ESTRO 2020 Abstract Book

S264 ESTRO 2020

OC-0471 Optimization of a compact x-ray source for clinical microbeam radiation therapy J. Winter 1,2,3 , J.J. Wilkens 2,3 , S.E. Combs 1,2 , S. Bartzsch 1,2 1 Helmholtz Zentrum München GmbH - German Research Center for Environmental Health, Institute of Radiation Medicine, Neuherberg, Germany ; 2 Technical University of Munich - School of Medicine and Klinikum rechts der Isar, Department of Radiation Oncology, Munich, Germany ; 3 Technical University of Munich, Physics Department, Garching, Germany Purpose or Objective Microbeam radiation therapy (MRT) is a promising approach for treating inoperable tumors as preclinical studies showed lower side effects to healthy tissue with the same tumor control as conventional RT. The dose in MRT is spatially fractionated into arrays of planar, micrometer wide beamlets (peaks) with doses up to hundreds of Gray and low-dose valleys in between. A high peak to valley dose ratio (PVDR) and a high dose rate can yet only be achieved at large synchrotrons such as the ESRF in Grenoble, France. Currently, we are constructing a preclinical prototype of a compact MRT source [1] that may provide the technology for clinical treatments. Here we investigate parameters and performance of such a compact, divergent MRT source. Material and Methods For dose calculation, we used Monte Carlo simulations in Geant4. Electrons of 200, 300, 400, 500, 600, 800 keV hit an eccentric (10–500 µm x 30 mm) Gaussian shaped focal spot. The generated x-rays were filtered (0.8 mm Be and 0.4 mm Cu), traveled through a microbeam collimator (2 x 2 cm 2 field, divergent slits of 50 µm x 20 mm), and hit a water phantom. We optimized electron energy, spot width, and source-to-collimator distance (scd) for a high PVDR and steep penumbras. The performance of the MRT source for a brain tumor treatment was analyzed for 400 keV electrons. The head was represented by a spherical phantom: 1 mm skin and 6 mm bone surrounded water-equivalent brain tissue. We investigated a full-arc rotation of the beams around the phantom, like arc therapy in conventional RT, as a possibility to reduce peak entrance doses and to compensate for steep depth doses of low-energy photons. Results For a high PVDR and steep penumbras, the focal spot width (Gaussian standard deviation) should not be larger than the width of a single collimator slit, see figure 1(a). The scd should be at least 35 cm. Highest PVDRs were found for 400 keV electrons, see figure 1(b). For higher energies, the secondary electrons scatter into the valleys which increases the valley dose. In the water phantom, 225 keV electrons, a spot width of 50 µm, and an scd of 50 cm led to a PVDR of 30 in 10 mm (20 in 40–100 mm) water depth. As comparison, parallel microbeams from the ESRF spectrum (250 keV max. energy) led to a PVDR of 35 in 10 mm (25 in 40–100 mm) depth.

Figure 2 shows the peak dose distribution in the head phantom after a full-arc beam rotation. The dose ratio between the target (center of the phantom) and the bone (entrance dose) improved from 0.05 for a single field to 2.15 for a full-arc rotation.

Conclusion We found parameters for a compact, divergent MRT source leading to a PVDR above 20 in all depths, which is comparable to the same field size at the ESRF [2]. For constructing the source, these parameters need to be balanced with manufacturing requirements and clinical needs such as reduced peak entrance doses. References [1] Bartzsch S, Oelfke U. Phys Med Biol 2017. 62(22): 8600– 15. [2] Martínez-Rovira I, Sempau J, Prezado Y. Med Phys 2011. 39(1): 119–31.