ESTRO 2023 - Abstract Book

S1805

Digital Posters

ESTRO 2023

and delivery system complexity is currently being investigated in view of the recent developments in the field of electrons acceleration. In addition, several pre-clinical studies recently claimed that the toxicity in healthy tissues can be significantly reduced, while keeping the same efficacy in cancer killing, if the dose rate is radically increased ( ∼ 50 Gy/s) with respect to conventional treatments ( ∼ 0.01 Gy/s). Such effect is known as the FLASH effect. To investigate the potential of VHEE based RT, a VHEE Treatment Planning System (TPS) which include the evaluation of volumetric constraints is needed: once the beam model and the patient information is defined, the delivery parameters (such as position, energy and current of the electron beams) can be computed optimizing the absorbed dose maps. Materials and Methods With the aim of selecting the energy and the intensity of the electron beams, the TPS takes as input the dose maps generated by MC simulations performed with the FRED software for a given set of conditions: unit fluence, pre-defined energy and defined point of access inside the patient body. The goal is to minimize the cost function, which quantifies the degree to which a treatment plan meets its competing objectives, by modifying the dose distribution within the constraints of clinical deliverability. Minimization methods based on the Simulated Annealing and the Quantum Simulated Annealing algorithms are used, allowing to take into account more complex volumetric and dose-rate constraints. The minimization output are the optimized dose map of the patient and the corresponding values for each particle beam that the accelerator has to deliver. Results The tumor coverage and the dose absorbed by the organs at risk have been analyzed by means of Dose Volume Histograms for the case of a Head&Neck treatment. The comparison of the achieved results with the real proton therapy plan and the simulation of a conventional RT treatments has been performed. The results show that the plan optimized with the VHEE FLASH TPS is competitive with standard RT, and that it could allow a better sparing of the healthy tissues. Conclusion In this contribution the development of a TPS for VHEE treatments delivered at FLASH rates is presented: the obtained results show that the implemented algorithms allow to obtain electron plans compatible with standard TPS solutions and are suitable for the VHEE FLASH potential studies in which also the beam delivery strategy can be optimized. 1 Swiss Medical Network, Radiation Oncology, Geneva, Switzerland; 2 Swiss Medical Network, Radiation Onoclogy, Geneva, Switzerland Purpose or Objective To report our initial experience for right breast radiotherapy in prone position and to evaluate the dosimetric impact of residual setup errors for tomotherapy treatment delivery. Materials and Methods Three right breast patients were scanned in supine and prone position. Targets and OARs were delineated on both scans. Radiotherapy consisted of 50 Gy in 25 fractions to the whole breast, with a simultaneous boost to the tumor bed of 60 Gy. Treatment plans were prepared in RayStation 11A using a TomoHelical technique for supine and TomoDirect technique for prone position. We used the robustness function in RayStation to create an additional 1-cm leaf opening around the breast for both techniques. The radiation oncologist selected the best plan for treatment. Initial setup was performed using an optical surface monitoring system and skin marks on the right breast. Treatment was delivered on a Radixact equipped with kVCT for pre-treatment imaging verification. A second kVCT was acquired at the end of the first fraction to evaluate intrafraction motion. Dose was computed on the 25 daily pre-treatment kVCTs using the online registration from the treatment session, and deformed target and OAR volumes (checked and corrected by the radiation oncologist). The daily dose distributions were compared to the reference plan using dose volume parameters. In addition, each fraction dose was deformed back on the planning CT, and the accumulated summed dose was compared to the reference plan. Results Three prone TomoDirect plans were selected for treatment. For similar target coverage, dosimetric results were significantly better in prone position, with reduction in heart (0.5-prone vs. 2.2 Gy-supine), LADA (0.4 vs. 1.6 Gy), right lung (3.1 vs. 8.1 Gy) and left breast (0.5 vs. 1.5 Gy) mean doses for all patients. Median setup time (defined as the time between patient entering the treatment room and beam start) was 6.5 min (range 4.8-13.9 min). End of session kVCTs showed no clinically relevant shifts compared to the pre-treatment kVCT. Radiotherapy was delivered with 5 or 6 TomoDirect beams with total beam-on times ranging from 187 to 220 s. Dose calculations on pre-treatment images showed that the mean PTV V95% could be reduced below 95%, but that mean CTV V95% was always above 97% (Table 1), both for the boost and the whole breast PTV. The PTV V107% remained below 2%. Accumulated dose on the planning CT resulted in excellent dosimetric parameters for the CTVs and PTVs, and sparing of the right lung similar to the reference plan. PO-2032 Evaluation of treatment delivery quality for right breast tomotherapy in prone position M. Jaccard 1 , M. Fargier-Voiron 1 , N. Perichon 1 , C. De Marco 1 , O. Matzinger 1 , S. Bulling 1 , F. Caparrotti 2