SU‐C‐103‐01: An Optical Tracking and Positioning System for Proton Therapy

Abstract

Purpose: An optical tracking and positioning system (OTPS) for proton therapy was developed to study the characteristics of robotic couch movement and the trend of inter‐fractional displacements on positioning patients treated for prostate or brain cancers. Methods: For patient localization, required movements of couch must be precisely executed. Commercial robotic couch with software‐driven six‐degree isocentric freedom was utilized in our clinic. To align couch rotating center within 1 mm to proton beam, movements of couch without or with patient loads were accurately measured within 0.5 mm without and by the OTPS; including a stereoscopically infrared camera and rigid body tools. To investigate large 10 mm inter‐fractional couch displacements with 5.0 mm standard deviation (S.D.) observed on one patient using head mask for immobilization, a mathematic formula, referred to as 3PtTrack method, was developed for studying the trend of inter‐fractional patient movements within head mask. Results: Typical 4.0 mm circular couch displacements were measured at horizontal plane while displacements were 2.0 mm at vertical plane; however, large 20 mm circular displacements were observed at Procure first clinic couch. After applying software‐driven ISO alignment, residual displacements at couch rotation were 0.3 mm. 85% of movements with patient loads on couch were within 0.5 mm to required values at horizontal plane. The 3PtTrack method could place daily QA device to required location within 0.5mm over a period of 3 months. Movements within mask were found to be similar (both less 5.0 mm with 2.0 mm S.D.) for two patients with largely different couch displacements. Conclusion: The couch rotation was well aligned to proton beam by using OTPS. Movements with patient loads were accurately executed. Large couch displacements presented an issue of fixation on head mask to couch, while smaller movement measured by 3PtTrack indicate that patients still moved with masks.