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Dosimetric Effects of the Elekta IBEAM Evo Couchtop On Stereotactic Body Radiation Therapy (SBRT) Plans

Z Karakas1*, J Pursley2 , T Zhu1 , (1) University of North Carolina, Chapel Hill, NC, (2) Massachusetts General Hospital, Salem, MA


(Sunday, 7/29/2018) 3:00 PM - 6:00 PM

Room: Exhibit Hall

Purpose: To model the Elekta iBEAM evo Couchtop and quantify its dosimetric effects on SBRT treatment plans calculated with RayStation TPS.

Methods: CT images of the Elekta iBEAM evo Couchtop were acquired. The outer Carbon fiber and inner foam layers were segmented and imported into RayStation TPS as a structure template to create the couch model. Beam attenuation through the couch was measured with an ion chamber at different gantry angles, and compared to calculated values in RayStation. Changes in PTV coverage and maximum skin dose were analyzed with the couch included for 21 SBRT plans (6 lung, 10 bone, 3 abdomen, and 2 extremity). The impact of couch inclusion on QA pass rates was evaluated with ArcCheck and high-dose region ion chamber measurements.

Results: Based on CT numbers, the outer and inner layer of the couch model was assigned a mass density of 0.595 and 0.066 g/cm³, respectively. The value for Carbon fiber differed from manufacturer specifications of 1.18-1.22 g/cm³. Measurements of beam attenuation through the couch were within 2% of RayStation calculations, including measurements made with worst-case scenario oblique incident beams. In terms of dosimetric effects, the couch reduced PTV coverage and increased maximum skin dose with decreasing distance between the target and couch surface. For targets within 5 cm of the couch, PTV coverage decreased by a mean of 3%, while maximum skin dose rapidly increased up to approximately 20%. Ion chamber measurements showed better agreement with couch inclusion, particularly for beams passing mostly through the couch, but gamma pass rates with the ArcCheck did not change significantly.

Conclusion: The treatment couch can significantly affect PTV coverage and skin dose, and should be included in SBRT treatment planning for targets close to the couch surface.


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