André Wambersie

Which RBE for iodine 125 in clinical applications

The problem of the selection of a RBE value for 125I is discussed for clinical applications in interstitial therapy. Microdosimetric studies indicate that the emission of the 125I has a y spectra similar to X-rays of 12-140 keV (conventional X-rays), but differs from 60Co, 137Cs gamma-rays and high energy X-rays. The radiobiological data available show RBE values ranging from 1 to 2.4 for 125I (reference beam 60Co). Although obtained with different biological systems and endpoints, RBE values of 1.15-1.20 are in general observed for high doses and high dose rates. On the other hand, higher RBE values (up to 2) are observed at low doses or low dose rates, which is consistent with theoretical and microdosimetric data.

Application of the LQ model to the interpretation of absorbed dose distribution in the daily practice of radiotherapy

In 1991, the vast majority of radiotherapy centers are implemented with computer treatment planning systems (TPS), and it has become routine practice to compute full absorbed dose distribution (ADD) in almost all treatment situations. Usually the target is covered by the 100% isodose and the surrounding normal tissues receive a lesser dose than the tumor. It implies, that, as the dose per fraction of, say, 2 Gy is prescribed at the 100%, normal tissues receive a daily dose different than 2 Gy. The absorbed doses delivered at different organs have therefore not the same biological effectiveness and must be corrected according to the actual dose per fraction for a proper interpretation of the treatment planning. This is of great importance since most of the tolerance levels used in the practice have been determined for doses per fraction around 1.8-2 Gy. The linear-quadratic (LQ) model provides a simple method for establishing biological equivalencies and has been used throughout this article to establish the difference between the absorbed dose computed by the TPS and its biological equivalent. It is shown that normal tissues receiving less than 100% of the daily dose are relatively more protected than suggested by the ADD, and, inversely, that normal structures overdosed and thus receiving more than the 100% daily dose are relatively more at risk for complications than suggested from the ADD.

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 Characterization of the Epithermal Neutron Beam Produced at the Tsing Hua Open-Pool Reactor (THOR) for BNCT: Comparison with Other BNCT Facilities

Purpose: To determine the RBE of the BNCT epithermal neutron beam produced at THOR for a reference biological system and reference irradiation conditions. Materials and Methods: Intestinal crypt regeneration in mice was chosen as biological system. No boron compound was administrated. RBE was determined relative to cobalt-60 gamma rays for two irradiation conditions corresponding to different gamma and ”non-gamma” absorbed dose mixtures. Results: The RBE(subscript beam) (i.e. considering the beam ”as it is”, with all dose components and its actual dose rate) was found equal to 1.5 and 1.9 for absorbed dose mixtures comprising 75 and 70% of gamma respectively. Assuming that the RBE of the gamma dose component is equal to unity and deriving the RBE of the non-gamma dose component, RBE(subscript non-γ) values of 2.9 and 4.0 were obtained from the data of the first and second absorbed dose mixture respectively. Comparing the THOR data with those obtained from the same system in 6 other BNCT facilities worldwide, large differences between the RBE(subscript beam) are observed, even when comparing those beam configurations exhibiting similar absorbed dose mixtures. There is, however, a general trend for the RBE(subscript beam) to increase slightly with increasing non-gamma dose component contribution. The derived RBE(subscript non-γ) exhibit also large differences, but average 3.4, which value is close to the one currently apply in clinical practice for determining the ”total biologically weighted dose”. Discussion and Conclusion: The THOR data as well as those of the other facilities exhibit a certain degree of internal consistency in that the RBE(subscript beam) increases with the non-gamma dose component. However, the RBE differences between institutions appear too large to be explained only by the difference in the composition of the beams and in dose rate (power of the reactor). It is likely that lessening the dosimetric uncertainties and adopting common dosimetric protocols would restore some homogeneity in the radiobiological data. As such experiments constitue an overall check of the irradiation procedure (including dosimetry), they should be undertaken for the commissioning of new installations or on the occasion of any substantial upgrade.