Comparative dosimetry in narrow high-energy photon beams

Abstract

A comparison of the response of different dosimeters in narrow photon beams (phi > or = 4 mm) of 6 and 18 MV bremsstrahlung has been performed. The detectors used were a natural diamond detector, a liquid ionization chamber, a plastic scintillator and two dedicated silicon diodes. The diodes had a very small detection volume and one was a specially designed double diode using two parallel opposed active volumes with compensating interface perturbations. The characteristics of the detectors were investigated both for dose distribution measurements, such as depth-dose curves and lateral beam profiles, and for output factors. The dose rate and angular dependence of the diamond and the two diodes were also studied separately. The depth-dose distributions for small fields agree well for the diamond, the scintillator and the single diode, while the measured dose maximum for the double diode is about 1% higher and for the liquid chamber about 1% lower than the mean of the others when normalized at a depth of 10 cm. The plastic scintillator and the liquid ionization chamber detect a penumbra width that is slightly broadened due to the influence of their finite size, while the double diode may even underestimate the penumbra width due to its small size and high density. When corrected for the extension of the detector volume a good agreement with Monte Carlo calculated beam profiles was obtained for the plastic scintillator and the liquid ionization chamber. Profiles measured with the diamond show an asymmetry when positioned with the smallest dimension facing the beam, while the double diode, the scintillator and the liquid chamber measure symmetric profiles irrespective of positioning. Significant differences in the output factors were obtained with the different detectors. The natural diamond detector measures output factors close to those with an ionization chamber (less than 1% difference) for field sizes between 3 x 3 and 15 x 15 cm2, but overestimates the output factors for large fields and underestimates the output factors for the smallest field sizes. The single and double diodes overestimated the output factor for large field sizes by up to 7 and 12% respectively due to the high content of low-energy photons. The double diode, and to some extent the single diode, also showed a relative increase in response compared with the more water equivalent liquid chamber and plastic scintillator at the smallest fields where there is a lack of lateral electron equilibrium. Both the plastic scintillator and the liquid chamber also show responses that deviate from the ionization chamber for larger field sizes. The major deviations can be explained based on the characteristics of the sensitive materials and the construction of the detectors.

Measuring output factors of small fields formed by collimator jaws and multileaf collimator using plastic scintillation detectors

“…Westermark et al 36 applied cone collimators similar to those used in the Gamma Knife ® ͑ranging in size from 4 to 18 mm in diameter͒ to a Varian Clinac 2100C and studied the output factors of a 6 MV beam using a plastic scintillation dosimetry system based on BC-400 ͑Saint-Gobain Crystals, Hiram, OH͒, as well as a single diode detector, and a double diode, a diamond detector, and a liquid ionization chamber. At these field sizes, the scintillator was shown to consistently measure output factors that were Ն5% lower than the liquid scintillator and measured lower than the diode detectors at most of the field sizes.…”

Energy‐dependent OAR sparing and dose conformity for total marrow irradiation of obese patients

“…Potential disadvantages of increasing beam energy include larger penumbra and the production of secondary neutrons originating in the head of the linac. These neutrons can contribute to integral dose for energies greater than 10 MV, therefore this study will not escalate beam energy beyond this level 29, 30, 31, 32. This work explores the hypothesis that a more penetrating beam may be useful in achieving target coverage while increasing healthy tissue sparing for certain TMI patients.…”

The dosimetric effect of mixed‐energy IMRT plans for prostate cancer

“…It has been shown that the narrow penumbra of low‐energy beams results in tighter dose distributions around a target, minimizing irradiation of nearby OARs, although the regions near beam entry ports receive higher doses. ( 6 ) On the other hand, high‐energy beams have negative aspects such as increasingly diffused beam boundaries due to the long lateral range of secondary electrons ( 7 ) and generation of secondary neutrons. The secondary neutrons are generated from the interactions between photons and treatment head when using 10 MV or higher, due to the photoneutron effect.…”