PRESAGE 3D dosimetry accurately measures Gamma Knife output factors

Abstract

Small-field output factor measurements are traditionally very difficult because of steep dose gradients, loss of lateral electronic equilibrium, and dose volume averaging in finitely sized detectors. Three-dimensional (3D) dosimetry is ideal for measuring small output factors and avoids many of these potential challenges of point and 2D detectors. PRESAGE 3D polymer dosimeters were used to measure the output factors for the 4 mm and 8 mm collimators of the Leksell Perfexion Gamma Knife radiosurgery treatment system. Discrepancies between the planned and measured distance between shot centers were also investigated. A Gamma Knife head frame was mounted onto an anthropomorphic head phantom. Special inserts were machined to hold 60 mm diameter, 70 mm tall cylindrical PRESAGE dosimeters. The phantom was irradiated with one 16 mm shot and either one 4 mm or one 8 mm shot, to a prescribed dose of either 3 Gy or 4 Gy to the 50% isodose line. The two shots were spaced between 30 mm and 60 mm apart and aligned along the central axis of the cylinder. The Presage dosimeters were measured using the DMOS-RPC optical CT scanning system. Five independent 4 mm output factor measurements fell within 2% of the manufacturer's Monte Carlo simulation-derived nominal value, as did two independent 8 mm output factor measurements. The measured distances between shot centers varied by ± 0.8 mm with respect to the planned shot displacements. On the basis of these results, we conclude that PRESAGE dosimetry is excellently suited to quantify the difficult-to-measure Gamma Knife output factors.

Figure 1

Head phantom shown mounted into a Gamma Knife halo assembly. Also shown are the green PRESAGE dosimeter, white polystyrene centering sleeve, and the black rotation bracket, which angularly aligns the keyed cylindrical dosimeter within the head phantom.

Figure 2

Starting with coordinates centered on the 16 mm dose distribution the average optical densities within spheres of smaller and smaller radii were measured. A power law function (equation 1) was fit to this data in order to globally scale the optical density distribution corresponding to the peak planned dose (8 Gy) for convenience and visualization of the dose distribution.

Figure 3

After globally scaling the optical density distribution to the peak delivered dose a quadratic fit (equation 2) was found to best match the 4 mm dose distributions and a cubic equation (equation 3) was found to best match the 8 mm dose distributions. The best fit functions were used to extrapolate the dose response of a theoretical “zero volume detector” allowing accurate output factor measurements while avoiding the normal volume averaging issues found in finite point detector measurements.