A complementary dual-modality verification for tumor tracking on a gimbaled linac system.
OBJECTIVE
For dynamic tracking of moving tumors, robust intra-fraction verification was required, to assure that tumor motion was properly managed during the course of radiotherapy. A dual-modality verification system, consisting of an on-board orthogonal kV and planar MV imaging device, was validated and applied retrospectively to patient data.


METHODS
Real-time tumor tracking (RTTT) was managed by applying PAN and TILT angular corrections to the therapeutic beam using a gimbaled linac. In this study, orthogonal X-ray imaging and MV EPID fluoroscopy was acquired simultaneously. The tracking beam position was derived from respectively real-time gimbals log files and the detected field outline on EPID. For both imaging modalities, the moving target was localized by detection of an implanted fiducial. The dual-modality tracking verification was validated against a high-precision optical camera in phantom experiments and applied to clinical tracking data from a liver and two lung cancer patients.


RESULTS
Both verification modalities showed a high accuracy (<0.3mm) during validation on phantom. Marker detection on EPID was influenced by low image contrast. For the clinical cases, gimbaled tracking showed a 90th percentile error (E90) of 3.45 (liver), 2.44 (lung A) and 3.40 mm (lung B) based on EPID fluoroscopy and good agreement with XR-log file data by an E90 of 3.13, 1.92 and 3.33 mm, respectively, during beam on.


CONCLUSIONS
Dual-modality verification was successfully implemented, offering the possibility of detailed reporting on RTTT performance.
Department of Radiotherapy, Universitair Ziekenhuis Brussel, Vrije Universiteit Brussel, Belgium. Electronic address: kenneth.poels@uzbrussel.be.
Kenneth
K
Poels
Tom
T
Depuydt
Dirk
D
Verellen
Benedikt
B
Engels
Christine
C
Collen
Steffen
S
Heinrich
Michael
M
Duchateau
Truus
T
Reynders
Katrien
K
Leysen
Marlies
M
Boussaer
Femke
F
Steenbeke
Koen
K
Tournel
Thierry
T
Gevaert
Guy
G
Storme
Mark
M
De Ridder
D057918:Fiducial Markers; D006801:Humans; D009038:Motion; D009369:Neoplasms / Q000473:pathology* / Q000532:radiotherapy*; D010315:Particle Accelerators; D019047:Phantoms, Imaging; D011880:Radiotherapy Planning, Computer-Assisted / Q000379:methods*
Dynamic tumor tracking; Intra-fraction treatment verification; kV/MV fluoroscopy