Monte Carlo Investigation of Stereotactic Radiosurgery of Spinal Metastases

Abstract

In this work, we have investigated the dose discrepancy between the Monte Carlo method and a commercial treatment planning system for stereotactic radiosurgery (SRS) of spinal metastases. Two treatment plans were generated using the Eclipse Pencil Beam Convolution (PBC) and Analytical Anisotropic Algorithm (AAA) for each of three patients diagnosed with spinal metastases. Retrospective Monte Carlo dose calculations were performed on the same patient anatomy and beam setup as in Eclipse using the EGS4/MCSIM code. 3-D dose distributions and dose-volume histograms (DVH) were analyzed and compared between the plans calculated with Eclipse PBC, AAA, and the Monte Carlo method. Our results indicated that the target dose coverage of the Monte Carlo calculated plan was not as good as that in the Eclipse PBC plan with a 9% decrease in homogeneity index. The target dose was over-estimated in the PBC plans with the mean dose to the PTV on average 6% higher than that calculated by the Monte Carlo. On the other hand, the mean dose to the spinal cord was also over-estimated in the PBC plans by about 6% compared to the Monte Carlo results. AAA calculated dose distributions were between those of PBC and Monte Carlo, with closer similarity to PBC results. It is highly likely that the over-estimation of the PTV dose may lead to under-treatment of the tumor and increase the risk of recurrence of cancer in patients with spinal metastases. Therefore, Monte Carlo method should be used to provide more accurate dose distributions and dose-volume histogram analyses for stereotactic radiosurgery of spinal metastases.