KEY POINTS
- The study evaluated accelerated commissioning of Brainlab Elements using the vendor reference beam model on an Elekta Versa HD, with both 6 MV and 6 MV flattening-filter-free beams. A qualified medical physicist and medical physics resident completed the beam measurements and modeling in one day, compared with the 2–4 weeks per photon energy often required for conventional commissioning workflows.
- Commissioning used a deliberately reduced measurement dataset including percent-depth doses, beam profiles, MLC gap measurements and output factors down to 6 × 6 mm² fields. All subsequent AAPM MPPG 5.b validation tests passed, and patient-specific QA test plans achieved gamma passing rates above 90%.
- End-to-end testing used anthropomorphic cranial and spine phantoms and compared Monte Carlo with pencil-beam models from Elements versions 4.0 and 4.5. Scenarios included single-target cranial SRS, multiple brain metastases, spine SRS, air–bone heterogeneity and an air–bone environment containing a steel implant.
- Monte Carlo calculations were generally the most reliable: most end-to-end measurements agreed with calculated dose within approximately 3%, and nearly all clinically relevant measurements remained within the 5% MPPG 9.b tolerance. The algorithm was tested using a 1-mm dose grid and 2% calculation uncertainty.
- Pencil-beam performance was more vulnerable in difficult geometries. In the water-filled spine phantom, the 6 MV avoid plan differed from measurement by −16.6% with PB v4.0 and −9.6% with PB v4.5, whereas Monte Carlo showed 0.0% difference. For the 6FFF version, differences were −12.6%, −7.4%, and −3.6%, respectively.
- Heterogeneous environments were handled considerably better by Monte Carlo. In the air-filled spine phantom, Monte Carlo differed from measurements by only −0.55% to +1.30%, while in the steel-insert phantom differences ranged from −1.36% to −0.33%. Pencil-beam calculations happened to remain within 5% in these specific heterogeneous test plans, but showed larger problems in small-field and low-dose avoidance scenarios.
- The authors specifically caution against relying on pencil-beam calculations for very small fields and complex scatter conditions. Three of four avoid plans using PB v4.0 differed from measurements by more than 10%, and the updated v4.5 beam model improved—but did not eliminate—these discrepancies.
CLINICAL TAKEAWAY
A vendor reference beam model may reduce Brainlab Elements SRS commissioning from weeks to roughly one working day without eliminating the need for rigorous end-to-end validation. The more important physics message is that Monte Carlo remained substantially more dependable in small-field and challenging scatter conditions, while pencil-beam errors could become clinically unacceptable.