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Experimental Evaluation of the Water-Equivalence of Novel Plastic Materials in Clinical Proton Beams

A Lourenco

A Lourenco1,2*, D Shipley2, N Wellock3, R Thomas2, H Bouchard4, A Kacperek5, F Fracchiolla6, S Lorentini6, M Schwarz6,7 , N MacDougall3, G Royle1, H Palmans2,8, (1) University College London, London, UK, (2) National Physical Laboratory, Teddington, UK, (3) Barts Health NHS Trust, London, UK, (4) Universite de Montreal, Montreal, Canada, (5) Clatterbridge Cancer Centre, Wirral, UK, (6) Azienda Provinciale per i Servizi Sanitari (APSS), Trento, Italy, (7) National Institute of Nuclear Physics (INFN), Trento, Italy, (8) EBG MedAustron GmbH, Wiener Neustadt, Austria


SU-I-GPD-T-153 (Sunday, July 30, 2017) 3:00 PM - 6:00 PM Room: Exhibit Hall

Purpose: Water-equivalent plastics offer advantages for dosimetry over the reference material, water, such as the convenience of realizing the experimental setup and better positioning accuracy. These materials are well developed for conventional radiotherapy, such as high-energy photon and electron beams, however, no specific water-equivalent plastics have been designed for proton therapy beams and their use is generally not recommended for reference dosimetry of high-energy proton beams.The aim of this work was to evaluate experimentally the water-equivalence of new plastic materials designed for proton beam dosimetry.

Methods: Three plastic materials with varying atomic compositions were produced in-house and characterised with reference to water in 60 MeV and 226 MeV un-modulated proton beams. Measurements were carried outat the 60 MeV proton cyclotron at the Clatterbridge Cancer Centre (CCC), UK, and at the 226MeV proton cyclotron at the Trento Proton Therapy Center (TPTC), Italy. A plastic-to-water conversion factor, H_{pl,w}, was measured by laterally integrated depth-dose ionisation chamber measurements in a water phantom, with and without plastic slabs with variable thicknesses in front of the water phantom. This methodology allows efficient measurements since ionisation chamber readings need to be measured only in water.

Results: For the 60 MeV proton beam, maximum deviations from unity of experimental H_{pl,w} factors were of the order of 0.5% for all trial plastics. For the 226 MeV proton beam, experimental H_{pl,w} values were about 1% higher than unity for the three trial plastics. Experimental data showed no preference regarding the most water-equivalent plastic.

Conclusion: The results indicated that the novel plastics are suitable substitutes for the measurement of dose to water using ionisation chambers in clinical proton beams. This work will feed into the development of water- and tissue-equivalent materials for proton beams.

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