Effect of subpixel magnetic resonance imaging shifts on radiosurgical dosimetry for vestibular schwannoma.
OBJECTIVE
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 (approximately 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.
Borden
Jonathan A
JA
Department of Neurosurgery, Tufts University School of Medicine and Tufts-New England Medical Center, Boston, Massachusetts 02111, USA. jonathan@openhealth.org
Tsai
Jen-San
JS
Mahajan
Anita
A
D006309:Hearing; D006801:Humans; D008279:Magnetic Resonance Imaging / Q000379:methods* / Q000592:standards; D009464:Neuroma, Acoustic / Q000000981:diagnostic imaging* / Q000601:surgery*; D019047:Phantoms, Imaging; D011829:Radiation Dosage; D016634:Radiosurgery / Q000379:methods*; D020127:Recovery of Function; D015203:Reproducibility of Results; D012984:Software; D014057:Tomography, X-Ray Computed