Reference dosimetry of proton pencil beams based on dose-area product: a proof of concept.

This paper describes a novel approach to the reference dosimetry of proton pencil beams based on dose-area product ([Formula: see text]). It depicts the calibration of a large-diameter plane-parallel ionization chamber in terms of dose-area product in a 60Co beam, the Monte Carlo calculation of beam quality correction factors-in terms of dose-area product-in proton beams, the Monte Carlo calculation of nuclear halo correction factors, and the experimental determination of [Formula: see text] of a single proton pencil beam. This new approach to reference dosimetry proves to be feasible, as it yields [Formula: see text] values in agreement with the standard and well-established approach of determining the absorbed dose to water at the centre of a broad homogeneous field generated by the superposition of regularly-spaced proton pencil beams.

Experimental validation of beam quality correction factors for proton beams.

This paper presents a method to experimentally validate the beam quality correction factors (kQ) tabulated in IAEA TRS-398 for proton beams and to determine the kQ of non-tabulated ionization chambers (based on the already tabulated values). The method is based exclusively on ionometry and it consists in comparing the reading of two ionization chambers under the same reference conditions in a proton beam quality Q and a reference beam quality (60)Co. This allows one to experimentally determine the ratio between the kQ of the two ionization chambers. In this work, 7 different ionization chamber models were irradiated under the IAEA TRS-398 reference conditions for (60)Co beams and proton beams. For the latter, the reference conditions for both modulated beams (spread-out Bragg peak field) and monoenergetic beams (pseudo-monoenergetic field) were studied. For monoenergetic beams, it was found that the experimental kQ values obtained for plane-parallel chambers are consistent with the values tabulated in IAEA TRS-398; whereas the kQ values obtained for cylindrical chambers are not consistent--being higher than the tabulated values. These results support the suggestion (of previous publications) that the IAEA TRS-398 reference conditions for monoenergetic proton beams should be revised so that the effective point of measurement of cylindrical ionization chambers is taken into account when positioning the reference point of the chamber at the reference depth. For modulated proton beams, the tabulated kQ values of all the ionization chambers studied in this work were found to be consistent with each other--except for the IBA FC65-G, whose experimental kQ value was found to be 0.6% lower than the tabulated one. The kQ of the PTW Advanced Markus chamber, which is not tabulated in IAEA TRS-398, was found to be 0.997 ± 0.042 (k = 2), based on the tabulated value of the PTW Markus chamber.

Relative response of alanine dosemeters for high-energy electrons determined using a Fricke primary standard.

A significant proportion of cancer patients is treated using MeV electron radiation. One of the measurement methods which is likely to furnish reliable dose values also under non-reference conditions is the dosimetry using alanine and read-out via electron spin resonance (ESR). The system has already proven to be suitable for QA purposes for modern radiotherapy involving megavoltage x-rays. In order to render the secondary standard measurement system of the Physikalisch-Technische Bundesanstalt based on alanine/ESR useable for dosimetry in radiotherapy, the dose-to-water (D(W)) response of the dosemeter needs to be known for relevant radiation qualities. For MeV electrons, the D(W) response was determined using the Fricke primary standard of the Swiss Federal Office of Metrology. Since there were no citable detailed publications on the Swiss primary standard available, this measurement system is described in some detail. The experimental results for the D(W) response are compared to results of Monte Carlo simulations which model in detail the beams furnished by the electron accelerator as well as the geometry of the detectors. The agreement between experiment and simulation is very good, as well as the agreement with results published by the National Research Council of Canada which are based on a different primary standard. No significant dependence of the D(W) response was found in the range between 6 and 20 MeV. It is therefore suggested to use a unique correction factor k(E) for alanine for all MeV qualities of k(E) = 1.012 ± 0.010.