA)
Country of Publication:
United States
Language:
English
Subject:
46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND TECHNOLOGY; 60 APPLIED LIFE SCIENCES; ERRORS; LEAST SQUARE FIT; LINEAR ACCELERATORS; PHOTON BEAMS; RADIATION DOSES; SOCIO-ECONOMIC FACTORS; TWO-DIMENSIONAL CALCULATIONS

Citation Formats

Zhou, S, Fan, Q, Lei, Y, Li, S, Li, X, Ma, R, Wang, S, Wang, X, Zheng, D, Zhu, X, Enke, C, and Wu, Q. SU-G-TeP2-14: Universal Function Form for Photon Open Field In-Water Output Ratio. United States: N. p., 2016. Web. doi:10.1118/1.4957049.
Zhou, S, Fan, Q, Lei, Y, Li, S, Li, X, Ma, R, Wang, S, Wang, X, Zheng, D, Zhu, X, Enke, C, & Wu, Q. SU-G-TeP2-14: Universal Function Form for Photon Open Field In-Water Output Ratio. United States. doi:10.1118/1.4957049.
Zhou, S, Fan, Q, Lei, Y, Li, S, Li, X, Ma, R, Wang, S, Wang, X, Zheng, D, Zhu, X, Enke, C, and Wu, Q. Wed . "SU-G-TeP2-14: Universal Function Form for Photon Open Field In-Water Output Ratio". United States. doi:10.1118/1.4957049.
@article{osti_22649394,
title = {SU-G-TeP2-14: Universal Function Form for Photon Open Field In-Water Output Ratio},
author = {Zhou, S and Fan, Q and Lei, Y and Li, S and Li, X and Ma, R and Wang, S and Wang, X and Zheng, D and Zhu, X and Enke, C and Wu, Q},
abstractNote = {Purpose: In-Water-Output-Ratio (IWOR) plays a significant role in linac-based radiotherapy treatment planning, linking MUs to delivered radiation dose. For an open rectangular field, IWOR depends on both its width and length, and changes rapidly when one of them becomes small. In this study, a universal functional form is proposed to fit the open field IWOR tables in Varian TrueBeam representative datasets for all photon energies. Methods: A novel Generalized Mean formula is first used to estimate the Equivalent Square (ES) for a rectangular field. The formula’s weighting factor and power index are determined by collapsing all data points as much as possible onto a single curve in IWOR vs. ES plot. The result is then fitted with a novel universal function IWOR=1+b*Log(ES/10cm)/(ES/10cm)^c via a least-square procedure to determine the optimal values for parameters b and c. The maximum relative residual error in IWOR over the entire two-dimensional measurement table with field sizes between 3cm and 40cm is used to evaluate the quality of fit for the function. Results: The two-step fitting strategy works very well in determining the optimal parameter values for open field IWOR of each photon energies in the Varian data-set. Relative residual error ≤0.71% is achieved for all photon energies (including Flattening-Filter-Free modes) with field sizes between 3cm and 40cm. The optimal parameter values change smoothly with regular photon beam quality. Conclusion: The universal functional form fits the Varian TrueBeam open field IWOR measurement tables accurately with small relative residual errors for all photon energies. Therefore, it can be an excellent choice to represent IWOR in absolute dose and MU calculations. The functional form can also be used as a QA/commissioning tool to verify the measured data quality and consistency by checking the IWOR data behavior against the function for new photon energies with arbitrary beam quality.},
doi = {10.1118/1.4957049},
journal = {Medical Physics},
number = 6,
volume = 43,
place = {United States},
year = {Wed Jun 15 00:00:00 EDT 2016},
month = {Wed Jun 15 00:00:00 EDT 2016}
}