A quality factor to compare the dosimetry of gamma knife radiosurgery and intensity-modulated radiation therapy quantitatively as a function of target volume and shape

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✓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.

Article Information

Address reprint requests to: Jonathan A. Borden, M.D., Department of Neurosurgery, Proger 7, New England Medical Center, 750 Washington Street, Boston, Massachusetts 02111. email: jborden@mediaone.net.

© AANS, except where prohibited by US copyright law.

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Figures

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    Graphs showing the QF15 (y axis) as a function of VT and complexity and modality. The VTs (x axis) of 0.2 cm3, 0.5 cm3, 1.5 cm3, and 10 cm3 were used. Gamma knife, Beak IMRT (4 × 10—mm collimator array), conventional IMRT (10 × 10—mm collimator array) and LINAC radiosurgery systems were compared. Upper Left: The QF15 compared with VT for the spherical target. Upper Right: The QF15 compared with VT for the ellipsoid target. Lower Left: The QF15 compared with VT for the AVM/star target. Lower Right: The QF15 compared with VT for the horseshoe target. For each graph the QF is plotted as a function of VT.

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    Graphs showing the QF12 (y axis) as a function of VT and complexity and modality. The VTs (x axis) of 0.2 cm3, 0.5 cm3, 1.5 cm3, and 10 cm3 were used. Gamma knife, Beak IMRT, conventional IMRT, and LINAC radiosurgery systems were compared. Upper Left: The QF12 compared with VT for the spherical target. Upper Right: The QF12 compared with VT for the ellipsoid target. Lower Left: The QF12 compared with VT for the AVM/star target. Lower Right: The QF12 compared with VT for the horseshoe target. For each graph the QF is plotted as a function of VT.

  • View in gallery

    Graph showing the gamma knife QF12 as a function of target complexity. The VT sets (0.5, 1.5, and 10 cm3) are plotted individually.

  • View in gallery

    Upper: The Beak IMRT QF12 is plotted as a function of target complexity. The VT sets (0.5, 1.5, and 10 cm3) are plotted individually. Center: The IMRT QF12 is plotted as a function of target complexity. The VT sets (0.5, 1.5, and 10 cm3) are plotted individually. Lower: The LINAC QF12 is plotted as a function of target complexity. The 10-cm3 VT set was used.

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