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A Novel Approach to Verify Air Gap and SSD for Proton Radiotherapy Using Surface Imaging

X Wang*, C Ma , K Nie , R Davis , N Yue , Y Zhang , Department of Radiation Oncology, Rutgers-Cancer Institute of New Jersey, Rutgers-Robert Wood Johnson Medical School, New Brunswick, NJ


(Monday, 7/30/2018) 3:45 PM - 4:15 PM

Room: Exhibit Hall | Forum 2

Purpose: To develop a novel approach to accurately verify patient setup in proton radiotherapy, including verification of air gap and SSD, whose consistency and accuracy is extremely important.

Methods: Patient surface can be monitored by AlignRT system at treatment for improved accuracy. An in-house program was developed to reconstruct patient surface at treatment position from AlignRT reference capture and calculate the corresponding air gap and SSD. To validate this method, a mannequin was scanned on a CT simulator and proton plans were generated for a Mevion S250 Proton machine with 20 gantry/couch angle combinations, as well as two different snout sizes, in Varian Eclipse Treatment Planning Systems (TPS). The surface generated from CT was imported into AlignRT system for set up. AlignRT reference capture was taken at set up position after orthogonal kV imaging to confirm positioning. Air gap and SSD calculated from surface imported from CT (surface(CT)) and surface captured at set up (surface(VRT)) were compared to those calculated in TPS.

Results: The air gap and SSD calculated from our program agreed well with TPS values. Using surface(CT), the differences in air gap were 0.01mm ± 0.56mm for small snout, and 0.29mm ± 0.65mm for large snout, and differences in SSD were 0.64mm ± 0.48mm regardless of snout size. Using surface(VRT), differences in air gap were 1.10mm ± 1.24mm and 1.67mm ± 3.35mm for small and large snout, respectively, and differences in SSD were 0.45mm ± 0.81mm. The large discrepancy happened where there was sharp surface gradient at the snout boundary.

Conclusion: The results showed the feasibility of using surface imaging approach to conveniently determine the air gap and SSD in proton treatment, providing an accurate and efficient way to confirm the target depth at treatment. Further study of including patient data to validate the robustness is underway.


Protons, Optical Imaging, Setup Verification


IM- Other imaging modalities: Infrared imaging

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