SU-E-T-564: Statistical Variation and Acceptance of Non Invasive Immobilization Systems for Multi-Fraction Gamma Knife EXtend Stereotactic Radiosurgery




Gamma Knife perfexion (GKPFX) with extend system is being used for multi-fraction or fractionated stereotactic radiosurgery (fGKSRS). Patient surveillance Unit (PSU) is used to attain a non invasive immobilization for fractionation. Calibrated digital probe is used to measure encoded mechanical positions on repositioning check tool (RCT). The objective of this study is to check consistency and statistical position variation during multi-fraction stereotactic radiosurgery using GKPFX extend system.


31 fractions through 7 patients being delivered to check the positional variation during patient setup. Variation is calculated after sampling the collected data at same point measurement for all the patients. Average value of three stage measurements or the reference values are compiled with the sample data using SPSS (statistical software). Measurement of encoded points has been taken and various statistical parameters are observed. Reproducibility is checked, comparing the reference to measured values through encoded points before treatment. Intra-fraction variation is measured on compiling pre and post treatment RCT measurements while inter-fractionation variation compiled along with fractions before treatment.


Pre-treatment radial setup difference auto calculated by the system varies from 0.5mm to 0.8 mm with mean variation of 0.7 mm. Positional variation was observed for post-treatment and was found that it varies from 0.9 mm – 1.0 mm. Paired sample test results the paired difference mean from the reference (for pre and post measurements) was 0.26 mm (0.1775 mm- 0.3475 mm) with only 25% of error significance. The distribution of the random component of observer uncertainty was 0.06mm.


Post treatment positional variation is 20%-30% of pre treatment measurements, i.e. about treatment measurements, i.e. about 0.2 mm. Explaining complete procedure to patient, limiting head rotation/lift and preventing other movements, may maximize positional accuracy during fractionations. Contending positional accuracy, supports the extend system for fractionated stereotactic radiosurgery among other multisession radiosurgery techniques.