Room: Exhibit Hall | Forum 3
Purpose: To investigate a new metric, the prostate walk, to characterize and classify intra-treatment motion during stereotactic body radiotherapy (SBRT) and evaluate its correlation to pre-treatment rectal and bladder volumes.
Methods: Intra-treatment motion was evaluated using simultaneous MV/kV imaging for 43 prostate cancer patients undergoing hypofractionated SBRT (205 treatment sessions). Bladder, rectum, and rectal gas volumes were determined from each patientâ€™s planning scan as well as pre-treatment cone-beam CTs. MV/kV pairs were obtained at ~15 second intervals throughout two-arc VMAT and a trace was generated by registering gold seed fiducial positions on each pair to a planning reference. Position corrections applied during treatment delivery were retrospectively removed to generate the â€œnatural motionâ€? trace. The prostate walk in each direction was calculated as the cumulative motion over the treatment session after applying a Savitsky-Golay filter to minimize measurement uncertainty and/or clinically insignificant motion. Walk was compared to the average positional deviation from the daily reference position and the pre-treatment rectal, bladder and rectal gas volumes.
Results: Linear correlation coefficients between prostate walk and average positional deviations were moderate at 0.27, 0.36, 0.35 in the S/I, A/P, and 3D directions, respectively. Large prostate walks were not always associated with large average deviations. Statistically significant correlations were observed in the R/L (p=0.01) and A/P (p < 0.005) directions between walk and pre-treatment rectal volume but not bladder volume. Rectal gas volume was correlated with walk in the R/L direction only (p=0.011).
Conclusion: Pre-treatment rectal volumes correlated with prostate walk in the R/L and A/P directions while rectal gas volume correlated with prostate walk in the R/L direction only. No correlation between walk and pre-treatment bladder volume was observed. Additional study is required to determine if walk can serve as a robust metric to identify different types of intra-treatment motion and predict dosimetric impact.
Funding Support, Disclosures, and Conflict of Interest: MSKCC is the recipient of research funding from Varian Medical Systems and Philips Healthcare.