Room: Stars at Night Ballroom 4
Purpose: To evaluate the inhomogeneity effect in Gamma Knife dose calculation using different dosimetry tools for the validation of the convolution algorithm.
Methods: Plans and measurements were done on an inhomogeneous head phantom (CIRS Inc., model 038) with ion chamber and 3D dosimeter inserts following the standard work flow of mask based Gamma Knife radiosurgery. An Exradin A16 chamber was placed at 20 positions in the phantom that are at least 1cm apart and irradiated using single 16mm shot with the corresponding treatment times from TMR10 dose calculation for 20Gy maximum dose. Nineteen radiochromic Presage sheets of 6cm x 6cm x 3mm dimensions were stacked together in the cube for dosimetry inserts and irradiated with an acoustic neuroma treatment plan. To calibrate the dose response of the Presage dosimeters, 10 Presage sheets were put in the central region of the insert cube in the middle of dummy Presage sheets and irradiated with graded doses from 0-40Gy using 16 mm collimator.
Results: The mean, minimum, maximum differences between the measured doses from the Tg21 formalism and the calculated doses from the TMR algorithm are -5.74%, -4.65%, -8.43% respectively. The mean, minimum, maximum differences between the measured doses from the Tg21 formalism and the calculated doses from the convolution algorithm are 0.01%, -1.86%, 0.89% respectively. Using a 2mm & 1% criteria, the Gamma passing rates between the measured 3D dose distributions from the Presage sheets and the calculated distributions from the convolution algorithm are consistently higher than those between the measurement and the TMR calculation.
Conclusion: The TMR10 algorithm underestimates dose by up to 8.5% at the 20 measurement points. The convolution algorithm agrees with the chamber measurements within 2%. The isodose lines from the TMR10 algorithm may be distorted substantially in the vicinity of air/bone inhomogeneities.