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.

Purpose:To determine the potentialities of synthetic single crystal diamond Schottky diodes for accurate dose measurements in radiation therapy small photon beams. Methods: The dosimetric properties of a diamond-based detector were assessed by comparison with a reference microionization chamber. The diamond device was operated at zero bias voltage under irradiation with high-energy radiotherapic photon beams. The stability of the detector response and its dose and dose rate dependence were measured. Different square field sizes ranging from 1 × 1 cm 2 to 10 × 10 cm 2 were used during comparative dose distribution measurements by means of percentage depth dose curves (PDDs), lateral beam profiles, and output factors. The angular and temperature dependence of the diamond detector response were also studied.Results: The detector response shows a deviation from linearity of less than ±0.5% in the 0.01-7 Gy range and dose rate dependence below ±0.5% in the 1-6 Gy/min range. PDDs and output factors are in good agreement with those measured by the reference ionization chamber within 1%. No angular dependence is observed by rotating the detector along its axis, while ∼3.5% maximum difference is measured by varying the radiation incidence angle in the polar direction. The temperature dependence was investigated as well and a ±0.2% variation of the detector response is found in the 18-40• C range. Conclusions:The obtained results indicate the investigated synthetic diamond-based detector as a candidate for small field clinical radiation dosimetry in advanced radiation therapy techniques.

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on behalf of the ISIORT Europe

The present study confirms that parallel-plate ionization chambers can properly and accurately substitute ferrous sulphate detectors in reference dosimetry of LIAC and NOVAC mobile linear accelerators. Therefore, we hope that the most commonly used protocols for reference dosimetry in external-beam radiotherapy will be updated in order to provide guidance in the calibration of electron beams from linear accelerators dedicated to IORT, so that users may benefit from specific, authoritative and up-to-date recommendations.