ESTRO 36 Abstract Book

S879 ESTRO 36 _______________________________________________________________________________________________

during SBRT. SeedTracker, in conjunction with the Elekta XVI system, reads the monoscopic images acquired during treatment and calculates the position of the prostate by auto segmenting the radiopaque markers implanted in it. The accurate performance of the SeedTracker was validated using static and dynamic studies utilising the phantoms implanted with gold seeds. The system also has variable angle stereo image reconstruction functionality for the rapid determination of 3D offsets and position correction of patients in the situations where intrafraction motion is observed during treatment.SeedTracker was utilized for real time monitoring of prostate position for patients undergoing stereotactic boost treatment within the PROMETHEUS trial (UTN: U1111-1167-2997) in Sydney South West Local Health District, Australia. The necessary ethical and legal approvals were obtained from the local health district Human Research Ethics Committee Research Governance Office and Therapeutic Goods Administration, Department of Health, Australian Government before its clinical implementation.The dosimetric accuracy achieved by the utilization of SeedTracker was studied by incorporating the observed position offsets in the planned dose. Results The performance evaluation study of SeedTracker showed that the system demonstrated a minimum True Positive Rate of 88% in studied static and dynamic scenarios with mean (σ) difference of 0.2(0.5) mm in calculated position accuracy. At the time of writing this abstract SeedTracker had been utilized for the real time position monitoring of twenty six patients’ SBRT treatment (consisting of twenty two treatment sessions). Eleven occurrences of position deviations outside the acceptable tolerance limits (3mm) were observed that led to treatment interruption and position correction of the patient. The retrospective dose reconstruction study showed that the V98 to prostate would have decreased by a maximum 20% compared to the planned V98 if real time position monitoring had not been performed and position corrections were not undertaken. The stereo image based position correction available in SeedTracker was shown to be minimum 2 mins faster than the conventional orthogonal image based approach. Conclusion The SeedTracker system has been shown to enable the accurate real time position monitoring and position corrections during prostate SBRT. The occurance of real position deviations during dose delivery was identified by the SeedTracker leading to improved accuracy of dose delivery to the prostate. EP-1624 Respiratory gating of an Elekta linac using a Microsoft Kinect v2 system D. Edmunds 1 , K. Tang 2 , R. Symonds-Tayler 3 , E. Donovan 1 1 The Royal Marsden NHS Foundation Trust, Physics, Sutton, United Kingdom 2 University of Surrey, Physics, Guildford, United Kingdom 3 Institute of Cancer Research, Physics, Sutton, United Kingdom Purpose or Objective To investigate whether it is possible to gate radiation delivery from an Elekta linac, using a commercial off-the- shelf (COTS) depth sensor, based on data acquired from patients in a clinical study. The goal of this work is to achieve real time breath-hold monitoring and gating for voluntary breath-hold (VBH) treatments for breast cancer patients. Material and Methods Six participants from the UK HeartSpare trial who had received left breast radiotherapy while performing VBH were recruited for this study. The patients were set up on an Elekta Synergy in a radiotherapy treatment room

exactly as in their original treatment. They then performed a sequence of 3 breath holds for a period of approximately 20 seconds each, during a simulated whole breast treatment with both lateral and medial beams, plus a VMAT delivery. A Microsoft Kinect Version 2 (Kinect v2) commodity depth sensor was used to record breathing traces during this time. These breathing traces were then used as input to a programmable motion platform carrying a solid water phantom placed on the treatment couch, which was monitored with a Kinect v2. In-house C++ software (see Fig. 1) was used to set a gating threshold, and when the phantom moved outside of this threshold, radiation delivery was paused via signals sent through a fibre optic connection to the linac’s gating interface. Radiation dose was verified using a calibrated ionisation chamber and electrometer, with the chamber positioned inside the phantom. A dose measurement was performed for a 200 MU radiation delivery, both with and without gating in place.

Figure 1: Screenshot of in-house C++ Kinect v2 software, showing a depth image from the camera. The solid water phantom and linac head can be seen in the centre of the image. A region of interest (ROI) is drawn as a white rectangle, and the mean distance of pixels in the ROI is calculated at 60 Hz. Gating threshold can be set with software controls (top left), and a control signal is transmitted to the linac gating interface via fibre-optic Kinect v2 was able to acquire all breath holds from each patient successfully. Extracted traces from each patient depth file were sent to the motion platform. In the gating experiments (see Fig. 2), a Kinect v2 was able to track the phantom motion with a root mean square error of between 0.6 mm and 1.3 mm. The latency of our in-house gating software was found to range between 30 and 100 ms. In all cases, the gated radiation delivery dose agreed with the baseline dose measurement without gating to better than 0.4%. cables. Results