The energy response of agar-alanine phantom dosimeter to gamma radiation

Response of the alanine/ESR dosimeter to radiation from an Ir-192 HDR brachytherapy source

The response of the alanine dosimeter to radiation from an Ir-192 source with respect to the absorbed dose to water, relative to Co-60 radiation, was determined experimentally as well as by Monte Carlo simulations. The experimental and Monte Carlo results for the response agree well within the limits of uncertainty. The relative response decreases with an increasing distance between the measurement volume and the source from approximately 98% at a 1 cm distance to 96% at 5 cm. The present data are more accurate, but agree well with data published by Schaeken et al (2011 Phys. Med. Biol. 56 6625-34). The decrease of the relative response with an increasing distance that had already been observed by these authors is confirmed. In the appendix, the properties of the alanine dosimeter with respect to volume and sensitivity corrections are investigated. The inhomogeneous distribution of the detection probability that was taken into account for the analysis was determined experimentally.

In vivo dose evaluation during gynaecological radiotherapy using L-alanine/ESR dosimetry

The dose delivered by in vivo 3-D external beam radiation therapy (EBRT) was verified with L-alanine/electron spin resonance (ESR) dosimetry for patients diagnosed with gynaecological cancer. Measurements were performed with an X-band ESR spectrometer. Dosemeters were positioned inside the vaginal cavity with the assistance of an apparatus specially designed for this study. Previous phantom studies were performed using the same conditions as in the in vivo treatment. Four patients participated in this study during 20-irradiation sessions, giving 220 dosemeters to be analysed. The doses were determined with the treatment planning system, providing dose confirmation. The phantom study resulted in a deviation between -2.5 and 2.1 %, and for the in vivo study a deviation between -9.2 and 14.2 % was observed. In all cases, the use of alanine with ESR was effective for dose assessment, yielding results consistent with the values set forth in the International Commission on Radiation Units and Measurements (ICRU) reports.

Combined effects of high doses and temperature on radiation-induced radicals and their relative contributions to EPR signal in gamma-irradiated alanine

Electron Paramagnetic Resonance (EPR) study of irradiated l-alanine showed differences in dose-response curves obtained at low and high microwave power for a broad range of doses, up to 3000 kGy. A mathematical model was fitted to experimental data and calculated yields of generation and of destruction of radicals showed variations with microwave power. The calculations were applied to both double integrals of the total EPR signal and to its components reflecting contributions of radicals R1, R2 and R3 in the alanine EPR signal. The relative contributions of radicals R1, R2 and R3 varied with dose >100 kGy; an increase in relative contribution of R3 was accompanied by a decrease in contribution of R1 radicals. The observed fading of EPR signal intensity in samples annealed to 175-208 degrees C was a strong function of dose, and varied by 2-3 orders of magnitude in the dose range examined.

EPR study of light illumination effects on radicals in gamma-irradiated L-alanine

Exposure of gamma-irradiated L-alanine samples to sunlight and to light from a regular, fluorescent lamp resulted in significant changes in their EPR resonance patterns, both to spectral shapes and intensities. The experimental EPR spectra were numerically decomposed into three components reflecting contributions of three different radicals (R1-R3) generated by ionizing radiation in alanine. The light exposure caused a decay of the measured EPR signal intensity. For similar light intensities and exposure times the decay was much more pronounced in samples illuminated by sunlight than in samples illuminated by the fluorescent lamp. In both cases light-induced decay of R1 radicals was observed. Sunlight illumination resulted in a moderate decay of R2 radicals and in a doubling of the R3 radical population. On the other hand, fluorescent light caused a significant increase of R2 radicals and did not change the amount of R3 radicals. A quantitative analysis of the variations of the three radical contributions to the total EPR spectra upon fluorescent light exposure suggests a net R1-->R2 free radical transformation. These effects of light on the alanine dosimetric signal should be taken into account in dosimetry protocols, assuring protection of alanine dosimeters from extended exposure to light.

In vivo alanine/EPR dosimetry in daily clinical practice: a feasibility study

PURPOSE

The objective of this study was evaluation of accuracy of in vivo dosimetry using electron paramagnetic resonance (EPR) in alanine. Additionally, we aimed to identify sources of uncertainty in dose determination and quantitative assessment of physical factors that may result in discrepancies between the measured and planned single-fraction doses.

METHODS AND MATERIALS

The measurements were performed using detectors in a form of 1.6 cm x 1.6 cm polyethylene sachets filled with powdered L-alanine. The detectors were taped to the patient's skin and measured the entrance doses for (60)Co and electron beams. Some detectors were covered with buildup material, and some measured the "skin dose." The EPR measurements were performed with a Varian E-4 spectrometer.

RESULTS

The calculated uncertainty of EPR measured doses was dependent on measured doses and varied from 6.6% for 0.5 Gy to 3.2% for 2 Gy. The calculated uncertainty was in concordance with experimentally determined reproducibility of EPR signals. However, the deviations between measured and planned doses exceeded the uncertainty range of EPR measurements, which can be attributed to uncertainty in determination of actually delivered doses to the detectors, on the basis of treatment planning data.

CONCLUSION

The accuracy of dose determination by EPR measurements was shown to be achievable within the 5% limit recommended by the ICRU for doses above 0.7 Gy. The accuracy of in vivo verification of radiotherapy doses by in vivo EPR dosimetry can be improved by meticulous selection of measurement conditions, i.e., radiation fields and detector positions, ensuring accurate calculation of doses delivered to the dosimeters.

Radiation dose measurements with alanine/agarose gel and thin alanine films around a 192Ir brachytherapy source, using ESR spectroscopy

Alanine/agarose gel and alanine films in stacks have been used for measurements of absorbed dose around an HDR 192Ir source in a vaginal cylinder-applicator, with and without a 180 degrees tungsten shield. The gel and the films were analysed by means of ESR spectroscopy and calibrated against an ion chamber in a 4 MV photon beam to obtain absolute dose values. The gel serves as both dosimeter and phantom material, and the thin (130 microm) films are used to achieve an improved spatial resolution in the dose estimations. Experimental values were compared with Monte Carlo simulations using two different codes. Results from the measurements generally agree with the simulations to within 5%, for both the alanine/agarose gel and the alanine films.

Perturbation effects in dosimetry: Part I. Kilovoltage x-rays and electrons

Perturbation effects are defined as departures from ideal large-detector or Bragg-Gray cavity behaviour. Such effects are central to the use of practical dosimeters for accurate dose determination, as is required in external-beam radiotherapy. A theoretical framework for treating perturbation effects is established. In this first part of the review, perturbation in kilovoltage x-ray and megavoltage electron beams are treated in detail, with the emphasis on ionization chambers. The displacement factor for ion chambers in kilovoltage x-ray beams is discussed, starting with the early, pioneering work of Lamerton and Lidén. The evidence for the large values of the perturbation factor in medium-energy x-ray beams (between 100 and 300 kV) recommended in the 1987 IAEA dosimetry code is critically examined and revised, smaller values are given. In electron beams the theoretical approaches to the correction for the in-scattering correction in gas-filled cavities is discussed in detail. The evidence for negligible perturbation in low-energy electron beams in plane-parallel chambers with adequate guardring widths is critically reviewed, including the suggested correction for perturbation due to backscattering differences between the chamber-wall material and the medium. The various models for the response of thermoluminescent dosimeters in electron beams are discussed. It is concluded that Monte Carlo simulation of dosimeter response is likely to play an even bigger role in the future.