Deriving Planning Target Volume Margins for Intracranial Stereotactic Radiation Therapy

Purpose/Objective(s): To assess the need to add margin to gross target volume to account for setup inaccuracy and interfraction variations during hypofractionated intracranial radiosurgery (RS). Materials/Methods: Hypofractionated RS was performed with a vacuumassisted dental mold on a new stereotactic frame system for 12 targets in 12 patients. Patient setup measurements were performed with a digital probe and a stereotactic localizer box for 58 treatments (4-5 fractions for each target). Two to three sets of phantom measurements were performed for each target to assess patient-specific hardware alignment accuracy versus the radiologic center. Patient-specific dosimetric tolerable margins were determined for each treatment via simulating patient shifts by moving isocenters of all the radiation beams in increments of 0.2-0.3 mm along the x, y, and z directions until the resulting treatment plan had (1) less than 95% coverage of the target volume by the prescription dose, (2) more than 5% variation in conformity indices and/or (3) excessive maximum dose to critical structures (i.e., 10 Gy to brainstem or 8 Gy to optic chiasm). Results: The mean patient setup uncertainty for all 58 treatments was 0.29 0.20 (1s) mm, 0.25 0.26 mm, and 0.19 0.08 mm along the x-, y-, and z-axis, respectively. These uncertainties translated into 99% confidence levels of 0.88 mm, 0.99 mm, and 0.61 mm as calculated from the Cartesian Van Herk formula. The mean patient-specific hardware alignment uncertainty was 0.47 0.26 mm, resulting in whole-procedural 3D mean radiological shift vectors of 0.69 0.73 mm. The maximum dosimetrically tolerable shifts were determined to be 1.80 0.73 mm, 2.10 0.90 mm and 1.93 0.74 mm along the x-, y-, and z-axis, respectively. Conclusions: A one-millimeter margin is demonstrated to be clinically achievable and dosimetrically tolerable for multi-session hypofractionated RS. Author Disclosure: L. Ma: None. F. Shannon: None. D. Pinnaduwage: None. J. Nakamura: None. B. Igor: None. M. Michael: None. D.A. Larson: None. P.K. Sneed: None.