KEY POINTS
- The technical evaluation compared Environ with the established Encompass frameless SRS mask across beam attenuation, HyperArc dose-calculation accuracy and clinical setup reproducibility. Clinical positioning data included 67 fractions in 21 Encompass patients and 72 fractions in 24 Environ patients.
- Across seven static 6-FFF beams, Encompass attenuated 1.13% ± 0.59% more than Environ (P=.002), with the largest difference reaching 1.82% at 90°. In contrast, attenuation differed by only 0.06% during a 0°–180° arc, below the estimated 0.4% measurement uncertainty.
- Dose-calculation error with Encompass depended strongly on explicitly contouring the mask. With Acuros XB, mean calculated-versus-measured difference improved from 2.38% without mask contouring to 0.10% with contouring across single-target plans.
- Environ was less sensitive to this issue: Acuros XB already achieved a mean difference of only 0.24% without contouring, while including the mask shifted the mean difference to −1.04%. The smallest 1-mm-margin targets generated the largest discrepancies for both systems because of the steep dose gradient around the detector.
- Multi-target HyperArc plans showed the same general pattern. With Acuros XB and mask contouring, mean dose difference was 1.70% for Encompass versus −0.25% for Environ; without contouring, corresponding differences were 2.33% and 1.71%.
- Patient positioning also favored Environ in the vertical axis: mean signed vertical correction was 0.29 versus 0.86 mm, and within-patient vertical variability fell from 1.25 to 0.31 mm (P<.001). Pitch variability was likewise lower at 0.50° versus 0.99° (P=.009), while the remaining axes were comparable.
- The clinical setup analysis was retrospective and evaluated interfraction CBCT corrections rather than intrafraction motion, which is especially important for SRS. The authors therefore recommend institution-specific commissioning rather than assuming that mask-contouring requirements or positioning advantages will generalize unchanged.
CLINICAL TAKEAWAY
Environ appears to be a technically credible alternative to Encompass for HyperArc SRS, with lower static-beam attenuation and particularly good vertical positioning reproducibility. Its apparent ability to achieve accurate Acuros calculations without explicitly contouring the mask is attractive, but that finding should be locally validated before changing planning practice.