ESTRO 2020 Abstract book

S283 ESTRO 2020

low-X beam, the low-Z beam provided improved CNR for all four contrast objects across the investigated range of imaging doses for both thin and thick phantoms. At imaging doses above which the cortical bone, CB2-30%, and breast contrast objects are visible in the thick phantom (CNR~1), the low-Z CNR advantage becomes increasingly significant with increasing dose. For the thin phantom, this CNR trend is consistent for the breast and CB2-30% contrast objects, whereas the low-Z CNR advantage of cortical bone and brain decrease with increasing imaging dose. Conclusion This study marks the first investigation of a sintered diamond target in the target arm of a Truebeam linear accelerator. Compared to the commercial low-X beam, the low-Z beam offers improved CNR for low-, medium-, and high-contrast objects in both thin and thick phantoms across a range of clinically relevant imaging doses. The improvements in CNR of visible low-contrast and medium- contrast objects at higher imaging doses (~1 cGy) could prove beneficial in the case of MV volumetric imaging, which will be investigated in forthcoming work.

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 1 Modified Truebeam target arm containing 2.5 MV low-Z sintered diamond target.

Figure 2 CNR versus imaging dose for breast, brain, cortical bone and cortical bone-30% for a) thin and b) thick phantoms. 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