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

Journal Article (Journal Article)

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.

Full Text

Duke Authors

Cited Authors

  • Zhou, S; Wu, Q; Fan, Q; Lei, Y; Li, S; Li, X; Ma, R; Wang, S; Wang, X; Zheng, D; Zhu, X; Enke, C

Published Date

  • June 2016

Published In

Volume / Issue

  • 43 / 6

Start / End Page

  • 3666 -

PubMed ID

  • 28046449

Electronic International Standard Serial Number (EISSN)

  • 2473-4209

Digital Object Identifier (DOI)

  • 10.1118/1.4957049


  • eng

Conference Location

  • United States