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Inter- and Intrafractional Dose Uncertainty in Multifraction Gamma Knife Treatment

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T Kim

Taeho Kim1*, David Schlesinger1,2 , (1) Radiation Oncology, University of Virginia Health System, Charlottesville, VA, (2) Neurological Surgery, University of Virginia Health Systems, Charlottesville, VA


WE-G-BRD-4 Wednesday 4:30PM - 6:00PM Room: Ballroom D

The purpose of this study is to evaluate inter- and intra-fractional dose variations resulting from head position deviations for patients treated with the Extend relocatable frame utilized in multi-fraction Gamma Knife radiosurgery (GKRS). While previous reports characterized the residual setup and intra-fraction uncertainties of the system, the dosimetric consequences have not been investigated.

A digital gauge was used to measure the head position of 16 consecutive Extend patients (62 fractions) at the time of simulation, before each fraction, and immediately following each fraction. Radial inter-fraction (difference between simulation and pre-fraction positions) and intra-fraction (difference between post-fraction and pre-fraction positions) shifts in patient position were calculated. Planned dose distributions were shifted by the offset to determine the time-of-treatment dose. Variations in mean and maximum target and organ at risk (OAR) doses as a function of positional shift were evaluated.

The mean radial inter-fraction shift was 0.64 mm (SD: 0.25, maximum: 1.17). The mean intra-fraction shift was 0.39 mm (SD: 0.25, maximum: 1.44). The mean target dose variation was 0.66% (SD: 1.15%, maximum: 5.77) for inter-fraction shifts and 0.26 % (SD: 0.34, maximum: 1.85) for intra-fraction shifts. The maximum OAR dose variation: 7.15% (SD: 5.73, maximum: 30.59) for inter-fraction shifts and 4.07% (SD: 4.22, maximum: 17.04) for intra-fraction shifts. Linear fitting of maximum OAR dose variations as a function of position yielded dose deviations of 11.1%/mm for inter-fractional shifts and 9.3 %/mm for intra-fractional shifts.

While positional uncertainties when performing multi-fraction Gamma Knife radiosurgery are small (<1 mm), dose deviations resulting from shifts in position can be significant (especially to OARs) due to the steep dose gradients characteristic of GKRS. Dosimetric consequences of positional uncertainties should be considered when using the Gamma Knife Extend System.

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