Author: Ahmad K. Al-Basheer, Andrew Clarke Grice, Kelly E. Hosier, Mark Logsdon, Evan A. Silverstein, Chris Tunnicliff, Chuan Wu 👨🔬
Affiliation: Sutter Medical Foundation, Sutter Health 🌍
Purpose: For Prostate SBRT Boost treatments, determine whether a uniform 2mm CTV to PTV margin is sufficient to give 95% dose coverage to the CTV in 90% of patients via a modified Van Herk Margin Formula to include angular contributions (aVHMF).
Methods: ExacTrac x-ray fiducial tracking was utilized during treatment for quantification of prostate motion. Retrospectively, C# scripts extracted 6D shifts relative to isocenter for 24 patients undergoing 3-fraction SBRT boost treatments, totaling 136 beam arcs. Systematic and random errors were quantified via statistical analysis of ExacTrac shifts used in our aVHMF. Each patient’s original CTV was modified in MIM using the average magnitude 6DoF shifts in both positive and negative directions. Dose was exported into MIM for evaluation of the minimum cumulative dose to the CTV.
Results: Application of the standard VHMF gave left/right, ant/post, and sup/inf margins of 0.81mm, 1.01mm, and 1.13mm whereas aVHMF resulted in 0.96mm, 1.55mm, and 1.67mm, respectively. Margins calculated from the standard VHMF resulted in 50% of patients receiving a minimum cumulative dose of 95%, whereas margins calculated from aVHMF resulted in 100% of patients meeting this goal. Average minimum dose differed by 2.3%.
Conclusion: Results indicate a 2mm uniform PTV margin is sufficient for prostate SBRT boosts at our site. Standard VHMF margins would result in unacceptable CTV coverage for large portion of patients, necessitating the inclusion of angular contributions. Our dataset underestimates ExacTrac’s benefit as exposures were not taken immediately following the end of arcs when prostate offset would be at maximum. Analysis of a larger population size with exposures at the end of each arc will be incorporated into future work to identify patient specific factors affecting margin calculations.