Gradient corrections for reference dosimetry using Farmer-type ionization chambers in single-layer scanned proton fields.

The local depth dose gradient and the displacement correction factor for Farmer-type ionization chambers are quantified for reference dosimetry at shallow depth in single-layer scanned proton fields. Integrated radial profiles as a function of depth (IRPDs) measured at three proton therapy centers were smoothed by polynomial fits. The local relative depth dose gradient at measurement depths from 1 to 5 cm were derived from the derivatives of those fits. To calculate displacement correction factors, the best estimate of the effective point of measurement was derived from reviewing experimental and theoretical determinations reported in the literature. Displacement correction factors for the use of Farmer-type ionization chambers with their reference point (at the center of the cavity volume) positioned at the measurement depth were derived as a ratio of IRPD values at the measurement depth and at the effective point of measurement. Depth dose gradients are as low as 0.1-0.4% per mm at measurement depths from 1 to 5 cm in the highest clinical proton energies (with residual ranges higher than 15 cm) and increase to 1% per mm at a residual range of 4 cm and become larger than 3% per mm for residual ranges lower than 2 cm. The literature review shows that the effective point of measurement of Farmer-type ionization chambers is, similarly as for carbon ion beams, located 0.75 times the cavity radius closer to the beam origin as the center of the cavity. If a maximum displacement correction of 2% is deemed acceptable to be included in calculated beam quality correction factors, Farmer-type ICs can be used at measurements depths from 1 to 5 cm for which the residual range is 4 cm or larger. If one wants to use the same beam quality correction factors as applicable to the conventional measurement point for scattered beams, located at the center of the SOBP, the relative standard uncertainty on the assumption that the displacement correction factor is unity can be kept below 0.5% for measurement depths of at least 2 cm and for residual ranges of 15 cm or higher. The literature review confirmed that for proton beams the effective point of measurement of Farmer-type ionization chambers is located 0.75 times the cavity radius closer to the beam origin as the center of the cavity. Based on the findings in this work, three options can be recommended for reference dosimetry of scanned proton beams using Farmer-type ionization chambers: (a) positioning the effective point of measurement at the measurement depth, (b) positioning the reference point at the measurement depth and applying a displacement correction factor, and (c) positioning the reference point at the measurement depth without applying a displacement correction factor. Based on limiting the acceptable uncertainty on the gradient correction factor to 0.5% and the maximum deviation of the displacement perturbation correction factor from unity to 2%, the first two options can be allowed for residual ranges of at least 4 cm while the third option only for residual ranges of at least 15 cm.

Palmans H ，Medin J ，Trnková P ，Vatnitsky S ... -  《-》

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

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. 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. Ranges of Qpbs , Qhyp , and Qds are consistent within 0.4 mm. Flatnesses of dose distributions are better than 0.5%. Calculated kQpbs,Qhypfpbs,fref 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. The quality correction factors kQpbs,Qdsfpbs,fref 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. 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 suggests that ionization chamber quality correction factors in scanned or broad proton beams are indistinguishable within the calculation uncertainties provided dose distributions achieved by both modalities are similar and compliant with the TRS-398 reference conditions.

Sorriaux J ，Testa M ，Paganetti H ，Bertrand D ，Lee JA ，Palmans H ，Vynckier S ，Sterpin E ... -  《-》

Monte Carlo simulated beam quality and perturbation correction factors for ionization chambers in monoenergetic proton beams.

Beam quality correction factors provided in current codes of practice for proton beams are approximated using the water-to-air mass stopping power ratio and by assuming the proton beam quality related perturbation correction factors to be unity. The aim of this work is to use Monte Carlo simulations to calculate energy dependent beam quality and perturbation correction factors for a set of nine ionization chambers in proton beams. The Monte Carlo code EGSnrc was used to determine the ratio of the absorbed dose to water and the absorbed dose to the sensitive air volume of ionization chambers f Q 0 related to the reference photon beam quality (60 Co). For proton beams, the quantity f Q was simulated with GATE/Geant4 for five monoenergetic beam energies between 70 MeV and 250 MeV. The perturbation correction factors for the air cavity, chamber wall, chamber stem, central electrode, and displacement effect in proton radiation were investigated separately. Additionally, the correction factors of cylindrical chambers were investigated with and without consideration of the effective point of measurement. The perturbation factors p Q were shown to deviate from unity for the investigated chambers, contradicting the assumptions made in dosimetry protocols. The beam quality correction factors for both plane-parallel and cylindrical chambers positioned with the effective point of measurement at the measurement depth were constant within 0.8%. An increase of the beam quality correction factors determined for cylindrical ionization chambers placed with their reference point at the measurement depth with decreasing energy is attributed to the displacement perturbation correction factors p dis , which were up to 1.045 ± 0.1% for the lowest energy and 1.005 ± 0.1% for the highest energy investigated. Besides p dis , the largest perturbation was found for the chamber wall where the smallest p wall determined was 0.981 ± 0.3%. Beam quality correction factors applied in dosimetry with cylindrical chambers in monoenergetic proton beams strongly depend on the positioning method used. We found perturbation correction factors different from unity. Consequently, the approximation of ionization chamber perturbations in proton beams by the respective water-to-air mass stopping power ratio shall be revised.

Kretschmer J ，Dulkys A ，Brodbek L ，Stelljes TS ，Looe HK ，Poppe B ... -  《-》

Technical note: Experimental determination of the effective point of measurement of the PTW-31010 ionization chamber in proton and carbon ion beams.

The accurate knowledge of the effective point of measurement (Peff ) is particularly important for measurements in proximity to high dose gradients such as in the distal fall-off of particle beams. For plane-parallel ionization chambers (ICs), Peff is well known and located at the center of the inner surface of the entrance window. For cylindrical ICs, Peff is shifted from the chamber's center toward the beam source. According to IAEA TRS-398, this shift can be calculated as 0.75·rcyl for light ions with rcyl being the radius of the cavity. For proton beams and in absence of a dose gradient, no shift is recommended. We have experimentally determined Peff for the 0.125 cc Semiflex IC in both proton and carbon ion beams. The first method consisted of simultaneous irradiation of a plane-parallel IC and the Semiflex in a 4-cm wide spread-out Bragg peak. In the second method, a single-energy beam was used, and both ICs were positioned successively at the same measurement depths. For both approaches, the shift of the distal edge of the depth ionization distributions recorded by the two chambers at different reference points was used to calculate Peff of the Semiflex. Both methods were applied in carbon ion beams, and only the latter was applied in proton beams. Both methods yielded a similar Peff for carbon ions, 0.88·rcyl , and 0.84·rcyl , which results in a difference of only 0.1 mm. The difference to the recommended value of 0.75·rcyl is 0.4 and 0.3 mm, respectively, which is larger than the positioning uncertainty. In the proton beam, a Peff of 0.92·rcyl was obtained. The Peff for the 0.125 cc Semiflex IC is shifted further from the cavity center as recommended by IAEA TRS-398 for light ions, with the shift for proton beams being even larger than for carbon ion beams.

Barna S ，Resch AF ，Puchalska M ，Georg D ，Palmans H ... -  《-》

Dosimetry using plane-parallel ionization chambers in a 75 MeV clinical proton beam.

Palmans H ，Verhaegen F ，Denis JM ，Vynckier S ... -  《PHYSICS IN MEDICINE AND BIOLOGY》