SU‐E‐T‐638: Optimization of Prescription Isodose Selection for Stereotactic Radiation Therapy: Conventional Linac vs. Cyberknife

Purpose: To determine the optimal prescription isodose line that minimizes normal tissue dose for stereotactic irradiation of intracranial targets comparing conventional linear accelerator and robotic (Cyberknife) delivery. Material and Methods: An anthropomorphic head phantom with a centrally located target was scanned using a 16 slice large bore CT simulator. Two concentric 5mm spherical shells of normal tissue, the first from the target surface to 5 mm radically, and the other from 5 mm to 10mm, were identified/contoured around the target. For linac planning, 9 co‐planar, non‐opposing 3D conformal beams were designed for conformal dose delivery. By manipulating the block margin, varying prescription isodose lines were used to cover exactly 95% of tumor. After linac planning, the same CT images and contours were sent to the Cyberknife for treatment planning. Multiple plans were generated with different prescription isodose lines covering exactly 95% of the target. gEUD (generalized effective uniform dose) was used to evaluate normal tissue irradiation within the shells using different parameters for parallel and serial tissues. Results: For linac plans, the results show that the optimal prescription isodose line yielding maximum normal tissue sparing is located between 50%∼60% of the maximum dose when the organ displays parallel behavior. For serially functioning tissues, the optimal prescription isodose line increases to approximately 80%. The gEUD of normal tissue (predisposing to toxicity) increases greatly when prescription isodose lines lower than 50% or higher than 80% are used. For Cyberknife plans, the optimal prescription isodose line lies near 50% for tissue for both either parallel or serial structures. These results are consistent for different tumor sizes. Conclusion: Both conventional linac and robotic delivery provide conformal tumor coverage with excellent normal tissue sparing. To achieve optimal normal tissue sparing, a lower isodose line should be prescribed for Cyberkinfe® than for a conventional linac.