Retrospective evaluation of CTV to PTV margins using CyberKnife in patients with thoracic tumors

Abstract

The objectives of this study were to estimate global uncertainty for patients with thoracic tumors treated in our center using the CyberKnife VSI after placement of fiducial markers and to compare our findings with the standard CTV to PTV margins used to date. Datasets for 16 patients (54 fractions) treated with the CyberKnife and the Synchrony Respiratory Tracking System were analyzed retrospectively based on CT planning, tracking information, and movement data generated and saved in the logs files by the system. For each patient, we analyzed all the main uncertainty sources and assigned a value. We also calculated an expanded global uncertainty to ensure a robust estimation of global uncertainty and to enable us to determine the position of 95% of the CTV points with a 95% confidence level during treatment. Based on our estimation of global uncertainty and compared with our general margin criterion (5 mm in all three directions: superior/inferior [SI], anterior/posterior [AP], and lateral [LAT]), 100% were adequately covered in the LAT direction, as were 94% and 94% in the SI and AP directions. We retrospectively analyzed the main sources of uncertainty in the CyberKnife process patient by patient. This individualized approach enabled us to estimate margins for patients with thoracic tumors treated in our unit and compare the results with our standard 5 mm margin.PACS number: 87.55‐x

Evaluation of target coverage and margins adequacy during CyberKnife Lung Optimized Treatment

“…Several retrospective studies calculated the uncertainty in radiation delivery (i.e., tumor tracking errors) by analysing dynamic tracking log files produced by the CyberKnife system. 12,[15][16][17][18][19][20]28,29 Our results were in agreement with these works: limited tracking errors (median < 1 mm) were observed in all directions and a significant correlation was found between the errors magnitude and motion amplitude.…”

Target margin design for real-time lung tumor tracking stereotactic body radiation therapy using CyberKnife Xsight Lung Tracking System

“…Because, in some cases, we used parameters that were only indirectly associated with the uncertainties of the studied steps or we were not able to obtain statistical information directly. In such cases, uncertainties were assigned based on the available literature 14 , 21 , 24 . When simplifications were made, we were always conservative in our analysis in order not to underestimate the final combined uncertainty of the process.…”

“…The tracking errors of FTTS and XLTS has similar multiple sources: the geometric or targeting error of the CyberKnife radiation delivery system; the correlation error between internal tumor locations versus external respiratory surrogate positions; the prediction error for system delay compensation; the tumor segmentation error and the tumor deformation error. The deformation error comes from the inner deformation of the tumor during treatment 14 . However, there are certain differences between the tumor segmentation errors of the two systems.…”

“…In this study we evaluated the relationship between the tumor positions and tracking errors, calculated the margin using a previously validated formulation 17 , 20 and the uncertainty estimation 14 , 15 , 22 . The margin was calculated from whole fractions data, in order to analyze the un-periodic respiratory motion during the treatments.…”

Robotic radiosurgery for the treatment of liver metastases

“…Typically, a margin of at least 3-5 mm was added to the GTV to generate the clinical target volume (CTV). No additional margin was added for the planning target volume (PTV) due to the accuracy of CyberKnife ® system combined with Synchrony ® Respiratory Tracking system, 15 so in all cases PTV equals CTV. Adjacent critical structures were delineated.…”