ESTRO 2020 Abstract Book

S873 ESTRO 2020

fraction number. Without tracking, the rate of OE≥2mm would not be negligible for painful pts PO-1604 Quantitative analysis of treatment process time for MRI-LINAC system gating efficiency optimization L. Placidi 1 , V. Pollutri 2 , C. Votta 2 , D. Cusumano 1 , L. Boldrini 3 , G. Chiloiro 3 , L. Azario 1 , M. De Spirito 1 , V. Valentini 3 1 Fondazione Policlinico Universitario A. Gemelli IRCCS - Università Cattolica Sacro Cuore, Medical Physics, Rome, Italy ; 2 Fondazione Policlinico Universitario A. Gemelli IRCCS, Radiotherapy, Rome, Italy ; 3 Fondazione Policlinico Universitario A. Gemelli IRCCS - Università Cattolica Sacro Cuore, Radiotherapy, Rome, Italy Purpose or Objective Real-time gating Magnetic Resonance-guided radiotherapy (RGMRgRT) system allows moving tumours on-line tracking. When treatment is linked to gating system, beam delivery time can be longer than without gating. Quantitative analysis of the treatment process time (TPT) using data obtained from patient treatment logs, has been used to evaluate to following aims: 1. assess if the duration of the slots provided for gating breath hold (BH) inspiration patients (30 min) are sufficient or not, to then optimize the daily treatment room schedule 2. compare how long the TPT has been varied between Viewray MERIDIAN Cobalt-60 three-sources (VMC) and the Viewray MERIDIAN LINAC (VML) RGMRgRT system 3. compute the gating efficiency (GE) for both free breathing (FB) and BH patients, to evaluate how close the estimated TPT provided by the TPS (mechanical and beam- on) is to the actual delivered (mechanical, beam-on and gating) 4. assess possible correlation between the GE and some clinical/physical/mechanical parameters Material and Methods Based on a selected cohort of 210 patients, a preliminary investigation has been carried out on 60 patients, corresponding to 318 fractions delivered in the RGMRgRT system. TPT was computed using data from planning and mechanical logs for each patient and each treatment fraction. Comparison between TPT of the in clinical operation VML (27 patients) and the previously VMC (33 patients) system was also evaluated. Treatment planning data are summarized in Fig. 1. Gating treatment boundary is the margin from CTV to gating the treatment and percent ROI is the allowed percentage of the tracked ROI outside the defined boundary to deliver beam-on. Only 5 plans (8.3%) were prescribed as long course ( >10 fractions with a dose per fraction <3 Gy); the remaining 55 plans (91.7%) were prescribed as SBRT ( <5 fractions, with a dose per fraction in the range of 5-12.5Gy). GE (defined as the percentage difference of the total delivered time and the estimated total time, computed by the TPS) was computed for VML BH and FB dataset.

mean GE value for VML with FB (6.1 %, range 55% / -5.5%) and BH (39.1%, range 123.4% / -1.3%) dataset. No evidence correlation of GE to clinical and/or planning parameters were observed.

Conclusion Even though based on a small sample, quantitative analysis of TPT of the RGMRgRT system answers to the aims of the study. BH inspiration gating technique could be optimised in order to reduce the TPT. Once the analysis on the completed dataset of 210 patients will be completed, the results will more clearly indicate any correlations between the GE and some clinical/physical/mechanical parameters and if further gating techniques should be considered to improve and optimize the use of RGMRgRT. PO-1605 Can we ignore rotational errors in the positioning of patients with high-risk prostate cancer? M. Poncyljusz 1 , M. Piziorska 1 , B. Czyzew 1 , N. Piotrkowicz 1 1 The Central Clinical Hospital of the Ministry of The Interior and Administration in Warsaw, The Radiotherapy Center, Warsaw, Poland Purpose or Objective Image guided radiotherapy provides an excellent solution to quantitative assessment and correct for patient set-up errors in the modern radiotherapy. However, the majority of linear accelerators are equipped with conventional couches that can be moved in three translational directions and perform only yaw rotation. Uncorrected roll and pitch result in rotational set-up errors, particularly when the distance from the isocenter to CTV border is large. This study reports on the residual rotational set-up error after using daily cone beam computed tomography to position high-risk prostate cancer patients for radiotherapy treatment. The aim of this study was to analyse the shifts resulting from uncorrected roll and pitch rotations. Material and Methods 22 patients with prostate cancer treated with VMAT technique had daily CBCT scans (570 CBCTs in total) prior to treatment delivery. The rotational errors remaining after on-line correction were retrospectively analysed. The shifts caused by uncorrected rotations were calculated with the use of the rotation matrix. The point A was a hypothetical point. The (x y z) coordinates of the point A were equal to the maximum distance from the isocenter to the CTV border in the X, Y and Z axis directions, respectively (X axis was defined as superior- inferior, Y as left-right and Z as anterior-posterior direction). The segment connecting the isocenter to the point A was the radius of the sphere that covered CTV. This implies that the shift of each point of CTV was smaller than the shift of the point A. Results 139 and 36 of the 570 rotational errors were larger than 1,5 degrees in pitch and roll direction respectively. The angle of rotation was from -4.2 to 3.9 degrees and from - 3.8 to 1.6 degrees in pitch and roll directions respectively. 17 of the 22 patients had no treatment fractions with the shifts of the point A larger than 5 mm in X axis direction

Results TPT (mean±SD) - made up of gantry and MLC mechanical movement, beam-on and gating time - for VML and VMC are: VML_BH=10.8±3.0 min, VLM_FB=11.1±2.9 min, VMC_BH=10.8±8.4 min, VMC_FB=7.7±5.3 min. GE results are depicted in Fig. 2: the red dotted lines represents the