Evaluation of intrafractional head motion for intracranial stereotactic radiosurgery with a thermoplastic frameless mask and ceiling-floor-mounted image guidance device

•We evaluated intrafractional head motion in patients who received intracranial SRS.•The intentional head motion in translation was mostly suppressed within 1 mm.•The intentional head motion in rotation was mostly suppressed within 1°.•A sudden large deviation was observed with low frequency.•X-ray verification and correction are required during treatment. To evaluate intrafractional head motion (IFM) in patients who underwent intracranial stereotactic radiosurgery with the ExacTrac X-ray system (ETX) and a frameless mask. A total of 143 patients who completed a pre-treatment examination for IFM were eligible for this study. The frameless mask type B R408 (Klarity Medical & Equipment Co., Ltd., Guangzhou, China), which covers the back of the head, and the entire face, was used for patient immobilization. After the initial 6D correction and first X-ray verification (IFM1), X-ray verification was performed every 3 min for a duration of 15 min. The IFMp (2 ≤ p ≤ 6) was calculated as the positional difference from IFM1. In addition, the inter-phase IFM (IP-IFM) and IFMm were calculated. The IP-IFM was defined as |IFMp − IFMp-1|, and IFMm as the difference between the values after all patients were asked to move their heads intentionally with the frameless mask on. Both translational IFMp and IP-IFM exceeded 1 mm for a single patient, whereas, for all patients, the translational IFMm values were kept to within 1 mm in all directions. The proportions of the rotational IFMp, IP-IFM, and IFMm values within 0.5° were greater than 94.4%, 98.6%, and 90.2% for all of the rotational axes, respectively. A frameless mask achieved highly accurate patient positioning in combination with ETX and a 6°-of-freedom robotic couch; however, a deviation over 1 mm and 0.5° was observed with low frequency. Therefore, X-ray verification and correction are required during treatment.