ESTRO 2021 Abstract Book

S1542

ESTRO 2021

Conclusion Fusion dosiomics model combining the primary tumor, lymph nodes, and 3mm peritumoral information showed better predictive performance than single modality models for different outcome predictions among all ROIs, which is helpful to develop the personalized treatment plans for patients with nasopharyngeal carcinoma. PO-1812 The price of FLASH beyond doubt in stereotactic transmission-beam PT of early-stage lung cancer S. Habraken 1,2 , S. Breedveld 1 , J. Groen 1 , J. Nuyttens 1,3 , M. van Vulpen 3 , M. Hoogeman 1,2 1 Erasmus MC Cancer Institute, University Medical Center Rotterdam, Department of Radiotherapy, Rotterdam, The Netherlands; 2 HollandPTC, Department of Medical Physics and Informatics, Delft, The Netherlands; 3 HollandPTC, Department of Radiation Oncology, Delft, The Netherlands Purpose or Objective Radiobiological experiments indicate that, besides a dose-rate threshold of 40 – 100 Gy/s, the FLASH effect comes with a (fraction) dose threshold of 8 - 10 Gy. Clinically, it may, therefore, be attained only in hypofractionated treatments with single-beam-per-fraction delivery. This comes at the expense of healthy-tissue sparing by fractionation. We investigate the trade-off between the FLASH effect and fractionation in single-beam vs. multi-beam per fraction delivery in stereotactic proton therapy of small lung lesions. Materials and Methods Proton treatment plans for 12 patients were generated in our in-house developed software for treatment planning. GTV delineations were available and a 5 mm GTV-PTV margin was applied. Only PTVs not exceeding 10cc (range 4.4 - 10.1cc) were included. Equiangular co-planar arrangements of 244 MeV proton transmission (shoot-through) beams, in which the overall highest dose rate may be achieved, were used. These avoided serial organs, e.g., spinal cord and oesophagus. To facilitate single beam per fraction delivery, the number of fractions was equal to the beam number. A single field uniform dose approach was used with: D 95%,PTV = 100% D pres per beam. Isoeffective fractionation regimens based on 54 Gy with 3 beams in 3 fractions and a/b = 10 Gy for the GTV, were used, i.e., 65.5 Gy/5, 73.7 Gy/7 and 80.0 Gy/9. The FLASH effect was modeled with a constant FLASH enhancement ratio (FER > 1), applied to the absorbed healthy-tissue voxel fraction dose, i.e., d j → d j /FER, for all voxels j and independent of dose. All plans were evaluated in terms of the equivalent dose in 2 Gy fractions (EQD2) to ipsilateral lung, excluding the GTV, assuming a/b = 3 Gy. Voxel EQD2s were calculated for both FLASH-enhanced single-beam per fraction treatments and conventional multi-beam per fraction treatments. Results Population median mean doses to ipsilateral lung minus GTV of 6.3 Gy, 5.6 Gy, 5.5 Gy, 5.4 Gy were respectively achieved in plans with 3, 5, 7 and 9 beams. The dependence on FER of the mean EQD2 to lung minus GTV is shown in figure 1. FLASH- enhanced single-beam per fraction delivery comes at the expense of healthy tissue sparing of fractionation. To outweigh this, a median FER of respectively 1.28 (population range 1.23-1.34), 1.30 (1.23-1.38), 1.29 (1.21-1.37), 1.28 (1.22-1.36) for plans with 3, 5, 7 and 9 beams/fractions is required.

Fig 1: Dependence on FER of the population minimum, median and maximum mean EQD2 to ipsilateral lung minus GTV in FLASH-enhanced single-beam per fraction treatments, normalized for each patient to conventional delivery of all beams in each fraction. Conclusion Whether or not the FLASH effect outweighs fractionation critically depends on the FER. Therefore, it is of utmost importance to determine the actual FER and the exact dose threshold at which the FLASH effect occurs.

PO-1813 On the dose response of SABR treatments of oligometastatic tumours A. Carver 1 , S. Green 1 , G. Webster 2 , Q. Ghafoor 3

1 University Hospitals Birmingham NHS Foundation Trust, Medical Physics, Birmingham, United Kingdom; 2 Worcestershire Acute Hospitals NHS Trust, Worcestershire Oncology Centre, Worcester, United Kingdom; 3 University Hospitals Birmingham NHS Foundation Trust, Oncology, Birmingham, United Kingdom Purpose or Objective The SABR (Stereotactic Ablative Radiotherapy) CtE (Commissioning through Evaluation) study measured the outcomes of patients treated with up to three oligo-metastatic lesions. Outcomes and DICOM planning data were submitted to PROPEL