Purpose Targeted 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. Methods Two 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 h. 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 modeled 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. Results Experimental 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. Conclusions This 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.

Objective: Small animal image-guided irradiators have recently been developed to mimic the delivery techniques of clinical radiotherapy. A dosemeter adapted to millimetric beams of medium-energy X-rays is then required. This work presents the characterization of a dosemeter prototype for this particular application. Methods: A scintillating optical fibre dosemeter (called DosiRat) has been implemented to perform real-time dose measurements with the dedicated small animal X-RAD® 225Cx (Precision X-Ray, Inc., North Branford, CT) irradiator. Its sensitivity, stem effect, stability, linearity and measurement precision were determined in large field conditions for three different beam qualities, consistent with small animal irradiation and imaging parameters. Results: DosiRat demonstrates good sensitivity and stability; excellent air kerma and air kerma rate linearity; and a good repeatability for air kerma rates .1 mGy s 21. The stem effect was found to be negligible. DosiRat showed limited precision for low air kerma rate measurements (,1 mGy s 21 ), typically for imaging protocols. A positive energy dependence was found that can be accounted for by calibrating the dosemeter at the needed beam qualities. Conclusion: The dosimetric performances of DosiRat are very promising. Extensive studies of DosiRat energy dependence are still required. Further developments will allow to reduce the dosemeter size to ensure millimetric beams dosimetry and perform small animal in vivo dosimetry. Advances in knowledge: Among existing point dosemeters, very few are dedicated to both medium-energy X-rays and millimetric beams. Our work demonstrated that scintillating fibre dosemeters are suitable and promising tools for real-time dose measurements in the small animal field of interest.

The biphenyl / POPOP couple is known for its scintillation properties, but we describe in this note its use for the discrimination between fast neutrons and gamma rays. More particularly, the influence of the matrix involved in this process is of interest, and for the first time pulse shape discrimination has been observed in non-common solvents such as heptane or ethanol. The discrimination efficiency of our systems is described in terms of factor of merit, angle between neutron and gamma lobes and scintillation performances.

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