Functional stereotactic radiosurgery involving a dedicated linear accelerator and gamma unit: a comparison study

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Object. The purpose of this work was to investigate the targeting and dosimetric characteristics of a linear accelerator (LINAC) system dedicated for stereotactic radiosurgery compared with those of a commercial gamma knife (GK) unit.

Methods. A phantom was rigidly affixed within a Leksell stereotactic frame and axial computerized tomography scans were obtained using an appropriate stereotactic localization device. Treatment plans were performed, film was inserted into a recessed area, and the phantom was positioned and treated according to each treatment plan. In the case of the LINAC system, four 140° arcs, spanning ± 60° of couch rotation, were used. In the case of the GK unit, all 201 sources were left unplugged. Radiation was delivered using 3- and 8-mm LINAC collimators and 4- and 8-mm collimators of the GK unit. Targeting ability was investigated independently on the dedicated LINAC by using a primate model.

Measured 50% spot widths for multisource, single-shot radiation exceeded nominal values in all cases by 38 to 70% for the GK unit and 11 to 33% for the LINAC system. Measured offsets were indicative of submillimeter targeting precision on both devices. In primate studies, the appearance of an magnetic resonance imaging—enhancing lesion coincided with the intended target.

Conclusions. Radiosurgery performed using the 3-mm collimator of the dedicated LINAC exhibited characteristics that compared favorably with those of a dedicated GK unit. Overall targeting accuracy in the submillimeter range can be achieved, and dose distributions with sharp falloff can be expected for both devices.

Article Information

Address reprint requests to: Timothy D. Solberg, Ph.D., Department of Radiation Oncology, 200 UCLA Medical Plaza, Suite B265, Los Angeles, California 90095–6951. email: solberg@radonc. ucla.edu.

© AANS, except where prohibited by US copyright law.

Headings

Figures

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    The dosimetry phantom, with recessed portion for film and small pinhole in the topmost portion, shown mounted in a Leksell frame.

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    The film plane is angled slightly with respect to the plane of the CT images to provide increased spatial resolution in the axial direction.

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    Dosimetric images for a single-shot, 4-mm GK unit helmet radiation (left) and a four-arc 3-mm collimator configuration delivered by the dedicated LINAC (right).

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    Profiles were constructed in the x (horizontal) and y (vertical) dimensions from which the nominal spot width (FWHM) and penumbra (80—20% width) were subsequently determined.

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    Horizontal (upper) and vertical (lower) film dosimetry profiles for the 3-mm LINAC collimator and the 4-mm GK unit helmet.

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    Horizontal (upper) and vertical (lower) film dosimetry profiles for the 8-mm LINAC collimator and the 8-mm GK unit helmet.

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    Targeting accuracy images for a single-shot, 8-mm GK unit helmet radiation (left) and a four-arc, 8-mm collimator configuration delivered by the dedicated LINAC (right).

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    Coronal T1-weighted MR images obtained in the primate brain at the time of radiosurgery treatment planning (left) and 4 months after stereotactic radiosurgery (right). The 50% dose wash, corresponding to a dose of 75 Gy, is shown superimposed on the planning MR image. The arrow on the follow-up MR image points to the location of the enhancing area.

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