(Sunday, 7/29/2018) 3:00 PM - 6:00 PM
Room: Exhibit Hall
Purpose: SRS treatments with cones deliver large doses with steep dose gradients and dose fall off. Positional accuracy, detector resolution, and calculation resolution are very important in conducting dose measurements and calculations. End to end tests will provide the best understanding of the total cone based SRS program.
Methods: Two treatment planning systems ‘Varian Eclipse Cone Planning’ and ‘Brainlab iPlan’ are evaluated based on the SRS cone beam models, dose calculation algorithms, and dose resolution. The calculated doses are compared with measured outputs in liquid water phantom setup, film (EBT-XD) based planar dose gamma analysis in head phantoms, and diode measurements in solid water and liquid water phantoms for non-coplanar arc plans simulating SRS cone patient treatment plans.
Results: The output factors compared within 3% between treatment planning systems and diode measurements. The uncertainty in the dose measured with diode was evaluated (ref: IAEA Techical Report #483) in this study. The End to End tests measurement results compared well with 90% passing points for film based measurements with gamma criteria of 3% 1mm. Diode measurements compared within 5% for 40 degree arcs. Further, the uncertainty in the ISO center at different couch and gantry angles translates into uncertainty in dose measured with diodes for small cones e.g. 5mm and 4mm. Output factors calculated in Eclipse cone planning system are sensitive to dose matrix resolution for smaller size cones. It is important to note that heterogeneity is accounted for in iPlan and not in Eclipse cone planning.
Conclusion: For a given clinical LINAC system, the accuracy of treatment delivery is to be established based on the End to End test results. Lack of End to End tests for SRS cone programs will potentially result in inferior quality of the program and larger uncertainties in dose cloud position in space.
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