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Alpha-DaRT is a new method for treating solid tumors with alpha particles,relying on the release of the alpha-emitting daughter atoms of radium-224 fromsources inserted into the tumor. The most important model parameters forAlpha-DaRT dosimetry are the diffusion lengths of radon-220 and lead-212, andtheir estimation is essential for treatment planning. The aim of this work isto provide first experimental estimates for the diffusion length of radon-220.The diffusion length of radon-220 was estimated from autoradiographymeasurements of histological sections taken from 24 mice-borne subcutaneoustumors of five different types. Experiments were done in two sets: fourteenin-vivo tumors, where during the treatment the tumors were still carried by themice with active blood supply, and ten ex-vivo tumors, where the tumors wereexcised before source insertion and kept in a medium at 37 degrees C with thesource inside. The measured diffusion lengths of radon-220 lie in the range0.25-0.6 mm, with no significant difference between the average values measuredin in-vivo and ex-vivo tumors: 0.40 $\pm$ 0.08 mm for in-vivo vs. 0.39 $\pm$0.07 mm for ex-vivo. However, in-vivo tumors display an enhanced spread ofactivity 2-3 mm away from the source. This effect is not explained by thecurrent model and is much less pronounced in ex-vivo tumors. The averagemeasured radon-220 diffusion lengths in both in-vivo and ex-vivo tumors lieclose to the upper limit of the previously estimated range of 0.2-0.4 mm. Theobservation that close to the source there is no apparent difference betweenin-vivo and ex-vivo tumors, and the good agreement with the theoretical modelin this region suggest that the spread of radon-220 is predominantly diffusivein this region. The departure from the model prediction in in-vivo tumors atlarge radial distances may hint at potential vascular contribution.

First measurements of radon-220 diffusion in mice tumors, towards treatment planning in diffusing alpha-emitters radiation therapy