Real-Time Motion Tracking and Correction During Tomotherapy for Prostate Cancer: Impact of Fiducial Migration.

Purpose/Objective(s) A real-time motion tracking technique has been recently implemented in our clinic to manage intrafraction prostate motion (IPM) during helical tomotherapy for prostate cancer. It involves detecting motion of multiple fiducials implanted in prostate based on real-time x-ray radiographs acquired at various beam angles and correcting the motion using dynamic jaw and MLC. The purpose of this study was to study interfractional fiducial migration (IFM) and its impact on intrafraction motion tracking. Materials/Methods Daily MVCT and radiographs acquired during tomotherapy with real-time motion tracking for 5 prostate cancer patients were analyzed to quantify IFM and IPM. All patients were treated with either 70 Gy in 28 or 75.6 Gy in 42 fractions based on plans generated on planning CTs acquired 14-70 days after the implantation of 4 fiducials for each patient. Time intervals between the first treatment and the planning CT acquisition ranged 4-16 days. The IPM was measured based on the maximum target position difference between the radiographs. Daily migration of each fiducial was obtained by (1) rigidly registering the daily MVCT with the planning CT based on prostate matching, and (2) calculating coordinate differences between the fiducial positions on the daily and plan images. To investigate the impact of fiducial migration on motion tracking that uses fiducial positions on the planning CT as baseline, we compared the maximum fiducial pair distance change (RigidBody, a tracking parameter) calculated from the planning CT to radiographs (default) to the value calculated from daily MVCT to radiographs. The daily target offsets (another tracking parameter) determined between the planning CT and radiographs were assessed if the fiducial migrations were removed. Results The average daily IPM was 3.2 ± 2.9 (up to 5.8 ± 4.0 for a patient) mm. The average IFM was 2.0 ± 1.1 mm (ranging 1.5-2.5 mm among the patients) with median and interquartile range (IQR) values of 1.8 and 1.2 mm, respectively. The fiducials migrated differently in the longitudinal (std/IQR = 1.8/2.2 mm), vertical (1.2/1.6 mm), and lateral (0.7/0.8 mm) directions. The daily fiducial COM deviated from its plan position by 1.6 ± 0.8 mm (ranging 1.3-2.0 mm among the patients) with median/IQR = 1.5/1.0 mm. The overall average of RigidBody reduced from 2.6 ± 0.7 mm (default) to 1.3 ± 0.4 mm if from daily MVCT. The average daily target offsets decreased from 3.2 ± 1.5 to 2.2 ± 1.0 mm if the fiducial migrations were removed. Conclusion The fiducial-based real-time prostate motion tracking during tomotherapy is advantageous, even in the presence of the measurable IFMs. Our results indicate that, to reduce/eliminate the effect of the fiducial migration on motion tracking stability and accuracy, the fiducial positions on the daily MVCT, instead of the planning CT in the current setting, should be used as the baseline. Author Disclosure G. Chen: North Central Chapter of AAPM, Medical College of Wisconsin. A. Tai: None. E.A. Omari: None. M. Bedi: None. C.A. Lawton: None. A. Li: Research Grant; Elekta AB, Accuray Inc, Siemens Healthineers, Manteia Med. Honoraria; Elekta AB, Accuray Inc. Patent/License Fees/Copyright; Manteia Med.