Prescription Dose Guideline Based on Physical Criterion for Multiple Metastatic Brain Tumors Treated With Stereotactic Radiosurgery

Abstract

Existing dose guidelines for intracranial stereotactic radiosurgery (SRS) are primarily based on single-target treatment data. This study investigated dose guidelines for multiple targets treated with SRS. A physical model was developed to relate the peripheral isodose volume dependence on an increasing number of targets and prescription dose per target. The model was derived from simulated and clinical multiple brain metastatic cases treated with the Leksell Gamma Knife Perfexion at several institutions, where the total number of targets ranged from 2 to 60. The relative increase in peripheral isodose volumes, such as the 12-Gy volume, was studied in the multitarget treatment setting based on Radiation Therapy Oncology Group 90-05 study dose levels. A significant increase in the 12-Gy peripheral isodose volumes was found in comparing multiple target SRS to single-target SRS. This increase strongly correlated (R(2) = 0.92) with the total number of targets but not the total target volumes (R(2) = 0.06). On the basis of the correlated curve, the 12-Gy volume for multiple target treatment was found to increase by approximately 1% per target when a low target dose such as 15 Gy was used, but approximately 4% per target when a high dose such as 20-24 Gy was used. Reduction in the prescription dose was quantified for each prescription level in maintaining the 12-Gy volume. Normal brain dose increases predictably with increasing number of targets for multitarget SRS. A reduction of approximately 1-2 Gy in the prescribed dose is needed compared with single target radiosurgery.