Markerless Real-Time 3D kV Tracking of Lung Tumors During Free Breathing Stereotactic Radiotherapy

Abstract Purpose/objective Accurate verification of tumor position during irradiation could reduce the probability of target miss. We investigated whether a commercial gantry mounted 2D kilo-voltage (kV) imaging system could be used for real-time 3D tumor tracking during volumetric modulated arc therapy (VMAT) stereotactic lung radiotherapy (SBRT). Markerless tumor tracking on kV fluoroscopic images was validated using a life-like moving thorax phantom and subsequently performed on kV images continuously acquired prior to and during free-breathing VMAT lung SBRT. Material/methods The 3D-printed/molded phantom containing 3 lung tumors was moved in 3D in TrueBeam developer mode, using simulated regular/irregular breathing patterns. Planar kV images were acquired at 7 frames/s during 11Gy/fraction 10MV FFF VMAT. 2D reference templates were created for each gantry angle using the planning 4DCT inspiration phase. kV images and templates were matched using normalized cross correlation to determine 2D tumor position and triangulation of 2D matched projections determined the 3rd dimension. 3D target tracking performed on CBCT projection data from 18 patients (20 tumors) and real-time on-line tracking data from 2 of the 18 patients, undergoing free-breathing VMAT lung SBRT is presented. Results For target 1 and 2 of the phantom (upper lung and middle/medial lung, mean density -130HU), 3D results within 2mm of the known position were present in 92% and 96% of the kV projections, respectively. For target 3, (inferior lung, mean density -478HU) this dropped to 80%. Benchmarking against the respiratory signal, 13/20 (65%) tumors (10.5±11.1cm3) were considered successfully tracked on the CBCT data. Tracking was less successful (≤50% of the time) in 7/20 (1.2±1.5cm3). Successful on-line tracking during lung SBRT was demonstrated. Conclusion 3D markerless tumor tracking on a standard LINAC using template matching and triangulation of free-breathing kV fluoroscopic images was possible in 65% of small lung tumors. The smallest tumors were most challenging.