Dosimetric comparison of absolute and relative dose distributions between tissue maximum ratio and convolution algorithms for acoustic neurinoma plans in Gamma Knife radiosurgery - Commentary

摘要

The authors are to be congratulated on conducting such a thorough and important study on radiosurgical basics. For about three decades, Gamma Knife treatment software has been based on the assumption that intracra-nial targets are surrounded by tissues with water-like properties as far as radiation physics are concerned. This is certainly appropriate for targets within the brain parenchyma such as metastases. However, most benign lesions, such as meningiomas, acoustic schwannomas, and pituitary adenomas, are located adjacent to bony structures and/or air-filled spaces. In this study, the authors compare two calculated doses delivered to acoustic schwannomas based on actual treatment plans. The two different calculations were done using identical shot placements from the original treatment plans. The authors can show that one of the calculations represents more adequately reality and the actually delivered dose. For that purpose, the authors first validated a new software tool correcting for heterogeneous tissues in close proximity to target volumes in phantoms with and without tissue heterogeneities. They then applied the new software tool in actual treatment plans for acoustic schwannomas, and compared the results with the original, clinically applied treatments based on the standard treatment software. The results show significant differences in dose distribution when comparing those treatment plans for acoustic schwannomas. Acoustic schwannomas are closely located to bone and air. Not surprisingly, the main differences they detected between the two dose distributions of the standard treatment software and the new software tool concern the dose delivered to the temporal bone, In addition, this study shows that the maximum dose delivered to the cochlea differs significantly for the two treatment plans. This is of great clinical importance, since the dose that is supposedly delivered to the cochlea, as calculated with the standard treatment software, seems to be significantly underestimated. This new software tool corrects for dose distortions in close proximity to hetreogeneities for which there is no correction in the standard software version. This is of great clinical importance in tumors of the skull base, which are in close proximity to heterogeneous tissues or air-filled spaces, and at the same time in close proximity to organs at risk, such as the cochlea or the optic apparatus. The authors' conclusion that the new software tool should be used in the future is certainly correct for lesions other than brain metastases, cavernous malformations, or arterio-venous malformations, which are not usually in close proximity to tissue heterogeneities.