ESTRO 2020 Abstract Book

S869 ESTRO 2020

also be used straightforward to validate adaptive workflows also for higher field strength or other motion management strategies, such as tracking. 1 C Baldock et al 2010 Phys. Med. Biol. 55 R1 PO-1597 Identifying inconsistent breathing patterns for increased monitoring during breath-hold treatment T. Nano 1 , M. Feng 1 , T.D. Solberg 1 , A. Witztum 1 1 University of California- San Francisco, Department of Radiation Oncology, San Francisco, USA Purpose or Objective To analyze spirometry-based breathing waveforms in patients undergoing voluntary breath-hold (BH) radiotherapy for detection of potential errors that may be missed clinically. Material and Methods 198 total breath-hold waveforms (average of 7.9 breath- holds per treatment fraction) from 16 patients with pancreas or liver disease undergoing stereotactic body radiation therapy in 3-5 fractions were included in the analysis. During each treatment, patients undergo multiple breath-holds using the SDX system (Dyn’r, Medical Systems, Aix-en-Provence, France). For each breath hold a breathing waveform is recorded (Figure 1A) from which the BH time is determined (Figure 1B). The average breath-hold time and range, number of missed holds (defined as BHs shorter than 2 seconds) and number of failed holds (defined as BHs longer than 2 seconds but shorter than the maximum BH minus 5 seconds) during each treatment fraction were extracted as quantitative metrics to assess treatment quality (Figure 1C). It is assumed the beam is not turned on during missed holds (since therapists wait >2 seconds to initialize beam), whereas failed holds require therapist intervention to abort the beam. A summary report is generated for each treatment (Figure 2). Results Out of 25 treatments, 14 were treated with inspiration breath-hold and 11 with expiration. For all treatments, breath-hold time was greater than beam-on time. Missed holds were observed in 2 BHs from 2 different patients, and failed holds were identified in 6 BHs from 4 different patients. For patients with multiple treatment information available (n=4), the average BH ranges between treatments were 0, 0.5, 0.4, and 0.6 seconds. Across all treatments the average (maximum) range of successful BHs was 1.4 (4.6) seconds. Conclusion Manual response to patients failing breath-hold is critical for safe treatment of patients undergoing SDX voluntary breath-hold and is necessary in the absence of full software integration with treatment delivery system. Identifying and documenting patients who are at increased risk of, or have previously fallen out of breath-hold, can better prepare therapists for future treatments.

PO-1598 Investigating the use of RayPilot for motion management during prostate SBRT: initial experience M. Trainer 1 , B. Nailon 2 , L. Carruthers 1 , M. Kirby 3 1 Edinburgh Cancer Centre, Oncology Physics, Edinburgh, United Kingdom ; 2 University of Edinburgh, School of Engineering, Edinburgh, United Kingdom ; 3 Liverpool University, Department of Radiotherapy, Liverpool, United Kingdom Purpose or Objective Discrepancies between the position of a patient on a planning CT and their treatment position can impact the dosimetry of their radiotherapy treatment, especially for prostate SBRT 1 . Real-time positional verification is therefore an important consideration in the safe delivery of SBRT. Here we perform an initial assessment of the viability of the RayPilot system (Micropos Medical, 2019) for intra-fractional motion tracking for our SBRT technique and benchmark it against local imaging protocol using kV planar imaging and CBCT, focusing on the first three The RayPilot real-time tracking system consists of a table- top array containing an antennae, and a small transmitter inserted trans-perineally into the prostate. Here it was configured on a Varian Truebeam linear accelerator (Varian Medical Systems, Palo Alto, CA, USA). For our first 3 prostate cancer patients, SBRT treatment plans were prepared on the Eclipse Treatment Planning System (v.13.6) following the PACE trial protocol (36·25 Gy in 5 fractions (fx) over 1–2 weeks). The main imaging method used for positional verification was kV orthogonal pairs matched to fiducial markers with additional pre & post treatment CBCTs. In parallel, the RayPilot system was used to monitor changes in the transmitter position during treatment with the beam halted if the positional discrepancy was more than 0.2 cm. The 3-D difference in the position of defined measurement points following image registration in Eclipse was recorded and analysed. Results The mean displacement of the transmitter was below the local imaging threshold tolerance (0.2cm) for the first 2 patients (Table 1). The mean CT (1.2cm) & CBCT (-0.24cm) displacements for Patient 3 were outside the imaging tolerance in the z-direction. In Table 2 the CT Vs CBCT displacement was found to be more than 0.2 cm in x,y & z: 0%, 6.3%, 56.3% based on RayPilot; and 14.6%, 11.5%, 25% based on fiducials. The pre vs post CBCT displacements were 5.6%, 11.1%, 27.8% based on RayPilot and 1.9%, 7.4%, 11.1% based on seeds. patients in the study. Material and Methods