Dosimetric Study of Hippocampal Avoidance (HA) in Patients Treated With Stereotactic Radiosurgery (SRS) for Brain Metastases

Abstract

EBT3 film pieces inserted in between the 3 mm sheets. All the measurements were compared with the results from the GammaPlan calculations using both the convolution algorithm and the TMR10 algorithm. Results: The mean, minimum, maximum differences between the ion chamber measured doses from the Tg 21 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 Tg 21 formalism and the calculated doses from the convolution algorithm are 0.01%,-1.86%, 0.89% respectively. The calculated 2D dose distributions from the TMR10 algorithm are in general about 5% more than those from the measurements and the convolution algorithm for the same shot arrangement and treatment times, as the result of neglecting the excessive attenuation from the skull bones. Using a 2 mm & 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. Implementation of the convolution algorithm in routine Gamma Knife radiosurgery treatment planning may be desired for optimal clinical outcome.