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Orfit Attenuation Measurement and Clinical Correction

W Feng1,2*, F Zhang1 , H Sha1 , A Chu3 , (1) Bayhealth Medical Center, Dover, DE, (2) St. Barnabas Medical Center, Livingston, NJ, (3) Stony Brook University, Stony Brook, NY


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

Room: Exhibit Hall

Purpose: To quantify the attenuation of Orfit headrest with CT, chamber and mapcheck, and to test clinical correction method.

Methods: Orfit high density and low density headrests and base plate are evaluated in the study. PTW N30013 Farmer chamber and Standard Imaging SuperMax electrometer are used for chamber measurement, radiation dose is measured at depth 1.5cm in CRIS plastic water with Trubeam 06X 10x10cm beam. SunNuclear Mapcheck 2 is used for two 2D measurement, thinnest part of headrest #5 is center with mapcheck isocenter and measured with Trubeam 06X 30x30cm beam. Varian Eclipse AAA algorithm is used for dose calculation and correction, IGRT couch is also added in the external beam planning for dose calculation. Since only CT HU inside of body contour is included for dose calculation, dose calculation with fixed MU, DVH difference are evaluated with original body contour and with body contour extended to include Orfit.

Results: CT HU of Orfit High Density headrest is -125 at surface and -885 inside.With chamber measurement, Orfit High Density headrest attenuates 2.8% at center and 4.2 at edge; orfit base plate has max attenuation of 7.3% at grip hold edge.With Mapcheck 2D measurement, the max attenuation is 10.74% for high density headrest and 1.25% for low density headrest; minimum attenuation is 0.66% for high density headrest and 0.15% for low density headrest.With Eclipse AAA algorithm dose calculation, PTV mean dose is 2.1% less with high density headrest

Conclusion: Even Orfit high density headrests have low CT HU internally, attenuation is as high as 2.1% due to its size, it should not be ignored during treatment planning. It is recommended that extending the body contour to cover headrest and Orfit base plate so as to include the Orfit in dose calculation and correct its attenuation.


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