Evaluation of Setup Uncertainties for Single-Fraction SRS by Comparing the Two Different Mask-Creation Methods

The purpose of this study was to evaluate the setup uncertainties for single-fraction stereotactic radiosurgery (SF-SRS) based on the clinical data with the two different mask-creation methods using pretreatment CBCT imaging guidance. Dedicated frameless fixation BrainLAB masks for 23 patients were created as a routine mask (R-mask) making method, as explained in the BrainLAB user manual. The alternative masks (A-mask) which were created by modifying the cover range of the R-mask for the patient head were used for 23 patients. The systematic errors including the each mask and stereotactic target localizer were analyzed and the errors were calculated as the mean and standard deviation (SD) from the LR, SI, AP, and yaw setup corrections. In addition, the frequency of three-dimensional (3D) vector length were also analyzed. The values of the mean setup corrections for the R-mask in all directions were small; < 0.7 mm and < 0.1 degree, whereas the magnitudes of the SDs were relatively large compared to the mean values. In contrast to the R-mask, the means and SDs of the A-mask were smaller than those for the R-mask with the exception of the SD in the AP direction. The mean and SD in the yaw rotational direction in the R-mask and A-mask system were comparable. The 3D vector shifts of a larger magnitude occurred more frequently for the R-mask than the A-mask. The setup uncertainties for each mask with the stereotactic localizing system had an asymmetric offset towards the positive AP direction. The A-mask-creation method, which is capable of covering the top of the patient head is superior to that for R-mask, and thereby the use of the A-mask is encouraged for SF-SRS to reduce the setup uncertainties. Moreover, the careful mask making is required to prevent the possible setup uncertainties.

💡 Research Summary

The purpose of this study was to quantify and compare the setup uncertainties associated with single‑fraction stereotactic radiosurgery (SF‑SRS) when using two different frameless fixation masks produced by BrainLAB. Twenty‑three patients were immobilized with a routine mask (R‑mask) fabricated strictly according to the BrainLAB user manual, while another twenty‑three patients received an alternative mask (A‑mask) in which the coverage area was deliberately extended to include the top of the head. All patients underwent pretreatment cone‑beam CT (CBCT) imaging, and the translational corrections in the left‑right (LR), superior‑inferior (SI), and anterior‑posterior (AP) axes, as well as the yaw rotational correction, were recorded. For each direction, the mean value and standard deviation (SD) of the required couch shifts were calculated, and the three‑dimensional (3‑D) vector magnitude of the total shift was also analyzed.

The results showed that the mean translational corrections for the R‑mask were all less than 0.7 mm, and the mean yaw rotation was less than 0.1°, indicating that systematic offsets were small for both mask types. However, the SDs for the R‑mask were relatively large, reflecting considerable random variation from fraction to fraction. In contrast, the A‑mask produced smaller means and SDs in the LR, SI, and AP directions, with the exception of the AP SD, which remained comparable between the two groups. Yaw rotational uncertainties were similar for both masks, suggesting that the mask design does not substantially affect rotational stability.

Analysis of the 3‑D vector shift distribution revealed that large magnitude shifts (≥3 mm) occurred more frequently with the R‑mask than with the A‑mask. This indicates a higher probability of exceeding clinically acceptable positioning tolerances when using the routine mask. Moreover, both mask systems exhibited an asymmetric systematic offset toward the positive AP direction, implying that the stereotactic localizer and mask interface may introduce a consistent forward bias.

The authors conclude that extending the mask coverage to the vertex of the patient’s head (the A‑mask) reduces both systematic and random components of setup error in SF‑SRS. They recommend the routine adoption of the A‑mask for single‑fraction treatments to improve geometric accuracy. Additionally, they stress that meticulous mask fabrication—accurate molding of the patient’s scalp, careful control of pressure points, and verification of fit—is essential to prevent residual uncertainties, regardless of the mask design.

The study’s implications are twofold. First, it provides quantitative evidence that a relatively simple modification of the mask‑making protocol can yield clinically meaningful improvements in positioning precision, which is critical for high‑dose, single‑fraction treatments where sub‑millimeter accuracy directly impacts tumor control and normal‑tissue toxicity. Second, it highlights the importance of systematic quality‑control procedures during mask production, suggesting that institutions should standardize and audit their mask‑creation workflow to fully realize the benefits of the A‑mask design. Future work could explore the integration of the A‑mask with real‑time surface‑guided imaging or adaptive workflow strategies, as well as the development of patient‑specific mask algorithms that automatically optimize coverage based on individual head geometry.