ESTRO 2022

Session Item

Saturday

May 07

10:30 - 11:30

Room D2

Dosimetry

Chair: Claus Andersen, Denmark;

Co-Chair: Cristina Garibaldi, Italy

Session Code: 1250

Session Type: Proffered Papers

Track: Physics

10:40 - 10:50

MLC modelling assessment with a set of standardized tests: Results from a multicentric study

Jordi Saez, Spain

Presentation Number: OC-0120

Abstract

Abstract Title:

MLC modelling assessment with a set of standardized tests: Results from a multicentric study

Authors:

Jordi Saez¹, Raquel Bar-Deroma², Evelien Bogaert³, Romain Cayez⁴, Tom Chow⁵, Marco Esposito⁶, Vladimir Feygelman⁷, Angelo F. Monti⁸, Julia Garcia-Miguel⁹, Eduard Gershkevitsh¹⁰, Jo Goossens¹¹, Carmen Herrero¹², Mohammad Hussein¹³, Catherine Khamphan¹⁴, Wolfgang Lechner¹⁵, Matthieu Lemire¹⁶, Alexander Nevelsky², Daniel Nguyen¹⁷, Lucia Paganini¹⁸, Marlies Passler¹⁹, Luis Isaac Ramos Garcia²⁰, Serenella Russo⁶, John Shakeshaft²¹, Laure Vieillevigne²², Victor Hernandez²³

¹Hospital Clínic de Barcelona, Radiation Oncology, Barcelona, Spain; ²Rambam Health Care Campus, Department of Radiotherapy, Division of Oncology, Haifa, Israel; ³Ghent University Hospital and Ghent University, Department of Radiation Oncology, Ghent, Belgium; ⁴Oscar Lambret Center, Department of Medical Physics, Lille, France; ⁵Juravinski Hospital and Cancer Centre at Hamilton Health Sciences, Department of Medical Physics, Ontario, Canada; ⁶USL Toscana Centro, Medical Physics Unit, Florence, Italy; ⁷Moffitt Cancer Center, Department of Radiation Oncology, Tampa, USA; ⁸ASST GOM Niguarda, Department of Medical Physics, Milano, Italy; ⁹Consorci Sanitari de Terrassa, Department of Medical Physics, Terrassa, Spain; ¹⁰North Estonia Medical Centre, Department of Medical Physics, Tallinn, Estonia; ¹¹Iridium Kankernetwerk, Department of Medical Physics, Antwerp, Belgium; ¹²Centro Médico de Asturias-IMOMA, Department of Radiation Oncology, Gijón, Spain; ¹³National Physical Laboratory, Metrology for Medical Physics Centre, London, United Kingdom; ¹⁴ Institut Sainte-Catherine, Department of Medical Physics, Avignon, France; ¹⁵Medical University of Vienna, Department of Radiation Oncology, Vienna, Austria; ¹⁶Hôpital Maisonneuve-Rosemont, Department of Medical Physics, Montréal, Canada; ¹⁷Centre de Radiothérapie de Mâcon, Department of Medical Physics, Mâcon, France; ¹⁸Humanitas Clinical and Research Center, Radiotherapy and Radiosurgery Department, Rozzano, Italy; ¹⁹Lake Constance Radiation Oncology Center, Department of Radiation Oncology, Singen-Friedrichshafen, Germany; ²⁰Clínica Universidad de Navarra, Department of Oncology, Pamplona, Spain; ²¹Gold Coast University Hospital, ICON Cancer Centre, Gold Coast, Australia; ²²Institut Claudius Regaud—Institut Universitaire du Cancer de Toulouse, Department of Medical Physics, Toulouse, France; ²³Hospital Sant Joan de Reus, Department of Medical Physics, Reus, Spain

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Purpose or Objective

Careful modelling of the MLC is crucial for IMRT/VMAT. It has been shown that TPS modelling errors were present in most centres failing end-to-end dosimetry audit [1] and a large centre-to-centre variability in MLC TPS parameters has been reported [2]. The goals of this study are to evaluate the feasibility of using a common set of tests to evaluate the MLC models implemented in TPSs and a common characterization of MLCs across vendors.

Material and Methods

24 centers worldwide were invited to participate in this pilot study. DICOM plans or scripts for the tests were supplied. The set of tests included reference, transmission, sweeping gap (SG) and asynchronous sweeping gap (aSG) fields for the 10 and 20 mm gaps. SG fields are sensitive to the transmission and rounded-leaf-end of the MLC [3,4], while aSG are sensitve to the tongue and groove (TG) width and the leaf tip-TG region [5,6]. In the aSG tests the gap is constant but adjacent leaves are shifted different distances ‘s’ to expose the leaf sides, causing a dose reduction as ‘s’ increases.

All centers used a Farmer-type ionization chamber with its long axis perpendicular to leaf movement with an isocentric set-up in water or solid water (SSD 90 cm, depth 10 cm) with a 6 MV photon beam. The DICOM set included images of a water phantom and an evaluation structure mimicking the ion chamber. Centers reported the average dose per field to the evaluation structure calculated by their TPS. Both measured and calculated doses were normalized to the reference field dose for comparison within the same MLC type.

Results

A summary of MLC and TPSs evaluated is given in Fig1. Normalized doses for the aSG fields for 2 representative MLC types are shown in Fig2. For each type:

Measured normalized dose curves were nearly identical. The standard deviation with respect to the mean were at most 1% (Agility), 1% (HD120) 1.2% (Millennium) and 0.4% (Halcyon). The slope and shape of the dose reduction caused by the TG depended on the type. These results reflect that these tests are sensitive to the dosimetric details of each type.
Calculated dose curves for TPSs showed large differences with respect to measured doses for all MLCs except for the Halcyon: 8% (Agility), 8% (HD120) and 3.5% (Millennium).

Conclusion

The proposed method constitutes the first successful attempt for an evaluation and assessment of MLC modelling with TPS-independent tests. Our study demonstrates that it is feasible to obtain information on the MLC characteristics with simple measurements. Only small variations were observed in the measurements within a given MLC type, while high variations were found in TPS dose calculations, which highlights the challenges faced by the community to configure MLCs in TPSs. This procedure can be readily incorporated in IMRT audits performed by national and international groups as part of the credentialing required for QA and clinical trials.

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Arnfield, MedPhys 2000
Hernandez, PMB 2017
Saez, PMB 2020