(Saturday, 4/7/2018)
Room: Foyer
Purpose: Stereotactic Radiosurgery (SRS) utilizes precise patient positioning and a higher single radiation dose fraction delivered to a focused area. However, due to highly irregular treatment fields with steep dose gradients, it is challenging to perform patient specific quality assurance (PSQA) using conventional methods, e.g. using two-dimensional (2D) diode arrays. EBT3 films with submillimeter resolution have been used to resolve this problem. To increase confidence in SRS plans, it is important to evaluate dose difference in 3D. In this study the PRESAGE 3D dosimeter was used to conduct PSQA of a SRS plan to treat a spinal metastasis with field sizes as small as 1.5 1.5 cm2.
Methods: This work consists of four major compnents, (1) Simulation: CT images of a 3D dosimeter with metal ball bearings were acquired using a Siemens Somatom CT scanner and imported into Varian Eclipse for verification plan generation. (2) Irradiation: The verification plan was delivered on a Varian TrueBeam linear accelerator to the dosimeter. (3) Dose reconstruction: The irradiated dosimeter was scanned using a parallel beam optical scanner with submillimeter resolution and images were reconstructed using filtered back-projection. (4) Analysis: 3D gamma analysis was performed and a 3D view of planned and measured dose distributions with failed dose points marked was created using MATLAB.
Results: The passing rates of 3D gamma comparison are 97.1% and 85.4% with criteria of 3%, 3 mm and 2%, 2mm, respectively. Comparison of the reconstructed 2D image at the isocenter with the planned dose using DoseLab yielded a passing rate of 98.4% with 2%, 2mm criteria. 3D visualization of failed dose points relative to organs at risk can be clinically used to inform the decision to accept a plan.
Conclusion: This study demonstrates the capabilities of the PRESAGE 3D dosimeter for PSQA of clinical SRS plans.
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