ESTRO 2021 Abstract Book

S1598

ESTRO 2021

radiochromic film strips are placed in the overlap region and in regions where repositioning is less reproducible. Results Coverage is greater than 98% of the external contour at 95% Dp. The lungs have a minimum dose of 7.1 Gy, mean of 7.5 Gy and maximum of 9.1 Gy. The average treatment time is 45min of irradiation and 14min of imaging. In the verification of the plans 98% of the points pass the gamma criterion. In the measurements in the gradient zone the dose variations are less than 2.5%.

Conclusion The treatment of TBI with tomotherapy allows the treatment to be performed with the patient in a comfortable position and with accurate positioning verification using the equipment's own imaging system.

PO-1877 Conformal energy filter optimization for Bragg peak FLASH proton therapy S. Deffet 1 , K. Souris 1 , R. Labarbe 2 , L. Hotoiu 2 , A. Pin 2 , E. Sterpin 3

1 Université catholique de Louvain, Molecular Imaging, Radiotherapy and Oncology (MIRO), Louvain-la-Neuve, Belgium; 2 Ion Beam Applications SA, Research, Louvain-la-Neuve, Belgium; 3 Katholieke Universiteit Leuven, Laboratory of Experimental Radiotherapy, Leuven, Belgium Purpose or Objective To meet dose rate objectives in FLASH proton therapy it is highly desirable to get rid of delays caused by switching between energy layers. To this end, we investigate the use of patient-specific ridge shifters of complex geometry hereafter named Conformal Energy Filter (CEF) to create Spread-Out-Bragg-Peak (SOBP) from mono-energetic beams. A CEF is typically composed of many peaks to degrade the initial energy into those required for dose conformity. With such a geometry, scattering has a major impact on the dose distribution and must be taken into account to optimize the CEF. Materials and Methods The inverse problem of computing the geometric characteristics of the CEF on the basis of an IMPT plan is particularly ill- posed. To solve it, we investigate the use of several priors: an assumption on the cross-sectional shape of the peaks (eg. hexagonal or square) or a sparse representation of the CEF in an adequate basis. With this regularization scheme, the difference between the dose map obtained with the CEF and the original dose map corresponding to the IMPT plan is iteratively minimized considering an initial fluence map estimated from the original plan. To perform the optimization in acceptable computational time, we approximate the dose using an analytical model which relies on the use of successive convolutions for scattering and straggling. The CEF obtained after optimization is finally inserted into the patient's CT scan and the final dose is estimated by Monte-Carlo simulations from a single energy plan whose beamlet weights were determined on the basis of the IMPT plan. Results Preliminary results on a prostate case show that optimizing the shape of the CEF by the method presented above while using large PBS spot (due to the range shifter) is a delicate exercise in finding the optimum spot spacing and spike step size in order to prevent an unacceptable degradation of the DVH of the target compared to a full IMPT plan (at conventional dose rate). These results indicate that one will have to balance the benefit of the DVH of a full IMPT plan (at conventional dose rate) versus the benefit of high dose rate of FLASH with a CEF. Conclusion The geometrical characteristics of a patient specific ridge filter were determined from the weight of each beamlet in the field, by optimizing a standard IMPT plan using a dedicated analytical algorithm inside MIROpt (http://openmiropt.org/), alongside the MCsquare dose engine (http://www.openmcsquare.org/). Accurate computation of these ridge filters makes it possible to use the full potential of the proton's Bragg peak for high dose rate FLASH irradiations. PO-1878 High quality planning technique for robust optimised VMAT in RayStation for IMC breast radiotherapy. A. Bird 1 , G. Webster 1 1 Worcestershire Oncology Centre, Worcestershire Acute Hospitals NHS Trust, Radiotherapy Physics, WORCESTER, United Kingdom Purpose or Objective In the UK IMC irradiation is becoming standard of care for patients considered at intermediate risk of recurrence. The development of VMAT techniques that deliver high plan quality and robustness are challenging. It is important to establish achievable plan quality to inform best practice before restricting access or treatment options. Materials and Methods