Kilovoltage triggered imaging for moderately hypofractionated prostate radiotherapy: Insights into intrafraction motion and intervention through log file analysis.

PURPOSE: To report on intrafraction motion and frequency of intervention observed during kilovoltage imaging and monitoring of implanted fiducial markers (FM) in patients receiving moderately hypofractionated prostate radiotherapy.

METHODS

Gold FM implanted in the prostate of 50 patients were tracked during volumetric-modulated arc therapy, delivered over 20 fractions, with the Varian intrafraction motion review (IMR) system. Images were acquired orthogonal to the treatment beam at 3 s intervals. A 5 mm radial tolerance was set around all FMs, resulting in interruption to treatment delivery and intervention when FM position exceeded this threshold. Data was programmatically extracted from system log files and analysed to retrospectively reconstruct the individual position and centroid of FM during treatment.

RESULTS

Log file data for 991 treatment fractions were analysed. Mid-treatment interventions were required on 12 % of fractions due to one or more FM exceeding the 5 mm tolerance. The maximum centroid displacement was always smaller than the maximum individual FM displacement. The mean added time with intervention was 3.1 ± 0.9 min. Almost 3 in 4 patients had ≥1 fraction requiring intervention. The mean intrafraction change in centroid position across all fractions was 1.8 ± 1.0 mm. The largest displacements were most-commonly observed in the anterior-posterior (AP) and superior-inferior (SI) directions.

CONCLUSION

Interrogation of IMR log files yielded quantitative data for a comprehensive analysis of intrafraction prostate motion not available at the clinical interface. Examining the relationship between centroid and individual FM positions can further guide the clinical use of the system.