Consistency in quality correction factors for ionization chamber dosimetry in scanned proton beam therapy.

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. This study aims to demonstrate, using Monte Carlo ( MC) simulations, that kQ factors computed/measured for broad beams can be used with scanned beams for similar reference dose distributions with no additional significant uncertainty. Methods We consider the Alfonso formalism13 usually employed for nonstandard photon beams. To approach reference conditions similar as IAEA TRS-398 and the associated dose distributions, PBS must combine many pencil beams with range or energy modulation and shaping techniques that differ from 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 10 × 10 cm2 field with a flat dose distribution for (a) a dedicated scanned pencil beam (Qpbs), (b) a hypothetical proton source (Qhyp), and (c) a double-scattering beam (Qds). The tested ionization chamber cavities are a 2 × 2 × 0.2 mm³ 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 [ABSTRACT FROM AUTHOR]