Energy response of agar-alanine free radical dosimetry to therapeutic electron beams

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.

The effect of dose on light-sensitivity of radicals in alanine EPR dosimeters

In this work we present a study of light-induced effects on free radicals and their transformations in gamma-irradiated pure L-alanine and in commercially available alanine detectors: rods, pellets and films. Samples irradiated to doses from 2 Gy to 4000 kGy were exposed to light from a fluorescent lamp and to ordinary daylight. The observed changes in EPR spectra of the samples were analyzed with regard to their intensity and shape. The shape analysis was based on numerical decomposition of the measured spectra into model spectra reflecting contributions of R1, R2 and R3 radical populations in the samples. The illumination of alanine dosimeters resulted in significant decrease of the central EPR line and was accompanied by distinct variations in the shape of EPR spectra. The rate of light-induced decay in spectra amplitude was found to be dependent on dose of ionizing radiation--the sensitivity to light was decreasing with increase in dose in all detectors in the 2-5x10(5) Gy dose range. The exposure of gamma-irradiated (to 300 Gy) alanine to normal, diffused daylight resulted in decay of the signal amplitude at rate about 0.5% per week. It was shown that decay in the R1 component was responsible for the observed reduction of the spectra amplitude. The observed increase in R2 contributions in samples exposed to light confirmed a hypothesis of R-->R2 radical transformations promoted by visible light. The reported effects indicate a necessity of protection of irradiated dosimeters from their prolonged exposure to light.

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.

An experimental and Monte Carlo investigation of the energy dependence of alanine/EPR dosimetry: II. Clinical electron beams

The energy dependence of alanine/EPR dosimetry for 8, 12, 18 and 22 MeV clinical electron beams was investigated by experiment and by Monte Carlo simulations. Alanine pellets in a waterproof holder were irradiated in a water phantom using an Elekta Precise linear accelerator. The dose rates at the reference point were determined following the TG-51 protocol using an NACP-02 parallel-plate chamber calibrated in a (60)Co beam. The EPR spectra of irradiated pellets were measured using a Bruker EMX 081 EPR spectrometer. Experimentally, we found no significant change in alanine/EPR response to absorbed dose-to-water over the energy range 8-22 MeV at an uncertainty level of 0.6%. However, the response for high-energy electrons is about 1.3 (+/-1.1)% lower than for (60)Co. The EGSnrc Monte Carlo system was used to calculate the ratio of absorbed dose-to-alanine to absorbed dose-to-water and it was shown that there is 1.3 (+/-0.2)% reduction in this ratio from the (60)Co beam to the electron beams, which confirms the experimental results. Alanine/EPR response per unit absorbed dose-to-alanine was also investigated and it is the same for high-energy electrons and (60)Co gamma-rays.

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.

Uncertainties in alanine dosimetry in the therapeutic dose range

A method for evaluating the overall uncertainty of alanine EPR transfer dosimetry in the therapeutic dose range is described. The method uses experimental data on EPR signal reproducibility from replicate dosimeters irradiated to low doses (1-5 Gy), estimates of Type B uncertainties, and Monte Carlo simulations of heteroscedastic orthogonal linear regression. A Bruker ECS106 spectrometer and Bruker alanine dosimeters have been used for this evaluation. The results demonstrate that alanine dosimetry can be used for transfer dosimetry in that range with the overall uncertainty 1.5-4% (1sigma) depending on the dose, the number of replicate dosimeters. and the duration of the calibration session (the session should not exceed one working day).