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The effect of influence quantities and detector orientation on small-field patient-specific IMRT QA: comparison of measurements with various ionization chambers

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Abstract

Intensity-modulated radiation therapy (IMRT) requires a patient-specific quality assurance (QA) program to validate the treatment plan and a high level of dosimetric accuracy in the treatment delivery. Dosimetric verification generally consists of both absolute- and relative-dose measurements in a phantom using ionization chambers. Measurements were carried out with three different ionization chambers (Scanditronix FC 65G, Exradin A18, and PTW PinPoint 31014) to assess the effects of influence quantities such as the stability, pre- and post-irradiation leakage, stem effect, polarity, and ion recombination on the IMRT point-dose verification with two different orientations. The Exradin A18 and PTW PinPoint ion chambers demonstrated noticeable leakage to magnitudes of 0.6 and 1.2%, whereas negligible leakage was observed with FC 65G ion chamber. Maximum deviations of 0.5 and 0.6% were noticed for the smallest field owing to the ion recombination effect with the PTW PinPoint ion chamber in the parallel and perpendicular orientations, respectively. The calculated total uncertainties of all influence quantities for the FC 65G, A18, and PTW PinPoint ion chambers were 0.5, 0.7, and 1.3%, respectively. The uncertainties determined for each chamber were incorporated into the point-dose measurements of 30 head and neck patient-specific QA plans, and the variation was found to be within ±3%. The magnitude of the leakage in a small-volume ion chamber indicated the significance of incorporating the correction factors in the absolute-dose measurement. A paired t test analysis indicated that the influence quantities significantly affect the point-dose measurements in the patient-specific IMRT QA.

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Acknowledgements

The authors are grateful to Dr. Jasmine Prasad for her valuable contribution towards the statistical analysis. We would also like to thank Dr. Shwetha for her support in this study. The contribution of Mr. Muhammed Shafeeque, Mr. Arun and Mr. Paranthaman towards the measurements on Linear Accelerators is greatly appreciated.

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Authors and Affiliations

  1. Department of Radiation Physics, Kidwai Memorial Institute of Oncology, Dr. M.H. Marigowda Road, Bangalore, 560 029, India

    Henry Finlay Godson, Ravikumar Manickam, Sathiyan Saminathan, Kadirampatti Mani Ganesh & Retna Ponmalar

  2. Department of Radiotherapy, Christian Medical College, Vellore, India

    Henry Finlay Godson & Retna Ponmalar

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  1. Henry Finlay Godson
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  2. Ravikumar Manickam
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Correspondence to Ravikumar Manickam.

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Godson, H.F., Manickam, R., Saminathan, S. et al. The effect of influence quantities and detector orientation on small-field patient-specific IMRT QA: comparison of measurements with various ionization chambers. Radiol Phys Technol 10, 195–203 (2017). https://doi.org/10.1007/s12194-016-0385-y

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  • DOI: https://doi.org/10.1007/s12194-016-0385-y

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