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A study into the relationship between the measured penumbra and effective source size in the modeling of the Pinnacle RTPS

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Abstract

The effect of detector size in the broadening of the penumbra on the model in the Pinnacle RTPS is investigated. A second order polynomial was devised to correlate the source size parameter with the RTPS-calculated penumbra. The optimal source size parameter was calculated for penumbra measurements based on the diamond detector and a standard ionization chamber (IC). This work was done for Jaw fields, MLC fields with a leaf end radius of 8 cm, and MLC fields with a leaf end radius of 12 cm. The optimum source size of the 8 cm MLC fields matched the jaw fields, and an average (based on field sizes studied) of 1.1 mm for the diamond detector data and 2.4 mm for the ionization chamber was established. The effect of this overestimation of the source size parameter based on detector-induced penumbra broadening was considered for a clinical IMRT prostate plan by using two models (diamond and IC). There were differences in the DVH of the PTV and of OARs but these effects were of negligible clinical significance. Dose difference distributions showed dose difference areas to be in penumbra regions of the segments, with larger dose differences where penumbras intersected and/or there was a significant weighting on the segment. Gamma analysis was also performed between the two plans, and was found to increase the amount of fail rates significantly for both 2%/2 mm and 3%/3 mm criteria. This decreases the sensitivity of IMRT QA in the detection of systematic errors.

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Acknowledgments

The authors acknowledge Robin Hill for the loan of the diamond detector from the Royal Prince Alfred Hospital, Camperdown. The authors would like to thank Australian Rotary Health and the NSW Cancer Institute Clinical Leaders program for funding assistance for NH and PM respectively. Research by the third author was undertaken, in part, thanks to funding from the Cancer Institute of NSW, Radiation Therapy Academic Leaders program.

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Authors and Affiliations

  1. Centre for Medical Radiation Physics (CMRP), University of Wollongong, Wollongong, NSW, 2500, Australia

    Johnson Yuen, Nick Hardcastle & Peter Metcalfe

  2. Cancer Care Centre, St George Hospital, Gray Street, Kogarah, NSW, 2217, Australia

    Johnson Yuen

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  1. Johnson Yuen
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  2. Nick Hardcastle
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  3. Peter Metcalfe
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Correspondence to Johnson Yuen.

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Yuen, J., Hardcastle, N. & Metcalfe, P. A study into the relationship between the measured penumbra and effective source size in the modeling of the Pinnacle RTPS. Australas Phys Eng Sci Med 34, 233–241 (2011). https://doi.org/10.1007/s13246-011-0070-8

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  • DOI: https://doi.org/10.1007/s13246-011-0070-8

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