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3-D Reconstruction of Radiation Isocenter for Stereotactic Radiosurgery

Michael Carlson

M Carlson1*, G Luo2, A Nelson3, G Ding4, (1) Vanderbilt University, Nashville, TN, (2) Vanderbilt Univ Medical Ctr, NASHVILLE, TN, (3) Vanderbilt University, Nashville, TN, (4) Vanderbilt University, NASHVILLE, TN

MO-B-Salon EF-3 Monday 10:00:00 AM - 12:00:00 PM Room: Salon EF

Purpose: To develop software that performs quantitative analysis from portal image data of the radiation isocenter shape and position in space, relative to the linac in-room laser, for SRS treatments.

Methods: A suitable phantom BB, i.e. Winston-Lutz (W-L) phantom, was aligned to laser isocenter while a radiosurgery cone is centered on the gantry crosshairs. The largest cone available (15 mm) was used to produce a suitable outline around the phantom. Portal images were created in cine mode during a 360 degree arc. The developed software analyzes each phantom image displacement, then creates a 3D back-projected volume representing the average isofluence from a complete arc. The surface demarcated by 80% fluence intensity is used to represent mechanical/radiation isocenter. The program output includes shifts required to bring the mechanical and laser isocenters into coincidence. Tests were conducted to assess sensitivity, accuracy, and reproducibility of the method.

Results: The projected portal image pixel size at isocenter for 140cm SID is 0.5mm. The algorithm uses 50% intensity averages to find the cone and phantom centers to a measured precision of 43 microns for a static setup. This far exceeds measured radial magnitude of cone mount setup error which was found to be 0.37mm +/- 0.14mm. The software, from reconstruction of isocenter, was able to detect intentional phantom displacements (3.0mm, 2.0mm, and 1.0mm in the coronal, transverse, and sagittal dimensions respectively) to within a 0.3mm range of visual detectability.

Conclusion: The software analysis provides both more accurate quantitative data than the traditional W-L test and a 3D rendering of average radiation isocenter over an entire arc. As a result, it can quantitatively assess the degree of gantry mechanical displacement using the fluence back-projection, and generate laser shifts which allow for easy and accurate readjustment of linac room lasers.

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