Uncertainty in the positioning of patients receiving treatment for brain metastases and wearing surgical mask underneath thermoplastic mask during COVID‐19 crisis

Thermoplastic masks, used along with surgical masks, enable immobilization methods to reduce the risk of infection in patients undergoing intracranial stereotactic radiosurgery and stereotactic radiotherapy (SRS/SRT) during the COVID‐19 crisis. The purpose of this study was to investigate the feasibility of thermoplastic mask immobilization with a surgical mask using an ExacTrac system. Twelve patients each with brain metastases were immobilized using a thermoplastic mask and a surgical mask and only a thermoplastic mask. Two x‐ray images were acquired to correct (XC) and verify (XV) the patient’s position at a couch angle of 0°. Subsequently, the XC and XV images were acquired at each planned couch angle for non‐coplanar beams. When the position errors were detected after couch rotation for non‐coplanar beams, the errors were corrected at each planned couch angle until a clinically acceptable tolerance was attained. The position errors in the translational and rotational directions (vertical, lateral, longitudinal, pitch, roll, and yaw) were retrospectively investigated using data from the ExacTrac system database. A standard deviation of XC translational and rotational position errors with and without a surgical mask in the lateral (1.52 vs 2.07 mm), longitudinal (1.59 vs 1.87 mm), vertical (1.00 vs 1.73 mm), pitch (0.99 vs 0.79°), roll (1.24 vs 0.68°), and yaw (1.58 vs 0.90°) directions were observed at a couch angle of 0°. Most of patient positioning errors were less than 1.0 mm or 1.0° after the couch was rotated to the planned angle for non‐coplanar beams. The overall absolute values of the translational and rotational XV position errors with and without the surgical mask were less than 0.5 mm and 0.5°, respectively. This study showed that a thermoplastic mask with a surgical mask is a feasible immobilization technique for brain SRS/SRT patients using the ExacTrac system.