A Paradigm Shift: Monte Carlo Could Play a Primary Role While Measurement Serves a Validation Purpose In Small Field Dosimetry
Abstract
Purpose
Small field dosimetry has been challenging without reasonable data. However, we've reached the point that the Monte Carlo approach can play a primary role in providing comprehensive beam data (including field output factors(FOFs), %dd and dose profiles, etc.) while the experimental measurement can serve as a secondary role for validation purposes.
Methods
We compared the Monte Carlo generated beam data with measurements for commonly used beams in modern radiotherapy treatments (SRS, SBRT), including 6 MV-FFF, 6MV-SRS, 6 MV, and 10 MV-FFF beam energies for circular fields defined by cones and square fields defined by jaws ranging from 4 mm to 30 mm dimensions.
Results
A good agreement was found between the Monte Carlo generated and measured FOFs for circular fields while measured values using different detectors were scattered for square fields. Findings highlight the sensitivity of small field FOF on the field size, where consistency was seen for fields defined by fixed circular cones while larger variations were observed for fields collimated by jaws. At a field size of 5×5 mm2, 0.1 mm variation defined by jaws lead to a 2% output change. This is due to source occlusion that is poorly quantified.
Conclusion
Commissioning beam data of an SRS/SBRT beam requires both expertise and suitable detectors. Obtaining accurate beam data is challenging due to setup uncertainties and detector corrections, especially for extremely small fields. Utilizing Monte Carlo simulations could obtain accurate data and reduce both the time required for small field commissioning as well as by eliminating setup uncertainties and detector perturbations. Since the beams produced from modern accelerators meet the beam specifications, the variation among different accelerators for the same beam is generally negligible. The Monte Carlo generated comprehensive data can serve as reference and be generally applicable to beams that meet the manufacturers’ specifications.