The first goal of this paper is to clarify the reference conditions for the reference dosimetry of clinical proton beams. A clear distinction is made between proton beam delivery systems which should be calibrated with a spread-out Bragg peak field and those that should be calibrated with a (pseudo-)monoenergetic proton beam. For the latter, this paper also compares two independent dosimetry techniques to calibrate the beam monitor chambers: absolute dosimetry (of the number of protons exiting the nozzle) with a Faraday cup and reference dosimetry (i.e. determination of the absorbed dose to water under IAEA TRS-398 reference conditions) with an ionization chamber. To compare the two techniques, Monte Carlo simulations were performed to convert dose-to-water to proton fluence. A good agreement was found between the Faraday cup technique and the reference dosimetry with a plane-parallel ionization chamber. The differences-of the order of 3%-were found to be within the uncertainty of the comparison. For cylindrical ionization chambers, however, the agreement was only possible when positioning the effective point of measurement of the chamber at the reference measurement depth-i.e. not complying with IAEA TRS-398 recommendations. In conclusion, for cylindrical ionization chambers, IAEA TRS-398 reference conditions for monoenergetic proton beams led to a systematic error in the determination of the absorbed dose to water, especially relevant for low-energy proton beams. To overcome this problem, the effective point of measurement of cylindrical ionization chambers should be taken into account when positioning the reference point of the chamber. Within the current IAEA TRS-398 recommendations, it seems advisable to use plane-parallel ionization chambers-rather than cylindrical chambers-for the reference dosimetry of pseudo-monoenergetic proton beams.

This work calculates beam quality correction factors (k Q ) in monoenergetic proton beams using detailed Monte Carlo simulation of ionization chambers. It uses the Monte Carlo code penh and the electronic stopping powers resulting from the adoption of two different sets of mean excitation energy values for water and graphite: (i) the currently ICRU 37 and ICRU 49 recommended I 75 eV w = and I 78 eV g = and (ii) the recently proposed I 78 eV w = and I 81.1 eV g =. Twelve different ionization chambers were studied. The k Q factors calculated using the two different sets of I-values were found to agree with each other within 1.6% or better. k Q factors calculated using current ICRU I-values were found to agree within 2.3% or better with the k Q factors tabulated in IAEA TRS-398, and within 1% or better with experimental values published in the literature. k Q factors calculated using the new I-values were also found to agree within 1.1% or better with the experimental values. This work concludes that perturbation correction factors in proton beamscurrently assumed to be equal to unity-are in fact significantly different from unity for some of the ionization chambers studied.

Proton, as well as other ion, beams applied by electro-magnetic deflection in pencil-beam scanning (PBS) are minimally perturbed and thus can be quantified a-priori by their fundamental interactions in medium. This a-priori quantification permits an optimal reduction of characterizing measurements on a particular PBS delivery system. The combination of a-priori quantification and measurements will then suffice to fully describe the physical interactions necessary for treatment planning purposes. We consider, for proton beams, these interactions and derive a “Golden” beam data set. The Golden beam data set quantifies the pristine Bragg peak depth dose distribution in terms of primary, multiple Coulomb scatter, and secondary, nuclear scatter, components. The set reduces the required measurements on a PBS delivery system to the measurement of energy spread and initial phase space as a function of energy. The depth doses are described in absolute units of Gy(RBE).mm2.Gp−1, where Gp equals 109 (giga) protons, thus providing a direct mapping from treatment planning parameters to integrated beam current. We used this Golden beam data on our PBS delivery systems and demonstrate that it yields absolute dosimetry well within clinical tolerance.

This work calculates beam quality correction factors ( ) in both modulated and unmodulated proton beams using the Monte Carlo (MC) code . The latest ICRU 90 recommendations on key data for ionizing-radiation dosimetry were adopted to calculate the electronic stopping powers and to select the mean energy to create an ion pair in dry air ( ). For modulated proton beams, factors were calculated in the middle of a spread-out Bragg peak, while for monoenergetic proton beams they were calculated at the entrance region. Fifteen ionization chambers were simulated. The factors calculated in this work were found to agree within 0.8% or better with the experimental data reported in the literature. For some ionization chambers, the simulation of proton nuclear interactions were found to have an effect on the factors of up to 1%; while for some others, perturbation factors were found to differ from unity by more than 1%. In addition, the combined standard uncertainty in the MC calculated factors in proton beams was estimated to be of the order of 1%. Thus, the results of this work seem to indicate that: (i) the simulation of proton nuclear interactions should be included in the MC calculation of factors in proton beams, (ii) perturbation factors in proton beams should not be neglected, and (iii) the detailed MC simulation of ionization chambers allows for an accurate and precise calculation of factors in clinical proton beams.