Radiotherapy Current Awareness Bulletin January 2017

To quantify the performance of the Clarity ultrasound (US) imaging system (Elekta AB, Stockholm, Sweden) for real-time dynamic multileaf collimator (MLC) tracking. The Clarity calibration and quality assurance phantom was mounted on a motion platform moving with a periodic sine wave trajectory. The detected position of a 30 mm hypoechogenic sphere within the phantom was continuously reported via Clarity's real-time streaming interface to an in-house tracking and delivery software and subsequently used to adapt the MLC aperture. A portal imager measured MV treatment field/MLC apertures and motion platform positions throughout each experiment to independently quantify system latency and geometric error. Based on the measured range of latency values, a prostate stereotactic body radiation therapy (SBRT) delivery was performed with three realistic motion trajectories. The dosimetric impact of system latency on MLC tracking was directly measured using a 3D dosimeter mounted on the motion platform. For 2D US imaging, the overall system latency, including all delay times from the imaging and delivery chain, ranged from 392 to 424 ms depending on the lateral sector size. For 3D US imaging, the latency ranged from 566 to 1031 ms depending on the elevational sweep. The latency-corrected geometric root-mean squared error was below 0.75 mm (2D US) and below 1.75 mm (3D US). For the prostate SBRT delivery, the impact of a range of system latencies (400-1000 ms) on the MLC tracking performance was minimal in terms of gamma failure rate. Real-time MLC tracking based on a noninvasive US input is technologically feasible. Current system latencies are higher than those for x-ray imaging systems, but US can provide full volumetric image data and the impact of system latency was measured to be small for a prostate SBRT case when using a US-like motion input. 4. The impact of CBCT-imaging and verification time on prostate motion using 4D TPUS Clarity system Author(s): Pang P.P.E.; Boo H.S.A.; Loh M.Q.J.; Chan W.S.J.; Aryani S.N.; Tuan K.L.J.; Knight K.; Baird M. Source: Radiotherapy and Oncology; Apr 2016; vol. 119 Publication Type(s): Journal: Conference Abstract Abstract:Purpose or Objective: Accuracy of radiotherapy to the prostate is often challenged by geometrical uncertainties due to inherent organ motion attributed to daily variations of the bladder and rectal volumes and contents. This study aims to simulate the use of 4D Clarity ultrasound image guidance without CBCT imaging to analyse the magnitude and trend of prostate motion during treatment (74Gy given in 37 fractions). The impact of CBCT imaging and verification time on prostate motion will be analysed. Material and Methods: 175 intrafraction monitoring sessions from 5 patients who underwent radical prostate volumetric modulated arc therapy (VMAT) monitored using 4D transperineal ultrasound scan (TPUS) resulted in a total of 1461.2min of data (184,085 positioning points) being analysed. All patients were instructed to comply with a full bladder protocol (i.e. 300-450ml in 30-45min) without specific rectal preparation protocol. Mean prostate motion was calculated and analysed in relation to time in the subsequent fractions. Overall treatment time was defined from acquisition of CBCT to treatment beam off time and imaging time was defined from time of CBCT acquisition to first beam on. Imaging time was subtracted from the overall treatment time for analysis of prostate motion without CBCT for verification. The remaining duration was representative of treatment time using 4D Clarity ultrasound imagePurpose or Objective: Accuracy of radiotherapy to the prostate is often challenged by geometrical uncertainties due to inherent organ motion attributed to daily variations of the bladder and rectal volumes and contents. This study aims to simulate the use of 4D Clarity ultrasound image guidance without CBCT imaging to analyse the magnitude and trend of prostate motion during treatment (74Gy given in 37 fractions). The impact of CBCT imaging and verification time on prostate motion will be analysed. Material and Methods: 175 intrafraction monitoring sessions from 5 patients who underwent radical prostate volumetric modulated arc therapy (VMAT) monitored using 4D transperineal ultrasound scan (TPUS) resulted in a total of 1461.2min of data (184,085 positioning points) being analysed. All patients were instructed to comply with a full bladder protocol (i.e. 300-450ml in 30-45min) without specific rectal preparation protocol. Mean prostate motion was calculated and analysed in relation to time in the subsequent fractions. Overall treatment time was defined from acquisition of CBCT to treatment beam off time and imaging time was defined from time of CBCT acquisition to first beam on. Imaging time was subtracted from the overall treatment time for analysis of prostate motion without CBCT for verification. The remaining duration was representative of treatment time using 4D Clarity ultrasound image