A kinetic model of diode detector response to pulsed radiation beams.

Diodes dosimeters present a complex response to pulsed beams, with diode sensitivity varying with dose-per-pulse, monitor unit rate (time between pulses) or number of integrated pulses. Such a response is caused by the complex kinetics of the interplay among charge carriers, recombination-generation centers, which capture excess minority charge carriers and facilitate recombination with a majority charge carrier, and traps with energy levels close to the conduction/valence band, which can trap and release charge carriers. This behavior has been well characterized experimentally, and modeled with phenomenological models. In this work we present a kinetic multi-compartment model of the response of diode detectors, which includes the interplay among charge carriers, recombination-generation centers, and traps. The model can qualitatively fit experimental data extracted from the literature on diode response versus dose-per-pulse, monitor unit rate (time between pulses), or number of integrated pulses. In this regard, our work provides further insight on the response of diode detectors, and a theoretical framework for the development of simple phenomenological models.

Development and clinical characterization of a novel 2041 liquid-filled ionization chambers array for high-resolution verification of radiotherapy treatments.

PURPOSE: The aim of this study was to present a novel 2041 liquid-filled ionization chamber array for high-resolution verification of radiotherapy treatments. MATERIALS AND METHODS: The prototype has 2041 ionization chambers of 2.5 × 2.5 mm2 area filled with isooctane. The detection elements are arranged in a central square grid of 43 × 43, totally covering an area of 107.5 × 107.5 mm2 . The central inline and cross-line are extended to 227 mm and the diagonals to 321 mm to be able to perform profile measurements of large fields. We have studied stability, pixel response uniformity, dose rate dependence, depth and field size dependence and anisotropy. We present results for output factors, tongue-and-groove, garden fence, small field profiles, irregular fields, and verification of dose planes of patient treatments. RESULTS: Comparison with other detectors used for small field dosimetry (SFD, CC13, microDiamond) has shown good agreement. Output factors measured with the device for square fields ranging from 10 × 10 to 100 × 100 mm2 showed relative differences within 1%. The response of the detector shows a strong dependence on the angle of incident radiation that needs to be corrected for. On the other hand, inter-pixel relative response variations in the 0.95-1.08 range have been found and corrected for. The application of the device for the verification of dose planes of several treatments has shown gamma passing rates above 97% for tolerances of 2% and 2 mm. The verification of other clinical fields, like small fields and irregular fields used in the commissioning of the TPS, also showed large passing rates. The verification of garden fence and tongue-and-groove fields was affected by volume-averaging effects. CONCLUSIONS: The results show that the liquid filled ionization chamber prototype here presented is appropriate for the verification of radiotherapy treatments with high spatial resolution. Recombination effects do not affect very much the verification of relative dose distributions. However, verification of absolute dose distributions may require normalization to a radiation field which is representative of the dose rate of the treatment delivered.