A Quantitative Analysis of the Reliability of Aquaplast Mask Immobilization for Cranial Radiosurgery With TomoTherapy

摘要

Patient treatment setup accuracy for cranial radiosurgery (SRS) is dependent on a number of parameters; including localization and immobilization. In this study we carried out a quantitative analysis of the reliability of a radiosurgery immobilization mask system in maintaining patient position during SRS treatment sessions. Patients were fitted with a thermoplastic mask and a customized pillow designed for SRS. TomoTherapy MVCT, using 1 mm slice thickness, was carried out and the required shifts, based on bony anatomy, were applied to the patient position prior to radiotherapy. At the completion of the treatment a second MVCT was carried out to determine if any patient movement had taken place. Typical time on the treatment couch varied from 25-60 minutes. A total of 26 patients were analyzed. Some patients received more than one treatment fraction. A total of 38 MVCT measurements were included in this study. The mean absolute shift for the 38 measurements was 0.42 mm (SD=0.29) lateral, 0.75 mm (SD=0.94) longitudinal, and 0.44 mm (SD=0.42) in the vertical direction. Overall, the average vector displacement was 1.18 mm (SD=0.82). The variation in shifts between different patients was significant. Two patients had a vector shift of greater than 3.0 mm; largely due to a longitudinal displacement. Based on these results a margin of approximately 1 mm is sufficient for patient lateral and vertical motion but up to a 2 mm margin may be needed for longitudinal motion. Hence a PRV and PTV of 3 mm is required to account for patient position uncertainty. Care in the initial molding of the mask, and a customized pillow, may be key in ensuring minimal patient motion during cranial SRS.