Abstract
Significant deviations from the expected dose have been reported in the absolute dosimetry validation of an intensity modulated radiation therapy treatment when individual segments are analyzed. However, when full treatment is considered and all segment doses are added together, these discrepancies fade out, leading to overall dose deviations below a 5% action level. This contradictory behavior may be caused by a partial compensation between detector over-responding and under-responding for measurement conditions far from radiation equilibrium. We consider three treatment verification scenarios that may lead to ionization chamber miss-responding, namely: narrow beam irradiation, field penumbra location and multi-leaf collimator transmission contribution. In this work we have analyzed the response of three different ionization chambers with different active volume under these conditions by means of Monte Carlo (MC) simulation methods. Correction factors needed to convert the detector readout into actual dose to water were calculated by inserting the specific detector geometry (carefully modeled) into the simulations. This procedure required extensive use of parallel computing resources in order to achieve the desired level of uncertainty in the final results. The analysis of the simulations shows the relative contribution of each of the three previously mentioned miss-responding scenarios. Additionally, we provide some evidence on dose deviation compensation in multi-segment radiotherapy treatment verification.
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Acknowledgments
The authors are indebted to the Spanish “Fondo de Investigaciones Sanitarias” (FIS) as well as to the University Law (LOU) contract between the University of Seville and the Andalusian Health Service (SAS) for financial support. Javier Pena wants to acknowledge the Ramón Areces foundation for its financial support. The authors also wish to thank PTW for providing any required geometrical input data of the ionization chambers for the MC simulation.
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González-Castaño, D., Pena, J., Sánchez-Doblado, F. et al. The change of response of ionization chambers in the penumbra and transmission regions: impact for IMRT verification. Med Biol Eng Comput 46, 373–380 (2008). https://doi.org/10.1007/s11517-007-0249-z
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DOI: https://doi.org/10.1007/s11517-007-0249-z