Thermoluminescence in proton and fast neutron therapy beams

In photon radiotherapy, thermoluminescence dosemeters (TLD) are commonly used for in vivo dosimetry. In proton and neutron therapy, their use raises a particular difficulty since their responses vary significantly with the energy spectrum. With reference to Co-60, the response of CaF2:Tm (TLD-300) and (LiF)-Li-7:Mg,Ti (TLD-700) crystals was investigated in the 85 MeV proton beam and the p(65)+Be neutron beam produced at the cyclotron at Louvain-La-Neuve and used for therapy applications. In the proton beam, TLD dosemeters were irradiated in a phantom, at different positions in the unmodulated and modulated Bragg curve. In the fast neutron beam, TLDs were positioned inside the geometrical limits of the beam at different depths, and outside its limits behind five leaves of the multi-leaf collimator. peak height variation of glow curves was investigated and correlated with the variation of radiation quality

Entrance and exit dose measurements with semiconductors and thermoluminescent dosemeters: a comparison of methods and in vivo results.

BACKGROUND AND PURPOSE: In order to compare diodes and TLD for in vivo dosimetry, systematic measurements of entrance and exit doses were performed with semiconductor detectors and thermoluminescent dosemeters for brain and head and neck patients treated isocentrically with external photon beam therapy. MATERIAL AND METHODS: Scanditronix EDP-20 diodes and 7LiF thermoluminescent chips, irradiated in a 8 MV linac, were studied with similar build-up cap geometries and materials in order to assure an equivalent electronic equilibrium. Identical calibration methodology was applied to both detectors for the dose determination in clinical conditions. RESULTS: For the entrance dose evaluation over 249 field measurements, the ratio of the measured dose to the expected dose, calculated from tabulated tissue maximum ratios, was equal to 1.010 +/- 0.028 (1 s.d.) from diodes and 1.013 +/- 0.041 from thermoluminescent crystals. For the exit dose measurements, these ratios were equal to 0.998 +/- 0.049 and 1.016 +/- 0.070 for diodes and TLDs, respectively, after application of a simple inhomogeneity correction to the calculation of the expected exit dose. CONCLUSIONS: Thermoluminescence and semiconductors led to identical results for entrance and exit dose evaluation but TLDs were characterised by a lower reproducibility inherent to the TL process itself and to the acquisition and annihilation procedures

Spatial variation of radiation quality during moving beam therapy with 14 MeV [d(0.25)+T] neutrons

In conformal moving beam therapy with fast neutrons, the contributions to dose from the direct beam, scattered radiation and the gamma component vary with the position in the phantom. To determine this variation in radiation quality, microdosimetric measurements of energy deposition spectra were performed at different positions in a therapy phantom. Fixed beam irradiation at different incidence angles showed strong changes in the lineal energy spectrum. An increase of slow protons (20 < y < 110 keV.mum(-1)) and a decrease of fact protons (2 < y < 20 keV.mum(-1)) was seen for irradiation outside the direct beam. During moving beam irradiation, different positions on the same isodose curves (55% or 35%) showed differences in (y) over bar (D) of up to 5%. Variations in the quality parameter. R, determined by applying an empirical biological weighting function, were of similar magnitude. Thus, spatial variations in radiation quality should be taken into account in biological dose planning for moving beam neutron therapy

Response analysis of TLD-300 dosimeters in heavy-particle beams.

In vivo dosimetry is recommended as part of the quality control procedure for treatment verification in radiation therapy. Using thermoluminescence, such controls are planned in the p(65) + Be neutron and 85 MeV proton beams produced at the cyclotron at Louvain-La-Neuve and dedicated to therapy applications. A preliminary study of the peak 3 (150 degrees C) and peak 5 (250 degrees C) response of CaF2:Tm (TLD-300) to neutron and proton beams aimed to analyse the effect of different radiation qualities on the dosimetric behaviour of the detector irradiated in phantom. To broaden the range of investigation, the study was extended to an experimental 12C heavy ion beam (95 MeV/nucleon). The peak 3 and 5 sensitivities in the neutron beam, compared to 60Co, varied little with depth. A major change of peak 5 sensitivity was observed for samples positioned under five leaves of the multi-leaf collimator. While peak 3 sensitivity was constant with depth in the unmodulated proton beam, peak 5 sensitivity increased by 15%. Near the Bragg peak, peak 3 showed the highest decrease of sensitivity. In the modulated proton beam, the sensitivity values were not significantly smaller than those measured in the unmodulated beam far from the Bragg peak region. The ratio of the heights of peak 3 and peak 5 decreased by 70% from the 60Co reference radiation to the 12C heavy-ion beam. This parameter was strongly correlated with the change of radiation quality

Radiobiological Effectiveness of Radiation Beams With Broad Let Spectra - Microdosimetric Analysis Using Biological Weighting Functions

The evaluation of averaged parameters from microdosimetric spectra, measured with proportional counters and biological weighting functions of the quantity lineal energy, provides a practical solution to the specification of radiation quality, especially for radiations with broad LET spectra, such as those used for radiation therapy. This approach has already proved satisfactory for fast neutron therapy beams of different energy using a biological weighting function numerically determined on empirical and statistical bases using the results of a RBE-microdosimetry intercomparison study. The paper discusses the applicability of this procedure by extending the range of radiations to fast protons. For this purpose, the earlier neutron data were combined with preliminary results from RBE experiments and microdosimetric measurements performed within the framework of two proton therapy programmes. Different weighting functions were derived by unfolding calculations applied to data sets combining high energy gamma rays, fast neutrons and fast protons