Jonathan A. Borden, Julian K. Wu and William A. Shucart
✓ A classification is proposed that unifies and organizes spinal and cranial dural arteriovenous fistulous malformations (AVFMs) into three types based upon their anatomical similarities. Type I dural AVFMs drain directly into dural venous sinuses or meningeal veins. Type II malformations drain into dural sinuses or meningeal veins but also have retrograde drainage into subarachnoid veins. Type III malformations drain into subarachnoid veins and do not have dural sinus or meningeal venous drainage. The arterial supply in each of these three types is derived from meningeal arteries.
The anatomical basis of the proposed classification is presented with several cases that illustrate the three types of dural AVFMs. A rationale for the treatment of spinal and cranial dural AVFMs according to their anatomical characteristics is discussed.
Jonathan A. Borden, Anita Mahajan and Jen-San Tsai
✓The authors have developed a quality factor (QF) to compare gamma knife radiosurgery, linear accelerator radiosurgery, and intensity-modulated radiation therapy (IMRT) dosimetry. This QF relates the percentage of target covered (PTC) by the prescription radiation isodose, target volume (VT), and enclosed tissue volume, which receives greater than a particular dose (VX): QFX = PTC×VT/VX. The authors investigated target shape independent of volume in predicting radiosurgical complication rates.
Plastic targets of a defined volume (0.2, 0.5, 1.5, and 10 cm3) and four increasingly complex shapes (spherical, ellipsoid, simulated arteriovenous malformation [AVM], and horseshoe) were created. Dosimetry was studied on the Leksell GammaPlan, Adac/Pinnacle, and Nomos Corvus workstations. The dosimetry of a new 4 mm × 10—mm IMRT collimator array (the Nomos Beak) not yet validated for use in our clinical practice was studied.
Particularly for larger targets, the gamma knife and IMRT Beak plans show similar conformality (QF assuming 15-Gy volume [QF15]). Particularly for small and round targets the gamma knife plan quality is significantly higher (QF assuming 12-Gy volume [QF12]). As VT and complexity increase, the IMRT Beak QF12 approaches that of the gamma knife.
The QF12 of gamma knife dosimetry has an inverse correlation with target shape complexity independent of VT.
At a prescription dose of 15 Gy to the target margin, the QF15 is a conformality index. The 12-Gy volume (volume enclosed by 12-Gy surface/volume receiving at least 12 Gy) estimates the radiosurgical normal tissue complication rate for AVMs. When the target is well covered, the QF12 is inversely proportional to the complication risk and is a measure of the plan quality.
Jonathan A. Borden, Jen-san Tsai and Anita Mahajan
Object. The purpose of this study was to evaluate subpixel magnetic resonance (MR) imaging shifts of intracanalicular vestibular schwannomas (VSs) with respect to the internal auditory canal (IAC) as documented on computerized tomography (CT) scanning and to investigate the source of imaging-related localization errors in radiosurgery as well as the effect of such shifts on the dosimetry for small targets.
Methods. A shift of the stereotactic coordinates of intracanalicular VSs between those determined on MR imaging and those on CT scanning represents an error in localization. A shift vector places the tumor within the IAC and measures the CT scan/MR image discrepancy. The shift vectors were measured in a series of 15 largely intracanalicular VSs (all < 1.5 cm3 in volume). Using dose volume histogram measurements, the overlap between shifted and unshifted tumors and radiosurgical treatment plans were measured. Using plastic and bone phantoms and thermoluminescent dosimetry measurements, the correspondence between CT and MR imaging targets and treatments delivered using the Leksell gamma knife were measured. Combining these measurements, the correspondence between intended and actual treatments was measured.
Conclusions. The delivery of radiation to CT-imaged targets was accurate to the limits of measurement (∼ 0.1 mm). The MR imaging shifts seen in the y axis averaged 0.9 mm and in the z axis 0.8 mm. The corresponding percentage of tumor coverage with respect to apparent target shift decreased from 98 to 77%. This represents a significant potential error when targets are defined solely by MR imaging.