Radiation Physics

PV QA 3 - Poster Viewing Q&A 3

TU_3_3138 - Investigation of dosimetric impact arising from the Cyberknife Xsight Lung tracking system using different Synchrony models

Tuesday, October 23
1:00 PM - 2:30 PM
Location: Innovation Hub, Exhibit Hall 3

Investigation of dosimetric impact arising from the Cyberknife Xsight Lung tracking system using different Synchrony models
C. W. Kong IV1, H. Geng1, W. W. Lam2, B. Yang1, W. K. R. Wong1, Y. Ding1, W. K. Law2, T. Y. Lee3, K. Y. Cheung2, and S. K. Yu2; 1Hong Kong Sanatorium & Hospital, Happy Valley, Hong Kong, 2Medical Physics & Research Department, Hong Kong Sanatorium & Hospital, Happy Valley, Hong Kong, 3Hong Kong Sanatorium & Hospital, Hong Kong, Hong Kong

Purpose/Objective(s): The Cyberknife Synchrony System when operated in XSightLung Treatment mode can track the patient’s respiratory motion in real time and the linac is repositioned dynamically based on the Synchrony model. Treatment accuracy for the CyberKnife’s XSightLung Treatment can be evaluated using a EBT3 film-based verification method in XSightLung Treatment (XLT) tracking phantom. However this Synchrony evaluation method is based on an ideal Synchrony model that the motions of the moving platform, on where the infrared markers are placed, are in constant relation to that of the moving target. Clinically, the motion of the markers placed on the patient’s body are not in that ideal relation to the target motion, depending on the patient's condition and breathing method during treatment. In some extreme case the regression of the Synchrony model is as poor as below 0.7. The purpose of this study is to investigate the effect on the dose accuracy of XSightLung Treatment when there is an uncertainty in the synchrony model.

Materials/Methods: XLT tracking phantom was used in the study. To make the synchrony model deviate from the original pattern of the phantom, the infrared markers, instead of being placed directly on the moving platform, were placed on a device first and the whole device was then attached to the platform. The function of the device was to introduce perturbation to the synchrony model as it can move the markers back and forth with different amplitudes, resulting in an irregular motion that simulates the unpredictability during actual breathing. Synchrony models with different regression 0.9605, 0.8562 and 0.7546 were generated by varying the amplitude that the device moved the markers. A 2-view Lung Optimized Treatment plan with Iris collimator and Monte-Carlo dose calculation, was used to irradiate the XLT phantom using different Synchrony models. Effect on the treatment delivery was finally investigated by analyzing the exposure on EBT3 films inserted in the target.

Results: For the original synchrony model on the XLT tracking phantom without any perturbation imposed on the motion of the markers, the regression of the Synchrony model was 0.9753 and the average dose difference in the high dose region between the treatment delivery and the planning calculation was -1.5±0.5%. When different perturbations were added to the synchrony model such that its regression decreased to 0.9605, 0.8562 and 0.7546, the average dose difference in the high dose region were decreased to -2.1±0.7,% -6.7±2.2% and -11.3±5.5% respectively.

Conclusion: Our study showed that the amount of under-dose to the treatment target increased with the degree of uncertainty in the Synchrony model.

Author Disclosure: C. Kong: None. W. Lam: None. K. Cheung: None. S. Yu: None.

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TU_3_3138 - Investigation of dosimetric impact arising from the Cyberknife Xsight Lung tracking system using different Synchrony models



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