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Experimental verification the electron return effect around spherical air cavities for the MR-Linac using Monte Carlo
J Shortall1, E Vasquez Osorio1, A Aitkenhead1,2
1Department of Cancer Sciences, The University of Manchester, Manchester, UK.
Medical Physics
|March 8, 2020
Summary
Monte Carlo calculations accurately predict dose deposition around air cavities during magnetic resonance-guided radiotherapy (MRgRT). This verification is crucial for safe treatment planning, especially concerning the electron return effect (ERE) near organs at risk.
Area of Science:
- Medical Physics
- Radiation Oncology
- Dosimetry
Background:
- Unplanned air cavities in patients receiving magnetic resonance-guided radiotherapy (MRgRT) can alter dose deposition due to the electron return effect (ERE).
- This dosimetric uncertainty is particularly relevant for treatments involving organs at risk (OARs) adjacent to gas, such as the intestinal tract during pelvic radiotherapy.
- Accurate dose calculation is essential for ensuring treatment efficacy and minimizing toxicity.
Purpose of the Study:
- To verify the accuracy of Monte Carlo calculations used in treatment planning systems (TPS) for predicting dose deposition around air cavities.
- To assess the dosimetric impact of the electron return effect (ERE) in the presence of varying air cavity sizes.
- To validate TPS calculations against experimental measurements using radiochromic film.
Main Methods:
- Experimental measurements were performed using GafChromic EBT3 films embedded in polymethyl methacrylate (PMMA) phantoms with spherical air cavities (0.5, 3.5, 7.5 cm diameter).
- Irradiations were conducted on an Elekta Unity system using a 7-MV photon beam under a 1.5-T magnetic field.
- Monte Carlo dose distributions calculated by the Elekta Monaco TPS were compared to measured film doses using gamma analysis (3%/3 mm criteria).
Main Results:
- The study achieved high agreement (>95%) between calculated and measured dose distributions using a 3%/3 mm gamma analysis criterion.
- The agreement decreased with increasing air cavity size, with discrepancies concentrated within the air cavity region.
- Analysis using stricter gamma criteria (3%/2 mm, 2%/3 mm, 2%/2 mm) also indicated good agreement.
Conclusions:
- Monte Carlo-based treatment planning systems can accurately calculate the dose effects of the electron return effect around air cavities.
- The findings support the clinical use of current TPS for MRgRT, even in the presence of air cavities.
- Further investigation into stricter gamma criteria may be warranted for specific clinical scenarios.
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