SU‐E‐T‐480: Radiobiological Dose Comparison of Single Fraction SRS, Multi‐Fraction SRT and Multi‐Stage SRS of Large Target Volumes Using the Linear‐Quadratic Formula

Purpose: To compare the radiobiological effect on large tumors and surrounding normal tissues from single fraction SRS, multi‐fractionated SRT, and multi‐staged SRS treatment. Methods: An anthropomorphic head phantom with a centrally located large volume target (18.2 cm 3 ) was scanned using a 16 slice large bore CT simulator. Scans were imported to the Multiplan treatment planning system where a total prescription dose of 20Gy was used for a single, three staged and three fractionated treatment. Cyber Knife treatment plans were inversely optimized for the target volume to achieve at least 95% coverage of the prescription dose. For the multistage plan, the target was segmented into three subtargets having similar volume and shape. Staged plans for individual subtargets were generated based on a planning technique where the beam MUs of the original plan on the total target volume are changed by weighting the MUs based on projected beam lengths within each subtarget. Dose matrices for each plan were export in DICOM format and used to calculate equivalent dose distributions in 2Gy fractions using an alpha beta ratio of 10 for the target and 3 for normal tissue. Results: Singe fraction SRS, multi‐stage plan and multi‐fractionated SRT plans had an average 2Gy dose equivalent to the target of 62.89Gy, 37.91Gy and 33.68Gy, respectively. The normal tissue within 12Gy physical dose region had an average 2Gy dose equivalent of 29.55Gy, 16.08Gy and 13.93Gy, respectively. Conclusion: The single fraction SRS plan had the largest predicted biological effect for the target and the surrounding normal tissue. The multi‐stage treatment provided for a more potent biologically effect on target compared to the multi‐fraction SRT treatments with less biological normal tissue than single‐fraction SRS treatment.