The correction for charge recombination was determined for different plane-parallel ionization chambers exposed to clinical electron beams with low and high dose per pulse, respectively. The electron energy was nearly the same (about 7 and 9 MeV) for any of the beams used. Boag's two-voltage analysis (TVA) was used to determine the correction for ion losses, k(s), relevant to each chamber considered. The presence of free electrons in the air of the chamber cavity was accounted for in determining k(s) by TVA. The determination of k(s) was made on the basis of the models for ion recombination proposed in past years by Boag, Hochhäuser and Balk to account for the presence of free electrons. The absorbed dose measurements in both low-dose-per-pulse (less than 0.3 mGy per pulse) and high-dose-per-pulse (20-120 mGy per pulse range) electron beams were compared with ferrous sulphate chemical dosimetry, a method independent of the dose per pulse. The results of the comparison support the conclusion that one of the models is more adequate to correct for ion recombination, even in high-dose-per-pulse conditions, provided that the fraction of free electrons is properly assessed. In this respect the drift velocity and the time constant for attachment of electrons in the air of the chamber cavity are rather critical parameters because of their dependence on chamber dimensions and operational conditions. Finally, a determination of the factor k(s) was also made by zero extrapolation of the 1/Q versus 1/V saturation curves, leading to the conclusion that this method does not provide consistent results in high-dose-per-pulse beams.

Energy and angular distributions of electron beams with different energies were simulated by Monte Carlo calculations. These beams were generated by the NOVAC7 system (Hitesys, Italy), a mobile electron accelerator specifically dedicated to intra-operative radiation therapy (IORT). The electron beam simulations were verified by comparing the measured dose distributions with the corresponding calculated distributions. As expected, a considerable difference was observed in the energy and angular distributions between the IORT beams studied in the present work and the electron beams produced by conventional accelerators for non-IORT applications. It was also found that significant differences exist between the IORT beams used in this work and other IORT beams with different collimation systems. For example, the contribution from the scattered electrons to the total dose was found to be up to 15% higher in the NOVAC7 beams. The water-to-air stopping power ratios of the IORT beams used in this work were calculated on the basis of the beam energy distributions obtained by the Monte Carlo simulations. These calculated stopping power ratios, s(w,air), were compared with the corresponding s(w,air) values recommended by the TRS-381 and TRS-398 IAEA dosimetry protocols in order to estimate the deviations between a dosimetry based on generic parameters and a dosimetry based on parameters specifically obtained for the actual IORT beams. The deviations in the s(w,air) values were found to be as large as up to about 1%. Therefore, we recommend that a preliminary analysis should always be made when dealing with IORT beams in order to assess to what extent the possible differences in the s(w,air) values have to be accounted for or may be neglected on the basis of the specific accuracy needed in clinical dosimetry.

Dosimetry in radiotherapy with ionization chambers calibrated in 60Co gamma beams in terms of absorbed dose to water, DW, can be performed if a factor conventionally denoted as kQ is known. The factor kQ depends on the beam quality and the chamber characteristics. Calculated values of the kQ factors for many types of ionization chamber have been recently published. In this work the experimental determination of the kQ factors for various ionization chambers was performed for 6 MV and 15 MV photon beams and for a 14 MeV electron beam. The kQ factors were determined by a procedure based on relative measurements performed with the ionization chamber and ferrous sulphate solution in 60Co gamma radiation and accelerator beams, respectively. The experimental kQ values are compared with the calculated values so far published. Theoretical and experimental kQ values are in fairly good agreement. The uncertainty in the experimental kQ factors determined in this work is less than about 1%, that is, appreciably smaller than the uncertainty of about 1.5% reported for the calculated values.

No abstract

The primary standard of absorbed dose to water established at ENEA for the Co-60 gamma-ray quality is based on a graphite calorimeter and an ionometric transfer system. This standard was recently improved after a more accurate assessment of some perturbation effects in the calorimeter and a modification of the water phantom shape and size. The conversion procedure requires two corresponding depths, one in graphite and one in water, where the radiation energy spectra must be the same. The energy spectra at the corresponding points were determined by a Monte Carlo simulation in water and graphite scaled phantoms. A thorough study of the calorimeter gap effect corrections was also made with regard to their dependence on depth and field size. A comparison between the ionization chamber calibration procedures based on the standards of absorbed dose to water and of air kerma was also made, confirming the consistency of the two methods.