ESTRO 2020 Abstract book

S288 ESTRO 2020

MR defined case, the point dose measurement was taken in a target volume at the centre of the brain. In the multiple metastases treatment case, the point dose measurement was taken in a target volume approx. 5cm off central axis of the brain. Results The point dose results of the initial audit measurements are shown in Figure 2. For the classic SRS case, all plans were single isocentre and showed a maximum local dose discrepancy of 3.5%. For the MR defined multiple metastases case, plans included were both single and multiple isocentre, with a maximum local dose discrepancy of 1.5%. For the five target multiple metastases audit case, plans included were both single and multiple isocentre, with a large difference between the point dose results observed. The multiple isocentre plans showed closer agreement to measurement, with a maximum local dose discrepancy of -1.4%, compared to - 12.9% for the single isocentre plans.

Center DKFZ, Department of Translational Radiation Oncology, Heidelberg, Germany ; 7 German Cancer Research Center, Clinical Cooperation Unit Radiation Therapy, Heidelberg, Germany ; 8 University Hospital Heidelberg, Department of Radiation Oncology and Radiotherapy, Heidelberg, Germany ; 9 German Cancer Research Center, Department of Medical Image Computing, Heidelberg, Germany Purpose or Objective 12 C-ion radiotherapy (RT) holds great potential for the treatment of radioresistant tumors, however its differential effects on tumor vasculature compared to photons are still not fully understood. We used dynamic contrast-enhanced magnetic resonance imaging (DCE-MRI) and pharmacokinetic modelling to quantify vascular changes after 12 C-ion and photon irradiation at therapeutic and subtherapeutic doses of the anaplastic experimental prostate tumor R3327-AT1. Material and Methods The anaplastic prostate tumor subline Dunning R3327-AT1 was transplanted subcutaneously on both thighs of 12 male Copenhagen rats. When tumors reached diameters of 9 mm, the rats’ right tumor was irradiated with single doses of either photons (6 MeV) or 12 C-ions (20 mm spread-out Bragg-peak), while the tumor on the left leg served as an untreated control. Tumors were irradiated in four dose groups: with curative doses of 37 Gy 12 C-ions and 85 Gy photons leading to local tumor control and with sub- curative doses of 16 Gy 12 C-ions and 37 Gy photons not leading to local tumor control. DCE-MRI was performed one day prior to and 3, 7, 14, and 21 days after irradiation. DCE-MRI data was analyzed voxel-wise by pharmacokinetic modelling using the extended Tofts model (ETM). The image based arterial input function was extracted individually for each animal from the left ventricle. After the last imaging time point, tumors were dissected and histologically analyzed to assess quantitative information on the tumor morphology (H&E), vasculature (CD31), and proliferation (bromodeoxyuridine (BrdU)). Results Non-irradiated control tumors showed a decrease in all ETM parameters ( K trans , v e , v p ) over the observation period due to the progressive decline in perfusion in this rapidly growing tumor. 12 C-ion treated tumors showed earlier vascular changes in perfusion/permeability indicated by an increase in K trans and v e at day 3 while the response of photon treated tumors was prolonged until day 21. The vascular fraction v p exhibited reasonable values but no significant temporal changes. Histological results revealed that photon RT led to more heterogeneous tissue structures than 12 C-ion irradiation which was also reflected in the broader distribution of v e values for photon treated tumors after 21 days. Quantitative analysis showed lower vascular density and lower rates of proliferative active cells in 12 C-ion treated tumors than in photon treated tumors 21 days after RT. No dose dependency was found neither in the DCE-MRI data nor in the quantitative histology results. Conclusion 12 C-ions evoked an early and strong vascular treatment response compared to photons. As no dose-dependency was found in the results, the initial vascular response can only have a minor effect on local tumor control.

Analysis of EBT3 and XD film for 2D delivery accuracy is ongoing. Conclusion The ACDS has developed a comprehensive SRS dosimetry audit for a range of clinical scenarios. Preliminary results for treatment of multiple metastases show single isocentre plans may have reduced accuracy in the treatment of off- axis target volumes.

Poster Highlights: Poster highlights 15: Particles and microbeams

PH-0476 Impact of single dose photon or 12C-ion irradiation on rat prostate tumors assessed by DCE-MRI A. BENDINGER 1,2 , L. Seyler 3 , M. Saager 1,4 , C. Debus 5,6 , P. Peschke 1,4 , D. Komljenovic 2 , J. Debus 4,7,8 , J. Peter 2 , R. Floca 4,9 , C. Karger 1,4 , C. Glowa 1,4,8 1 German Cancer Research Center, Medical Physics in Radiation Oncology, Heidelberg, Germany ; 2 German Cancer Research Center, Medical Physics in Radiology, Heidelberg, Germany ; 3 University Hospital Erlangen- Friedrich-Alexander-Universität Erlangen-Nürnberg, Radiology, Erlangen, Germany ; 4 Heidelberg Institute for Radiation Oncology HIRO and National Center for Radiation Research in Oncology NCRO, HIRO and NCRO, Heidelberg, Germany ; 5 German Aerospace Center DLR, Department of High-Performance Computing- Simulation and Software Technology, Köln, Germany ; 6 National Center for Tumor Diseases NCT- German Cancer Research