ESTRO 2023 - Abstract Book

S754

Monday 15 May 2023

ESTRO 2023

1 Medical Radiation Physics, Lund University, Lund, Sweden; 2 Department of Hematology, Oncology and Radiation Physics, Skåne University Hospital, Lund, Sweden; 3 Department of Veterinary Clinical Sciences, University of Copenhagen, Frederiksberg, Denmark; 4 .decimal, LCC, Sanford, Florida, USA; 5 Division of Oncology and Pathology, Department of Clinical Sciences, Lund University, Lund, Sweden; 6 MRC Oxford Institute for Radiation Oncology, Department of Oncology, University of Oxford, Oxford, United Kingdom Purpose or Objective The increased differential effect between normal tissue and tumor in FLASH radiotherapy (FLASH-RT) as compared to conventional radiotherapy was first observed for high-dose single-fraction electron treatments. Electron radiotherapy can be useful for superficial and intraoperative treatments but is associated with inhomogeneous dose distributions in heterogeneous tissue, resulting in severe hot- and/or cold-spots. This requires careful consideration in high-dose electron FLASH-RT before clinical implementation. In this study we evaluate a possible solution: Bolus-Electron-Conformal-Therapy (BECT) with intensity-modulation for FLASH-RT. Materials and Methods Our beam model in the treatment planning system electronRT (.decimal®, LLC, Sanford, Florida, USA) was based on a 10 MeV electron beam, from a modified clinical Elekta Precise linear accelerator used for treating veterinary patients with FLASH-RT, at a source-to-surface distance of 70 cm. In electronRT, CT-based treatment planning can be utilized to design and subsequently fabricate individualized 3D bolus in machinable wax. Furthermore, the beams can be intensity-modulated using tungsten pins in the electron block cut-out. We had previously evaluated and verified the beam model and the customized optimized-thickness bolus in different phantoms using radiochromic film measurements. To evaluate the impact of the optimized-thickness bolus and intensity-modulation in superficial clinical electron treatments, a treatment planning study was conducted including five canine cancer patient cases with simulated superficial tumors. A conformity index (CI = VCTV/V95%), a target dose homogeneity index (HI = (D2%-D98%)/D50%), and the maximum relative dose outside the target (D_(max,Body-CTV)) was determined for treatment plans without bolus, with bolus, and with bolus plus intensity- modulation. For all patients, the prescribed dose was 30 Gy to 50% of the CTV, with a treatment margin of 5 mm. Results Without bolus the median D_(max,Body-CTV), HI, and CI of the dose distributions were 115%, 0.27, and 0.7, respectively. By adding a customized bolus, the dose distributions generally showed an increased conformity and target dose homogeneity (Tabel 1), with a median D_(max,Body-CTV), HI, and CI of 107%, 0.18, and 0.8, respectively. With the combination of customized bolus and intensity-modulation the target dose homogeneity was further improved (median HI of 0.15). Figure 1 illustrates one case where BECT with intensity-modulation improved the dose distribution. Tabel 1:

Figure 1:

Conclusion By using BECT and intensity-modulation, the conformity and target dose homogeneity of the dose distributions in canine patients with complex heterogeneous tissue could be improved. By utilizing this technique, we aim to decrease the dose outside the target volume and avoid hot-spots in future clinical electron FLASH-RT studies, thereby reducing the risk of radiation-induced toxicity. PD-0908 Suitability of biology-guided radiotherapy for metastatic renal cell carcinoma directed by PSMA PET M. Gaudreault 1 , D. Chang 2 , N. Hardcastle 1 , L. McIntosh 1 , P. Jackson 1 , T. Kron 1 , C. Udovicich 2 , M. Hofman 3 , S. Siva 2 1 Peter MacCallum Cancer Centre, Physical Sciences, Melbourne, Australia; 2 Peter MacCallum Cancer Centre, Radiation Oncology, Melbourne, Australia; 3 Peter MacCallum Cancer Centre, Cancer Imaging, Melbourne, Australia Purpose or Objective Prostate-specific membrane antigen (PSMA) positron emission tomography (PET) is a novel imaging modality for metastatic renal cell carcinoma (RCC) that demonstrated improved sensitivity than conventional imaging. Biology-guided radiotherapy