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Impact of Carbon Fiber Couchtop On TrueBeam Flattening-Filter-Free (FFF) SRS/SBRT Radiation Therapy

A Le

A Le*, M Huq, S Jang, UPMC CancerCenter, Pittsburgh, PA

SU-E-T-676 Sunday 3:00PM - 6:00PM Room: Exhibit Hall

To evaluate the attenuation of Flattening-Filter-Free (FFF) SRS/SBRT beams resulting from transmission through the ExacTrac carbon fiber couchtop and to validate the couchtop model in Eclipse Ver.11.

A cylindrical phantom of 8cm-diameter was scanned with an ion chamber placed in the center. Radiation doses of static fields (3x3, 5x5, and 10x10 cm2) with gantry angles ranging from 100 to 180 degrees were computed for 6X, 6XFFF, 10X, 10XFFF, and 15 MV photon beams with and without ExacTrac couchtop. The calculated attenuation of FFF beams with small fields was compared with measured data. Skin-sparing effect with a posterior-anterior FFF beam was evaluated with TLDs placed underneath a rectangular solid-water phantom on top of the carbon couchtop.

The largest measured couch attenuation was 17.7 %for a gantry angle of 100 degree, 6MV-FFF mode and 3x3 cm2 field size, in comparison to 6.7 % calculated by Eclipse TPS. The Eclipse TPS underestimates the couch attenuation with the largest difference between measured and calculated dose of 10%. The effect of couch attenuation is more prominent at gantry angle close to 100 degree, FFF beams and smaller field size.

There will be a significant dose difference due to high couch attenuation of EXACTRAC carbon fiber couchtop. Furthermore, the virtual couch modeled in TPS (Eclipse v11, Varian) underestimates the couch attenuation for this couch. Therefore, precaution and careful QA should be considered, especially when treating patients with posterior target using small field SBRT techniques and FFF mode for low energy photon beam.

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