SU-F-T-593: Technical Treatment Accuracy in a Clinic of Fractionated Stereotactic Radiosurgery.

PURPOSE: The purpose of this study is to estimate technical treatment accuracy in fractionated stereotactic radiosurgery (fSRS) using extend system (ES) of Gamma Knife (GK).

METHODS: The fSRS with GK relies on a patient specific re-locatable immobilization system. The reference treatment position is estimated using a digital probe and a repositioning check tool (RCT). The "calibration values" of RCT apertures were compared with measured values on RCT-QA tool to evaluate the standard error (SE) associated with RCT measurements. A treatment plan with single "4 mm collimator shot" was created to deliver a radiation dose of 5 Gy at the predefined plane of a newly designed in-house head-neck phantom. The plan was investigated using radiochromic EBT3 films. The stereotactic CT imaging of a designed mini CT phantom and distortion study of MR imaging, were combined to calculate imaging SE. The focal precision check for GK machine tolerance was performed using a central diode test tool.

RESULTS: Twenty observations of RCT and digital probe, shown the SE of +/-0.0186mm and +/-0.0002mm respectively. A mean positional shift of 0.2752mm (σ=0.0696mm) was observed for twenty similar treatment settings of head-neck phantom. The difference between radiological and predefined exposure point was 0.4650mm and 0.4270mm; for two independent experiments. The imaging studies showed a combined SE of +/- 0.1055mm. Twenty frequent runs of a diode test tool showed the tolerance SE of +/-0.0096mm. If, the measurements are considered to be at 95% of confidence level, an expanded uncertainty was evaluated as +/- 0.2371mm with our system. The positional shift, when combined with an expanded uncertainty, a trivial variation of 0.07mm (max) was observed in comparing resultant radiological precision through film investigations.

CONCLUSION: The study proposes an expression of "technical treatment accuracy" within "known uncertainties" is rational in the estimation of routine fSRS quality. The research work is supported by the research section of "All India Institute of Medical Sciences" - New Delhi, India under project no A-247.