Sorriaux, J; Testa, M; Paganetti, H; Bertrand, D; Lee, J A; Palmans, H; Vynckier, S; Sterpin, E (2017) Consistency in quality correction factors for ionization chamber dosimetry in scanned proton beam therapy. Medical Physics, 44 (9). pp. 4919-4927.

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Abstract

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) systems cannot approximate reference conditions using a single spot. However, dose distributions requested in TRS-398 can be reproduced with PBS using a combination of spots. The objective of the present study is to demonstrate, using Monte Carlo (MC), that kQ factors for broad beams can be used with scanned beams for similar reference dose distributions with no significant additional uncertainty.
Methods : We consider hereafter the Alfonso formalism (Alfonso et al, 2008) usually employed for non-standard photon beams. 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). In order to evaluate the impact of these differences on kQ factors, ionization chamber responses are computed with MC (Geant4 9.6) in three different proton beams, with their corresponding quality factors (Q), producing a 10x10cm2 field with a flat dose distribution for: i) dedicated scanned pencil beam (Qpbs), ii) hypothetical proton source (Qhyp), iii) double scattering beam (Qds). Tested ionization chamber cavity are a 2x2x0.2mm3 air cavity, a Roos-type ionization chamber and a Farmer-type ionization chamber.
Results and discussion : Ranges of Qpbs, Qhyp and Qds are consistent within 0.4 mm. Flatnesses of dose distributions are better than 0.5%. Calculated k^fpbs,fref_Qpbs,Qhyp is 0.999 ± 0.002 for the air cavity and the Farmer-type ionization chamber, and 1.001 ± 0.002 for the Roos-type ionization chamber. k^fpbs,fref_Qpbs,Qds is 0.999 ± 0.002 for the Farmer-type and Roos-type ionization chambers, and 1.001 ± 0.001 for the Roos-type ionization chamber.
Conclusion : The Alfonso formalism was applied to scanned proton beams. In our MC simulations, neither the difference in the beam profiles (scanned beam vs hypothetical beam) nor the different incident beam energies influenced significantly the beam correction factors. This suggest that ionization chamber quality correction factors in scanned or broad proton beam are indistinguishable whitin the calculation uncertainties provided dose distributions achieved by both modalities are similar and compliant with TRS-398 reference conditions.

Item Type:	Article
Keywords:	reference dosimetry, proton therapy, pencil beam scanning, beam quality correction factors
Subjects:	Ionising Radiation > Dosimetry
Divisions:	Chemical, Medical & Environmental Science
Identification number/DOI:	10.1102/mp.12434
Last Modified:	07 Mar 2018 15:05
URI:	https://eprintspublications.npl.co.uk/id/eprint/7776