In vivo surface dosimetry with a scintillating fiber dosimeter in preclinical image‐guided radiotherapy

CERVOXY COLLInternational audiencePURPOSE: New preclinical image-guided irradiators and treatment planning systems represent a huge progress in radiobiology. Nevertheless, quality control of preclinical treatments is not as advance as in clinical radiotherapy and in vivo dosimetry is little developed. In this study, we evaluate the use of a scintillating fiber dosimeter called DosiRat to verify the agreement between the doses planned with SmART-Plan and the measured doses during small animal irradiations.METHODS: In vivo dosimetry was first evaluated with DosiRat through dose measurements performed at the surface of a 3 × 9 × 3 cm3 phantom. Measured and planned doses were compared for different irradiation conditions (prescription point, anterior and posterior beams, 5 mm and 10 mm irradiation fields). In a second phase, measured and planned doses were compared for rat brain irradiations performed with anterior beams, with DosiRat positioned at the beam entrance. Comparisons were performed for different tube currents (1.3 and 13 mA), collimations (5, 10 and 25 mm diameter) and planned doses (0.1, 0.5, 2 and 10 Gy).RESULTS: In the case of the phantom irradiations, planned and measured doses showed discrepancies smaller than the 5 % accuracy of the TPS, except in cases in which the dosimeter was not centered in the irradiation field. The differences were larger for animal irradiations (from -3.3 % to 8.8 %) because of variations of the beam energy spectrum and the non-equivalence between materials at medium and low energy.CONCLUSIONS: This study highlighted the complexity to implement one-dimension in vivo dosimetry in orthovoltage millimetric beams. Nevertheless, DosiRat is well adapted to in vivo dosimetry because of its small volume and its direct reading and allowed in vivo control of planned doses for anterior beams down to 5 mm diameter

Experimental dosimetry of in vitro irradiation of tumor cells with 212Pb

International audienc

Experimental dosimetry of in vitro irradiation of tumor cells with 212Pb

International audienc

Experimental dosimetry of in vitro irradiation of tumor cells with 212Pb

International audienc

Application of a new spectral deconvolution method for in vitro dosimetry in assessment of targeted alpha therapy

International audienceBackgroundThe improvement of in vitro assessment of targeted alpha therapy (reproducibility, comparability of experiments horizontal ellipsis ) requires precise evaluation of the dose delivered to the cells. To answer this need, a previous study proposed an innovative dosimetry method based on alpha-spectroscopy and a specific deconvolution process to recover the spatial distribution of Pb-212 isotopes inside in vitro culture wells. Nevertheless, although promising, the deconvolution method was time consuming and only tested for a simple isotope decay chain. PurposeThe purpose of this work is to propose a new matrix deconvolution method of alpha spectra based on a constrained-non-negative-maximum-likelihood decomposition, both faster and offering a greater modelling flexibility, allowing to study independently the kinetics of each of the daughter nuclides of complex decay chains (illustrated here with Ra-223) in in vitro culture wells. MethodsFirstly, the performance of the new method was fully evaluated through Monte Carlo simulations of in vitro irradiations. Different spatial distributions of Pb-212 and Ra-223, the corresponding alpha spectra measured by a silicon detector and the doses delivered to the cells were simulated with Geant4. The deconvolution results were then compared to the simulation results. Secondly, measurements were carried out in culture wells without cells containing 15 kBq of Pb-212 or 9.3 kBq of Ra-223, placed above silicon detectors recording alpha spectra in real time. The matrix deconvolution was then applied to determine the spatial and temporal distribution of all alpha-emitting daughters of studied isotopes. ResultsThe matrix deconvolution was proved to recover the simulated distribution gradients, ensuring simulated doses within 3 % for both tested radionuclides, with errors on dose normally distributed around the reference value (consequently not exhibiting any bias), even in the case of complex decay chains as Ra-223. The experimental study of Pb-212 and Ra-223 showed highly inhomogeneous distributions and time evolution of the concentration gradients, consistent with the previous study. Furthermore, it highlighted the complex kinetics of Ra-223 with different distributions of its alpha-emitting daughters (Rn-219, Po-215, At-215, Bi-211, Po-211). ConclusionsThis study validates a new deconvolution method, fast and flexible, that proved to be accurate and reliable. This method allowed to reveal the complexity of isotopes kinetics in in vitro experiments, especially with complex decay chains. Experimental dosimetry, necessary to improve reliability of in vitro studies in targeted alpha therapy, is demonstrated to be feasible with the proposed method

Radionuclide spatial distribution and dose deposition for in vitro assessments of 212^{212}Pb-alphaVCAM-1 targeted alpha therapy

CERVOXY COLLInternational audiencePurposeTargeted alpha therapy (TAT) takes advantage of the short‐range and high linear‐energy‐transfer of α‐particles and is increasingly used, especially for the treatment of metastatic lesions. Nevertheless, dosimetry of α‐emitters is challenging for the very same reasons, even for in vitro experiments. Assumptions, such as the uniformity of the distribution of radionuclides in the culture medium, are commonly made, which could have a profound impact on dose calculations. In this study we measured the spatial distribution of α‐emitting 212Pb coupled to an anti‐VCAM‐1 antibody (212Pb‐αVCAM‐1) and its evolution over time in the context of in vitro irradiations.MethodsTwo experimental setups were implemented without cells to measure α‐particle count rates and energy spectra in culture medium containing 15 kBq of 212Pb‐α‐VCAM‐1. Silicon detectors were placed above and below cell culture dishes for 20 hours. One of the dishes had a 2.5‐µm‐thick mylar‐base allowing easy detection of the α‐particles. Monte Carlo simulations were performed to analyze experimental spectra. Experimental setups were modelled and α‐energy spectra were simulated in the silicon detectors for different decay positions in the culture medium. Simulated spectra were then used to deconvolute experimental spectra to determine the spatial distribution of 212Pb‐αVCAM‐1 in the medium. This distribution was finally used to calculate the dose deposition in cell culture experiments.ResultsExperimental count rates and energy spectra showed differences in measurements taken at the top and the bottom of dishes and temporal variations that did not follow 212Pb decay. The radionuclide spatial distribution was shown to be composed of a uniform distribution and concentration gradients at the top and the bottom, which were subjected to temporal variations that may be explained by gravity and electrostatic attraction. The absorbed dose in cells calculated from this distribution was compared with the dose expected for a uniform and static distribution and found to be 1.75 times higher, which is highly significant to interpret biological observations.ConclusionThis study demonstrated that accurate dosimetry of α‐emitters requires the experimental determination of radionuclide spatial and temporal distribution and highlighted that in vitro assessment of dose for TAT cannot only rely on a uniform distribution of activity in the culture medium. The reliability and reproducibility of future experiments should benefit from specifically developed dosimetry tools and methods