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On the Equivalence of the Quality Correction Factor for Pencil Beam Scanning Proton Therapy

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J Sorriaux

J Sorriaux1,2,3*, H Paganetti3 , M Testa3 , D Giantsoudi3 , J Schuemann3 , D Bertrand4 , J. Orban de Xivry 2, J Lee1,2 , H Palmans5,6 , S Vynckier7 , E Sterpin1 , (1) Center of Molecular Imaging, Radiotherapy and Oncology, Institut de recherche experimentale et Clinique, Universite catholique de Louvain, Avenue Hippocrate 54, 1200 Brussels, Belgium, (2) ICTEAM Institute, Universite catholique de Louvain, 1348 Louvain-la-Neuve, Belgium, (3) Departement of Radiation Oncology, Massachusetts General Hospital and Harvard Medical School, Boston, MA 02114, USA, (4) Ion Beam Applications S.A, Louvain-la-Neuve, Belgium, (5) EBG MedAustron GmbH, Wiener Neustadt, Austria, (6) National Physical Laboratory, Teddington, UK, (7) Departement de radiotherapie, Cliniques Universitaires Saint-Luc, Avenue Hippocrate 54, 1200 Brussels, Belgium


SU-E-T-464 Sunday 3:00PM - 6:00PM Room: Exhibit Hall

In current practice, most proton therapy centers apply IAEA TRS-398 reference dosimetry protocol. Quality correction factors (kQ) take into account in the dose determination process the differences in beam qualities used for calibration unit and for treatment unit. These quality correction factors are valid for specific reference conditions. TRS-398 reference conditions should be achievable in both scattered proton beams (i.e. DS) and scanned proton beams (i.e. PBS). However, it is not a priori clear if TRS-398 kQ data, which are based on Monte Carlo (MC) calculations in scattered beams, can be used for scanned beams. Using TOPAS-Geant4 MC simulations, the study aims to determine whether broad beam quality correction factors calculated in TRS-398 can be directly applied to PBS delivery modality.

As reference conditions, we consider a 10x10x10 cm³ homogeneous dose distribution delivered by PBS system in a water phantom (32/10 cm range/modulation) and an air cavity placed at the center of the spread-out-Bragg-peak. In order to isolate beam differences, a hypothetical broad beam is simulated. This hypothetical beam reproduces exactly the same range modulation, and uses the same energy layers than the PBS field. Ion chamber responses are computed for the PBS and hypothetical beams and then compared.

For an air cavity of 2x2x0.2 cm³, the ratio of ion chamber responses for the PBS and hypothetical beam qualities is 0.9991 ± 0.0016.

Quality correction factors are insensitive to the delivery pattern of the beam (broad beam or PBS), as long as similar dose distributions are achieved. This investigation, for an air cavity, suggests that broad beam quality correction factors published in TRS-398 can be applied for scanned beams.

Funding Support, Disclosures, and Conflict of Interest: J. Sorriaux is financially supported by a public-private partnership involving the company Ion Beam Applications (IBA).

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