ESTRO 2020 Abstract book

S122 ESTRO 2020

Diffusing Alpha-emitters Radiation Therapy (“DaRT”) is a new concept, which enables – for the first time - the treatment of solid tumors by alpha particles. The treatment utilizes implantable seeds, whose surface is embedded with a low activity of radium-224. Each seed continuously emits the short-lived alpha-emitting daughters of radium-224, which spread over several mm around it, creating a “kill region” of high alpha-particle dose. DaRT is presently tested in clinical trials, starting with locally advanced and recurrent squamous cell carcinoma (SCC) of the skin and head and neck, with promising results with respect to both efficacy and safety. This work aims to provide a simple model which can serve as a zero-order approximation for DaRT dosimetry. Material and Methods The model consists of diffusion equations for radon-220, lead-212 and bismuth-212, with the other short-lived daughters in local secular equilibrium. For simplicity, the medium is assumed to be homogeneous, isotropic and time-independent. Vascular effects are accounted for by effective diffusion and clearance terms. To leading order, the alpha particle dose can be described by simple analytic expressions, which shed light on the underlying physics. The beta and gamma dose is calculated using the EGSnrc Monte Carlo code. Animal studies were used to estimate the values of the key parameters of the model, using a combination of phosphor-imaging-based autoradiography (to record the spatial distribution of lead-212 in treated tumors) and gamma spectroscopy. Results The calculations demonstrate that, for a reasonable choice of model parameters, therapeutic alpha-particle dose levels are obtained over a region measuring 4-7 mm in diameter for sources carrying a few µCi of radium-224. In particular, for SCC tumors the macroscopic alpha particle dose is predicted to exceed 10 Gy over a ~5 mm diameter region. The beta and gamma dose falls below 5 Gy at a radial distance of ~2 mm from the seed. The model predictions served as the basis for treatment planning in the SCC clinical trial, where treatments employing DaRT seeds carrying 2 µCi of radium-224 and spaced 5 mm apart resulted positive response (30-100% shrinkage in volume) in all treated tumors. Conclusion The promising results of the SCC clinical trial indicate that in spite of its approximate nature, the simple diffusion- based dosimetry model provides a quantitative starting point for DaRT treatment planning. The parameters governing the alpha particle dose are expected to vary, to some extent, between different tumor types and should be evaluated in suitable animal models. The predictions concerning the beta and gamma dose are expected to be largely tumor-independent. OC-0219 Development and validation of an EGSnrc accelerator head model for a 1.5 T MR-Linac M. Friedel 1 , M. Nachbar 1 , D. Mönnich 1,2 , O. Dohm 3 , D. Zips 2,3 , D. Thorwarth 1,2 1 University Hospital Tübingen, Department of Radiation Oncology- Section for Biomedical Physics, Tübingen, Germany ; 2 German Cancer Consortium DKTK, partner site Tübingen- and German Cancer Research Center DKFZ Heidelberg, Tübingen, Germany ; 3 University Hospital Tübingen, Department of Radiation Oncology, Tübingen, Germany Purpose or Objective To develop a full EGSnrc accelerator head and cryostat model for a 1.5 T MR-Linac (MRL) to enable independent, high precision dose calculations accounting for magnetic field effects. In this work, an EGSnrc model of the 1.5 T Elekta Unity (Elekta AB, Stockholm, Sweden) was developed, implemented and validated against experimental data and a commercial treatment planning system (TPS).

Material and Methods Primary electron beam parameters for the implemented model were adapted to be in accordance with measured dose profiles of the 1.5 T Elekta Unity. Those parameters were the mean electron energy as well as the Gaussian radial intensity and energy distribution. Energy tuning was done comparing percentage depth dose (PDD) curves simulated with monoenergetic beams of varying energies from 6.7 MeV to 7.7 MeV with measured data for a 2×2 cm 2 field. The optimum radial intensity distribution was found by varying the radial full width at half maximum (FWHM) between 0.4 mm and 4.0 mm and comparing simulated to measured lateral profiles for a field size of 22×22 cm 2 . The influence of the energy distribution was investigated by comparing simulated lateral and depth dose profiles of 22×22 cm 2 fields with varying energy spreads (0% - 20%) to measured data. Additionally, PDDs and cross profiles were determined with the TPS Monaco (V4.40). Comparison of simulations and TPS with measurements was performed by calculating average and maximum local dose deviations with respect to the experimental data. In addition, output factors (OF) were compared for square fields from 2×2 to 22×22 cm 2 . Results The tuning of parameters for the EGSnrc accelerator and cryostat model of the 1.5 T Elekta Unity and comparison with experimental data showed that the optimum initial electron beam for MRL simulations was monoenergetic with an electron energy of (7.4 ± 0.2) MeV. The optimum Gaussian radial intensity distribution had a FWHM of (2.2 ± 0.3) mm. The average relative deviations of the simulations were below 1% for all simulated profiles with optimum electron parameters, whereas the largest maximum deviation of 2.07% was found for the 22×22 cm 2 cross-plane profile. For the TPS calculated profiles, mean and maximum deviations were <1% and 2.5%, respectively. Profiles were insensitive to energy spread variations. Figure 1 presents PDDs and profiles for exemplary fields. The mean relative difference (range) of simulation- or TPS- based OF with respect to measured data was 0.08% (-0.90% - 1.18%) and -0.19% (-0.88% – 0.44%), respectively (cf. Figure 2).

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