Performance of Two Open-Faced Thermoplastic Masks for SRS Immobilization
Abstract
Purpose
Open-faced thermoplastic masks are widely used for immobilization in stereotactic radiosurgery (SRS) to achieve submillimeter treatment accuracy. This study compared the performance of two commercially available mask designs for SRS patients: the dual-layer Orfit mask (33804/2MA/12MI+N) and the alternative single-layer Ring the Bell Medical mask (CO3-24VR-O).
Methods
Open-faced three-point masks were fit to an anthropomorphic phantom following vendor provided molding specifications. Mask shrinkage was quantified by measuring inter-point distances and reference clip locations immediately post-fabrication and at weekly serial time points up to 21 days post-fabrication. Immobilization performance was evaluated by applying 5-60 N forces to an anthropomorphic phantom at 11 reference locations to simulate patient motion. Resulting translational and rotational displacements were quantified using a Varian Identify six-degree-of-freedom surface guidance system.
Results
Both masks exhibited time-dependent shrinkage, with the greatest dimensional changes occurring within the first week for both mask types. Shrinkage magnitude and spatial patterns were comparable between the two designs, with maximum shrinkage between points on the order of 2-3 mm observed between fabrication and week two. Shrinkage on this order is generally accommodated by removing one shim in the treatment setup between simulation and the first fraction of treatment. Immobilization testing demonstrated similar force–displacement responses for both masks. For equivalent applied forces, maximum displacements and force-displacement curves were similar with both masks, with Ring the Bell mask potentially providing more rotational rigidity than the Orfit mask. Maximum rotational offsets remained below 0.5 degrees and translational below 2.5 mm under the most extreme force application.
Conclusion
Despite differences in construction, the single-ply Ring the Bell mask with integrated reinforcement demonstrated shrinkage behavior and immobilization performance comparable to the dual-layer Orfit system. These findings support the feasibility of the Ring the Bell mask as an alternative immobilization solution for open-faced intracranial SRS within a surface-guided radiotherapy workflow.