Program Information

Quality Correction Factors in Scanned Or Broad Proton Therapy Beams Are Indistinguishable

J Sorriaux^1,2, M Testa³ , H Paganetti³ , D Bertrand⁴ , J Orban de Xivry⁴ , J Lee^1,2 , H Palmans^5,6 , S Vynckier⁷ , E Sterpin¹ , (1) Molecular Imaging Radiotherapy & Oncology, Universite Catholique de Louvain, Brussels, Belgium (2) ICTEAM Institute, Universite catholique de Louvain, Louvain-la-Neuve, Belgium (3) Department of Radiation Oncology, Massachusetts General Hospital and Harvard Medical School, Boston, MA 02114, USA Massachusetts (4) Ion Beam Applications, Louvain-la-neuve, Brabant Wallon, (5) EBG MedAustron GmbH, Wiener Neustadt, Austria (6) National Physical Laboratory, Teddington, UK (7) Cliniques Universitaires Saint-Luc, Brussels, Belgium

Presentations

SU-F-BRD-15 (Sunday, July 12, 2015) 4:00 PM - 6:00 PM Room: Ballroom D

Purpose:
The IAEA TRS-398 code of practice details the reference conditions for reference dosimetry of proton beams using ionization chambers and the required beam quality correction factors (kQ). Pencil beam scanning (PBS) requires multiple spots to reproduce the reference conditions. The objective is to demonstrate, using Monte Carlo (MC) calculations, that kQ factors for broad beams can be used for scanned beams under the same reference conditions with no significant additional uncertainty. We consider hereafter the general Alfonso formalism (Alfonso et al, 2008) for non-standard beam.

Methods:
To approach the reference conditions and the associated dose distributions, PBS must combine many pencil beams with range modulation and shaping techniques different than those used in passive systems (broad beams). This might lead to a different energy spectrum at the measurement point. In order to evaluate the impact of these differences on kQ factors, ion chamber responses are computed with MC (Geant4 9.6) in a dedicated scanned pencil beam (Q_pcsr) producing a 10x10cm2 composite field with a flat dose distribution from 10 to 16 cm depth. Ion chamber responses are also computed by MC in a broad beam with quality Q_ds (double scattering). The dose distribution of Q_pcsr matches the dose distribution of Q_ds. k_(Q_pcsr,Q_ds) is computed for a 2x2x0.2cm³ idealized air cavity and a realistic plane-parallel ion chamber (IC).

Results:
Under reference conditions, quality correction factors for a scanned composite field versus a broad beam are the same for air cavity dose response, k_(Q_pcsr,Q_ds) =1.001±0.001 and for a Roos IC, k_(Q_pcsr,Q_ds) =0.999±0.005.

Conclusion:
Quality correction factors for ion chamber response in scanned and broad proton therapy beams are identical under reference conditions within the calculation uncertainties. The results indicate that quality correction factors published in IAEA TRS-398 can be used for scanned beams in the SOBP of a high-energy proton beam.

Funding Support, Disclosures, and Conflict of Interest: Jefferson Sorriaux is financed by the Walloon Region under the convention 1217662. Jefferson Sorriaux is sponsored by a public-private partnership IBA - Walloon Region

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