Introducing sitetrack: continuous patient motion monitoring during stereotactic radiotherapy for the head

Immobilization of the head during stereotactic radiotherapy (STR) has, until recently, been fundamental for ensuring accuracy. We developed a continuous motion monitoring system (SiteTrack) to detect and quantify head movement during CyberKnife (Accuray, Inc., Sunnyvale, CA) STR and Leksell (Elekta AB, Stockholm, Sweden) frame fixation. In this study, we present findings obtained during tests of SiteTrack.

SiteTrack is composed of a potentiometer with 4 encoders connected through 4 threads tied to the plastic buttons attached to the thermoplastic mask, in the case of CyberKnife, or by 3 threads directly connected to the Leksell frame. The accuracy of SiteTrack was studied with a phantom using the target localization system of the CyberKnife. During CyberKnife treatment, SiteTrack software sends an emergency stop signal (E-stop) if the patient moves beyond a limit. Seventy-three cases of CyberKnife STR and 7 cases of Leksell frame fixation were monitored.

In the phantom study, regression analysis showed a significant correlation between SiteTrack and target localization system parameters. The expected root mean square position error at the moment of E-stop was 0.62 +/- 0.44 mm when the E-stop limit was set at +/-0.5 mm. Twenty-two (30%) of 73 patients kept still during CyberKnife treatment (<0.5 mm); 51 (70%) of 73 patients moved more than 0.5 mm and, thus, caused E-stops. SiteTrack monitoring during frame fixation showed motion beyond +/-1 mm in 4 cases and +/-0.5 mm in 3 other cases.

Significant head movement can occur during CyberKnife treatment or fixation with a Leksell frame. SiteTrack may improve the accuracy of the CyberKnife as well as frame-based STR.