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Anita Mahajan, Jonathan Borden and Jen-san Tsai

Object. The purpose of this study was to identify possible risk factors leading to carcinomatous meningitis in patients with a known brain metastasis and who were treated with gamma knife radiosurgery (GKS).

Methods. Two hundred eighty lesions in 101 patients were treated during 121 GKS procedures. The clinical and neuroimaging history, tumor histology, and follow-up studies were reviewed for all patients. Evidence as demonstrated by magnetic resonance imaging criteria and/or cerebrospinal fluid (CSF) cytology of carcinomatous meningitis was evaluated. The data were then analyzed to identify potential risk factors for the development of CSF dissemination.

Conclusions. It appears that carcinomatous meningitis is exclusively identified in patients with adenocarcinoma, in particular with primary lung cancer. Furthermore, the incidence is higher if surgery is performed and the interval to subsequent GKS is prolonged.

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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.