The role of volume averaging effects, beam hardening, and phantom scatter in dosimetry of grid therapy
At a Glance
Section titled âAt a Glanceâ| Metadata | Details |
|---|---|
| Publication Date | 2024-10-29 |
| Journal | Physics in Medicine and Biology |
| Authors | Ahtesham Ullah Khan, Bishwambhar Sengupta, Indra J. Das |
| Institutions | Northwestern University, Northwestern Memorial Hospital |
Abstract
Section titled âAbstractâAbstract Objective . Current reference dosimetry methods for spatially fractionated radiation therapy (SFRT) assume a negligible beam quality change, perturbation, or volume-averaging correction factor. Therefore, the aim of this work was to investigate the impact of the grid collimators on the dosimetric characteristics of a 6 MV photon beam. A detector-specific correction factor, <mml:math xmlns:mml=âhttp://www.w3.org/1998/Math/MathMLâ overflow=âscrollâ> <mml:mrow> <mml:msubsup> <mml:mi>k</mml:mi> <mml:mrow> <mml:msub> <mml:mi>Q</mml:mi> <mml:mrow> <mml:mtext>grid</mml:mtext> </mml:mrow> </mml:msub> <mml:mo>,</mml:mo> <mml:mtext> </mml:mtext> <mml:mtext> </mml:mtext> <mml:msub> <mml:mi>Q</mml:mi> <mml:mrow> <mml:mtext>msr</mml:mtext> </mml:mrow> </mml:msub> </mml:mrow> <mml:mrow> <mml:mtext> </mml:mtext> <mml:msub> <mml:mi>f</mml:mi> <mml:mrow> <mml:mtext>grid</mml:mtext> </mml:mrow> </mml:msub> <mml:mo>,</mml:mo> <mml:msub> <mml:mi>f</mml:mi> <mml:mrow> <mml:mtext>msr</mml:mtext> </mml:mrow> </mml:msub> </mml:mrow> </mml:msubsup> </mml:mrow> </mml:math> , was proposed. Several dosimeters were evaluated for their ability to measure both reference dose and grid output factors (GOFs). Approach . A Monte Carlo model of a grid collimator was created to study the change in the depth dose characteristics with the grid collimator. The impact of the collimator on the percent depth dose (PDD), electron contamination, and average photon energy was investigated. The <mml:math xmlns:mml=âhttp://www.w3.org/1998/Math/MathMLâ overflow=âscrollâ> <mml:mrow> <mml:msubsup> <mml:mi>k</mml:mi> <mml:mrow> <mml:msub> <mml:mi>Q</mml:mi> <mml:mrow> <mml:mtext>grid</mml:mtext> </mml:mrow> </mml:msub> <mml:mo>,</mml:mo> <mml:mtext> </mml:mtext> <mml:mtext> </mml:mtext> <mml:msub> <mml:mi>Q</mml:mi> <mml:mrow> <mml:mtext>msr</mml:mtext> </mml:mrow> </mml:msub> </mml:mrow> <mml:mrow> <mml:mtext> </mml:mtext> <mml:msub> <mml:mi>f</mml:mi> <mml:mrow> <mml:mtext>grid</mml:mtext> </mml:mrow> </mml:msub> <mml:mo>,</mml:mo> <mml:msub> <mml:mi>f</mml:mi> <mml:mrow> <mml:mtext>msr</mml:mtext> </mml:mrow> </mml:msub> </mml:mrow> </mml:msubsup> </mml:mrow> </mml:math> correction factors were calculated for two reference-class micro ion chambers. The reference dose and GOFs were measured with a grid collimator using six ion chambers, two silicon diodes, and a diamond detector. Main results. The PDD in the presence of the grid was observed to be steeper compared to the open field. The average photon energy increased from 1.33 MeV to 1.74 MeV with the presence of the grid collimator. The dose contribution by scattered photons was significantly higher at deeper regions for the open field compared to the grid field. The <mml:math xmlns:mml=âhttp://www.w3.org/1998/Math/MathMLâ overflow=âscrollâ> <mml:mrow> <mml:msubsup> <mml:mi>k</mml:mi> <mml:mrow> <mml:msub> <mml:mi>Q</mml:mi> <mml:mrow> <mml:mtext>grid</mml:mtext> </mml:mrow> </mml:msub> <mml:mo>,</mml:mo> <mml:mtext> </mml:mtext> <mml:mtext> </mml:mtext> <mml:msub> <mml:mi>Q</mml:mi> <mml:mrow> <mml:mtext>msr</mml:mtext> </mml:mrow> </mml:msub> </mml:mrow> <mml:mrow> <mml:mtext> </mml:mtext> <mml:msub> <mml:mi>f</mml:mi> <mml:mrow> <mml:mtext>grid</mml:mtext> </mml:mrow> </mml:msub> <mml:mo>,</mml:mo> <mml:msub> <mml:mi>f</mml:mi> <mml:mrow> <mml:mtext>msr</mml:mtext> </mml:mrow> </mml:msub> </mml:mrow> </mml:msubsup> </mml:mrow> </mml:math> correction was calculated to be <0.5%. The reference dose for all detectors, except for the CC13 and CC04 chambers, was within 1% of each other. The CC13 under-responded up to 3.2% due to volume-averaging effects. The GOFs calculated for all detectors, except Razor and A16, were within 1% of each other. Significance . The phantom scatter dictates the change in the PDD with the presence of the grid. The micro ion chambers exhibit negligible <mml:math xmlns:mml=âhttp://www.w3.org/1998/Math/MathMLâ overflow=âscrollâ> <mml:mrow> <mml:msubsup> <mml:mi>k</mml:mi> <mml:mrow> <mml:msub> <mml:mi>Q</mml:mi> <mml:mrow> <mml:mtext>grid</mml:mtext> </mml:mrow> </mml:msub> <mml:mo>,</mml:mo> <mml:mtext> </mml:mtext> <mml:mtext> </mml:mtext> <mml:msub> <mml:mi>Q</mml:mi> <mml:mrow> <mml:mtext>msr</mml:mtext> </mml:mrow> </mml:msub> </mml:mrow> <mml:mrow> <mml:mtext> </mml:mtext> <mml:msub> <mml:mi>f</mml:mi> <mml:mrow> <mml:mtext>grid</mml:mtext> </mml:mrow> </mml:msub> <mml:mo>,</mml:mo> <mml:msub> <mml:mi>f</mml:mi>
Tech Support
Section titled âTech SupportâOriginal Source
Section titled âOriginal SourceâReferences
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