This study developed a dedicated real-time monitoring system to detect intra-fractional head motion in intracranial radiotherapy using pressure sensors. The dedicated real-time monitoring system consists of pressure sensors with a thickness of 0.6 mm and a radius of 9.1 mm, a thermoplastic mask, a vacuum pillow, and a baseplate. The four sensors were positioned at superior-inferior and right-left sides under the occipital area. The sampling rate of pressure sensors was set to 5 Hz. First, we confirmed that the relationship between the force and the displacement of the vacuum pillow follows Hook's law. Next, the spring constant for the vacuum pillow was determined from the relationship between the force given to the vacuum pillow and the displacement of the head, detected by Cyberknife target locating system (TLS) acquisitions in clinical application. Finally, the accuracy of our system was evaluated by using the 2  ×  2 confusion matrix. The regression lines between the force, y, and the displacement, x, of the vacuum pillow were given by y = 3.8x, y = 4.4x, and y = 5.0x when the degree of inner pressure was  -12 kPa,-20 kPa, and  -27 kPa, respectively. The spring constant of the vacuum pillow was 1.6 N mm(-1) from the 6D positioning data of a total of 2999 TLS acquisitions in 19 patients. Head motions of 1 mm, 1.5 mm, and 2 mm were detected in real-time with the accuracies of 67%, 84%, and 89%, respectively. Our system can detect displacement of the head continuously during every interval of TLS with a resolution of 1-2 mm without any radiation exposure.

Daily maximal IFPD changed day by day. Although maximal IFPD was more than 10 mm, IFPD of ≥3 mm was observed in a comparatively small proportion of treatment time. Through the course of conventionally fractionated radiotherapy, fractions with IFPD of ≥5 mm were infrequent.

The purpose of this study was to evaluate the relationship between pressure on the occipital region and intrafraction motion using an individualized vacuum pillow and a thermoplastic mask for intracranial treatment. We calculated head displacement during treatment from 8811 image verifications in 59 patients and divided them into two groups according to the magnitude of the mean and standard deviation (SD) of the displacement in the 59 patients. Pressure was compared between the small (n = 29) and large (n = 30) displacement groups using Welch’s t-test for the mean and SD of displacement. The mean head displacement in the small and large groups was (0.3, 0.3, 0.4) and (0.5, 0.6, 0.7) (unit: mm) for the vector length and 10 mm and 30 mm radius targets, respectively. The mean SD of head displacement in the small and large groups was (0.2, 0.2, 0.2) and (0.3, 0.3, 0.4) (unit: mm) for the vector length and 10 mm and 30 mm radius targets, respectively. Significant differences were observed in the SD of the displacement in the vector length and 10 mm radius target between the two groups. The SD of the displacement under a pressure of 15 kPa was smaller than that under a pressure of 11 kPa. The intrafraction motion under a high-pressure level on the occipital region was less than that under a low-pressure level. Management of pressure on the occipital region may result in less intrafraction motion in clinical practice.

The newly developed phantom was useful to evaluate the accuracy of immobilization systems for a Cyberknife system for intracranial lesions.