ESTRO 38 Abstract book

S56 ESTRO 38

analyzed the impact of the slit widths and the collimator position relative to the X-ray tube. Modifications of the emission angle and respective reduction of the projected focal spot size were evaluated as an innovative approach increasing the PVDR. Apart from the PVDR, we considered a high peak dose rate, as well as small beam penumbra widths as figures of merit. Based on the simulations, the collimator setup was built and is now subject to experimental validation with radiochromic film dosimetry.

Conclusion The PSMA PET response in salivary glands after radiotherapy demonstrates a significant dose-effect relationship on the glandular and voxel level, consistent with the hypothesized loss of glandular cells. The accuracy of this method appears to allow detection of differences in radiosensitivity between salivary gland types and between individual patients. If these dose-effect relations can be modelled and related to patient-reported outcomes, this could contribute to new dose constraints and potentially, a lower incidence of xerostomia in H&N RT patients. PV-106 An optimized compact microbeam source for preclinical studies F. Treibel 1,2,3 , J.J. Wilkens 1,2 , S. Bartzsch 2,3 , S.E. Combs 2,3 1 Technical University of Munich, Physics Department, Garching, Germany; 2 Technical University of Munich, Department of Radiation Oncology, Munich, Germany; 3 Helmholtz Zentrum München, Institute of Innovative Radiotherapy, Neuherberg, Germany Purpose or Objective Microbeam Radiation Therapy (MRT) is based on spatial fractionation of the dose in arrays of highly collimated microbeams. In preclinical investigations MRT showed high normal tissue tolerance while preserving the degree of tumor control. Currently, MRT can only be delivered at large synchrotrons due to beam properties such as high dose rate and low beam divergence. To promote research on MRT, we developed a collimator for an alternative microbeam source using a small animal irradiator (SARRP by Xstrahl). In the design special attention was paid to an optimzation of the peak-to-valley dose ratio (PVDR) and Considering the beam divergence of a conventional X-ray source, we developed a collimator with tilted slits. A movable middle plate framed by two fixed plates allows for variable slit widths between 0 and 100 μm (Fig.1). Monte-Carlo simulations in Geant4 were performed to optimize the design of the setup with regard to variations in dose and improvement in PVDR in a water phantom. We the achieved dose rate. Material and Methods

Results Significant improvements were introduced compared to the previous design of Bartzsch, Cummings, Eismann and Oelfke in 2016 in Medical Physics . The source distance of the collimator was increased from 70 mm to 212 mm. We achieved a very high PVDR of up to 25 in 1 cm depth representing an enhancement of 62% versus the PVDR measured at 70 mm by Bartzsch et al. Simultaneously, the divergence of the microbeams is reduced. Larger treatment depths can be obtained, enabling in vivo studies in rodents. Increased flexibility in beam width is achieved by the three-layered collimator design. The observed sensitivity of the dose to variations in slit width demands accuracies below 2 μm which are obtained by use of piezo actuators. The projected focal spot width has a strong effect on the dose profile. At 2 cm depth in water, comparisons of 20° and 12° target angle revealed up to 22% deviation for the peak dose and 3% for the valley. In average, the PVDR was increased by 17% for the smaller projected focal spot size (Fig.2).

Conclusion Our investigations led to a new microbeam source. Precisely controlled positioning of the collimator and of the slit widths are crucial to provide reproducible dose profiles. Modification of the source distance and the