ESTRO 2021 Abstract Book

S1462

ESTRO 2021

Medicine, Department of Radiation Oncology, sayama, Japan

Purpose or Objective The Synchrony respiratory tracking system (SRTS) is a real-time respiratory tracking system and has recently installed in Radixact TM which is an updated version of helical tomotherapy. The SRTS enables real-time tracking of moving targets by modeling the correlation between the targets and external surrogate LED- markers placed on the patient’s chest. However, optimal threshold of model parameters such as Potential Diff and MeasuredΔ which can be obtained in the treatment course was poorly investigated. The purpose of this study was to investigate the optimal threshold of model parameters on the motion-tracking accuracy of the SRTS. Materials and Methods The plan that had a virtual target at the center of the SRS MapCHECK TM (Sun Nuclear Corporation, USA) was generated using the Precison TM treatment planning system. Acrylic cube phantom for motion tracking was attached to the SRS MapCHECK TM . The phantom was moved with the 4th power of a sinusoidal wave with breathing cycles 4s, and an amplitude of 1 cm. To simulate irregular breathing, the respiratory cycle was varied with Gaussian random numbers scaled to adjust their standard deviation to 10%. Motions with respiratory phase shifts between the target and surrogate motion were also tested. The delivered coronal dose distributions were compared with the planned ones, using gamma analysis method. The local gamma passing rates were evaluated using acceptance criteria: 2% /2 mm, 10% threshold. Results above 95% were considered passing. We also calculated the Potential Diff and MeasuredΔ that could be tracked with a probability in excess of 95% ( PD95 and MD95, respectively) and investigated the relationship between gamma passing rates and model parameters. Results Figure 1a shows the relationship between phase shift and gamma passing rate as measured with the SRS- MapCHECK TM . The decrease of gamma passing rate was large when the LED marker delayed to the motions compared to when LED marker motion preceded target motion. Furthermore, gamma passing rate of unstable breathing model was significantly lower compared to stable model (Table 1). Effect of phase shift was more pronounced in the unstable breathing model. Figure 1b shows the relationship between the model parameters value and the gamma passing rate. The PD95 showed significant linear correlation with the gamma passing rate , but MD95 did not. The correlation coefficients (R 2 ) were 0.687 and 0.056 for PD95 and MD95 , respectively ( P <0.001 for all data). By setting the Potential diff threshold to 3 mm or less, a gamma passing rate of 95% or more was achieved. Conclusion Phase shift effected on tracking accuracy. Coaching stable breathing will be useful for improvement of motion tracking accuracy. PD95 was the best indicator of tracking accuracy. Potential Diff threshold of 3 mm was reasonable to maintain tracking accuracy and can minimize treatment pauses.

PO-1740 Statistical process control for routine QA of medical linear accelerators: a 14 years experience P. Viola 1 , M. Craus 1 , C. Romano 1 , G. Macchia 2 , F. Deodato 3 , M. Buwenge 4 , S. Cammelli 4 , S. Bisello 4 , V. Valentini 5 , A.G. Morganti 4 , S. Cilla 1 1 Gemelli Molise Hospital, Medical Physics Unit, Campobasso, Italy; 2 Gemelli Molise Hospital, Radiation Oncology Unit, Campobasso, Italy; 3 Gemelli molise Hospital, Radiation Oncology Unit, Campobasso, Italy; 4 IRCCS Azienda Ospedaliero-Universitaria di Bologna, Radiation Oncology Department, Bologna, Italy; 5 Fondazione Policlinico Universitario A. Gemelli, IRCCS, Radiation oncology Department, Roma, Italy Purpose or Objective To perform a large retrospective evaluation of daily Quality Assurance (QA) for two different linacs using Statistical Process Control (SPC) strategy.

Materials and Methods