Two-Staged Radiosurgical Treatment of Large Arteriovenous Malformations

Abstract

When the size of an arteriovenous malformation (AVM) in any one direction is more than 4 cm, an attempt is made to treat it in two stages, separated by 6 months. The first stage treats one-half of the AVM and the remainder is treated in the second stage, thus giving rise to a higher prescription dose for each stage. Therefore, the AVM, as a whole, could be given a higher dose than if the entire AVM were treated in a single fraction. In order to reproduce, in each stage, the isocenter coordinates determined from a pair of orthogonal films, two techniques are applied. One is the use of BrainLAB's mask system, which maintains the isocenter accuracy within ± 1 mm between the two stages. The other is the use of four 3-mm long titanium screws (fiducial markers) embedded in the patient's skull. The coordinate transformation matrix established for the fiducial markers between the first- and second-treatment stages can be applied to the respective isocenter coordinates. Thus, the original isocenter can be reestablished in the coordinate system of the second-treatment stage. Application of this method was first tested with four 5-mm diameter aluminum balls (BBs) attached as fiducial markers to the external surface of the Rando head phantom and three 3-mm tungsten balls embedded as isocenters inside the skull. One patient case was also studied in terms of predicted accuracy, since the isocenters cannot be pinpointed in terms of anatomical structure (like the three tungsten balls). The two-staged radiosurgical approach with small screws embedded in the skull maintains the accuracy required for stereotactic radiosurgery, thereby facilitating the treatment of large AVMs. Since rigid fixation of the head, as is used with traditional stereotactic radiosurgery, is not used with this two-staged approach, treatment does not have to be delivered shortly after diagnostic images are acquired for treatment planning. This gives the physicist, radiation oncologist, and neurosurgeon additional time to optimize the treatment plan, if necessary. By dividing the treatment into two stages, it is hoped that a higher dose of radiation can be safely delivered to large AVMs, thereby increasing the likelihood of cure.