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Retrospective analysis of portal dosimetry pre-treatment quality assurance of hybrid IMRT breast treatment plans

Published online by Cambridge University Press:  27 February 2020

Meghan Koo ,
Johnson Darko Open the ORCID record for Johnson Darko [Opens in a new window]  and
Ernest Osei Open the ORCID record for Ernest Osei [Opens in a new window]
Meghan Koo
Affiliation:
Department of Physics and Astronomy, University of Waterloo, Waterloo, ON, Canada
Johnson Darko*
Affiliation:
Department of Physics and Astronomy, University of Waterloo, Waterloo, ON, Canada Department of Medical Physics, Grand River Regional Cancer Centre, Kitchener, ON, Canada Department of Clinical Studies, Ontario Veterinary College, University of Guelph, Guelph, ON, Canada
Ernest Osei
Affiliation:
Department of Physics and Astronomy, University of Waterloo, Waterloo, ON, Canada Department of Medical Physics, Grand River Regional Cancer Centre, Kitchener, ON, Canada Department of Clinical Studies, Ontario Veterinary College, University of Guelph, Guelph, ON, Canada Department of Systems Design, University of Waterloo, Waterloo, ON, Canada
*
Author for correspondence: Johnson Darko, Department of Medical Physics, Grand River Regional Cancer Centre, Kitchener, ONN2G 1G3, Canada. Tel: 519 749 4300x5793. E-mail: [email protected]
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Abstract

Background:

The purpose of this study is to evaluate the effectiveness and sensitivity of the Varian portal dosimetry (PD) system as a quality assurance (QA) tool for breast intensity-modulated radiation therapy (IMRT) treatment plans.

Materials and methods:

Four hundred portal dose images from 200 breast cancer patient IMRT treatment plans were analysed. The images were obtained using Varian PortalVision electronic portal imaging devices (EPIDs) on Varian TrueBeam Linacs. Three patient plans were selected, and the multi-leaf collimator (MLC) positions were randomly altered by a mean of 0·5, 1, 1·5 and 2 mm with a standard deviation of 0·1 mm on 50, 75 and 100% of control points. Using the improved/global gamma calculation algorithm with a low-dose threshold of 10% in the EPID, the change in gamma passing rates for 3%/3 mm, 2%/2 mm and 1%/1 mm criterion was analysed as a function of the introduced error. The changes in the dose distributions of clinical target volume and organ at risk due to MLC positioning errors were also analysed.

Results:

Symmetric and asymmetric breast or chest wall plan fields are different in delivery as well as in the QA. An average gamma passing rate of 99·8 ± 0·5 is presented for 3%/3 mm symmetric plans and 96·9 ± 4·5 is presented for 3%/3 mm asymmetric plans. An average gamma passing rate of 98·4 ± 4·3 is presented for 2%/2 mm symmetric plans and 89·7 ± 9·5 is presented for 2%/2 mm asymmetric plans. A large-induced error in MLC positioning (2·0 mm, 100% of control points) results in an insignificant change in dose that would be delivered to the patient. However, EPID portal dosimetry is sensitive enough to detect even the slightest change in MLC positioning error (0·5 mm, 50% of control points).

Conclusions:

Stricter pre-treatment QA action levels can be established for breast IMRT plans utilising EPID. For improved sensitivity, a multigamma criteria approach is recommended. The PD tool is sensitive enough to detect MLC positioning errors that contribute to even insignificant dose changes.

Keywords

breast cancerEPIDIMRT QAportal dosimetryquality assurance
Type
Original Article
Information
Journal of Radiotherapy in Practice , Volume 20 , Issue 1 , March 2021 , pp. 22 - 29
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Copyright
© The Author(s), 2020. Published by Cambridge University Press

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