ESTRO 2020 Abstract book

S776 ESTRO 2020

Consolidated suggest pretreatment verification of IMRT plans be employed as part of a patient-specific quality assurance (QA) program. The TomoTherapy LINAC is able to deliver complex treatments using a compressed-air driven 64-leaf binary MLC, which modulates the radiation while the source is rotating around the patient. An arc-shaped array of 640 CT xenon detectors is located opposite the linear accelerator on a rotating slip-ring gantry. After treatment delivery, exit detector data can be extracted and processed for patient-specific QA. We presented a feasibility study for a TomoTherapy phantomless pretreatment verification using exit detector data. Material and Methods The DICOM-RT files of TomoTherapy treatments on two different anatomical sites were exported using the Accuray TPS. A homemade software (Matlab, The Mathworks Inc.) was developed to read the DICOM-RT file and generate a TomoTherapy procedure reproducing the plan, except the position of couch, which was forced to not move. The couch was retracted to avoid any obstacle between source and detectors. The exit detector sinogram of the treatment projections was exported after delivery. A couple of ad-hoc procedures were created and executed in order to model the exit detectors array response to single and paired open leaves beams with couch retracted. The detectors array response s j,n due to a single open leaf j at projection n was extracted from the exit detector sinogram of the “single-leaf” procedure. The response rate S j was modelled by averaging s j,n over 51 projections (i.e. a complete gantry rotation), normalized to the leaf open time. In order to account for the leaf fluence output factor (LFOF) a second “paired-leaves” procedure was cretated to extract the detector response p j,n due to paired open leaf j, j+1 at projection n. Similarly, the response rate P j was defined by averaging s j,n over 51 projections, normalized to the leaves open time. Leaf open time and gantry rotation period were set to 0.314 and 20 s, respectively, for both procedures. Detector response rate contribution due to LFOF, was evaluated computing L j = P j – (S j + S j+1 ), see fig1. The expected exit sinogram of the plan was evaluated by superposition of Pj and Lj according to the programmed leaf open time after end of planning (EOF): indicating with T j,n the open time of leaf j at projection n, the expected sinogram was given by: recommendations [1-3]

where the entire clinical workflow is executed using different phantoms. Material and Methods A numerical expression for calculating the water-to- medium SPR from the energy depositions scored by GateRT-ion MC platform and the water and medium mass stopping powers of the particle was developed. The water- to-medium SPR for protons and carbon ion beams was calculated as the ratio of the computed dose to water and the dose to medium (in water). A new tool for the computation of the RE was implemented in GATE. It makes use of the RE tables of the alanine detector as a function of energy for the different particle types [1]. The comparison of the dose distributions corrected by the RE and the data obtained with ionization chambers and alanine pellets during the dosimetric end-to-end tests activities of the proton beam lines at MedAustron Ion Therapy Center was performed [2]. RE results from GateRT-ion simulations were also compared with those from the RayStation v7.99.20 Treatment Planning System (TPS). Results The water-to-alanine SPR was computed in a water volume for different spread-out Bragg peaks (SOBP). A value of 1.02 for protons with a decrease of 2% along the depth- dose curve was obtained. The RE distribution for a homogeneous dose distribution in water and in a homogeneous phantom was computed for alanine. A difference with RayStation TPS for protons and carbon ions of less than 1% was found up to the 80% distal range (Figure 1).

Figure 1. Relative Effectiveness of alanine calculated with GATE and RaySearch TPS for a proton SOBP. Conclusion The capabilities of GateRT-ion for the determination of the water-to-medium SPR have been demonstrated. The method was shown to be equivalent to the Bragg cavity theory. The validation of the RE calculation in clinically realistic fields for protons and carbon ion beams was performed in this work. A good agreement was obtained in comparison with the RE implementation available in RayStation TPS. References [1] HERRMANN, R., Diss. PhD thesis, Aarhus University, Denmark (2012). [2] CARLINO, A., et al., Physics in Medicine & Biology 63.5 (2018): 055001. PO‐1371 A feasibility study of using TomoTherapy exit detector data for pretreatment verification M. Parisotto 1 , V. Marco 1 , S. Maggi 1 1 Ospedali Riuniti, Medical Physics, Ancona, Italy

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