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Volumetric Indices to Aid Definition of Respiratory Gates with Particular Reference to Lung Stereotactic Body Radiotherapy (SBRT)

H Malhotra

H Malhotra*, J Gomez , Roswell Park Cancer Institute, Buffalo, NY


SU-E-J-184 (Sunday, July 12, 2015) 3:00 PM - 6:00 PM Room: Exhibit Hall

Purpose: Conventional definition of respiratory gates relies on tumor motion determination in limited planes. We are proposing a new method to define the RPM gates in a consistent manner ensuring that the tumor motion is restricted to 4 mm in 3D for lung SBRT patients. The method relies on studying the ratio of volumes obtained by GTVnn intersection with GTV50+2mm margin and GTVnn where GTVnn is the GTV volume in phase nn (=0,10,30..) while GTV50+2mm is a pseudo structure created by adding an isotropic margin of 2mm to GTV50. If for any phase nn, above ratio equals 1, it ensures that the tumor motion is ≤ ±2 mm in 3D from GTV50.

Methods: This method was tested for 50 patients (14-Central, 36-peripheral) to determine the RPM gates which were then compared with the gates used clinically. The minimum cut-off value of the above coefficient for its inclusion of a phase in RPM gate was taken as 0.97 for central and 0.95 for peripheral tumors.

Results: 15 (30%) of the patients did not require any change in the RPM gates w.r.t. gates defined using conventional motion assessment methods. In 15(30%) cases, the RPM gates could have been smaller while in remaining 20 patients, gates could have been larger. 5(/14) patient’s central tumors and 10 (/36) peripheral tumors did not need any gate change. 8(/50) patients could have RPM gate change of 30% while 10(/50) could have a gate change of up to 20%. 10, 20 & 30% RPM gate change could have happened for 11, 10 & 9 patients, respectively.

Conclusion: Proposed volumetric indices based method allows a consistent, scientific and objective method to decide optimal RPM gates which is free from any inter or intra person variability and satisfies the tumor motion limits as defined by AAPM TG-76 in totality.

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