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Transmission and Attenuation On 6DoF Couch Under All FFF and FF Photon Beams

R Foster* and M Xu, Northwestern Medicine Cancer Centers, Warrenville, IL and Geneva, IL

Presentations

(Sunday, 7/12/2020)   [Eastern Time (GMT-4)]

Room: AAPM ePoster Library

Purpose: The objective was to evaluate transmission/attenuation factors for all photon beams through 6DoF-couch in TrueBeam. The attenuation would have an impact on percentage depth dose and, therefore, skin dose dependent upon gantry-orientation.
Methods: A polystyrene-cylindrical-phantom (8-cm diameter) with an ionization-chamber(IC) at isocenter was positioned on the 6DoF-couch made of carbon fiber. A SAD setup with 6x6-cm² radiation field was employed for all IC charge measurements including 6, 10, 15-MV and, 6MV-FFF, 10MV-FFF photon beams. Transmission measurements were made using IC with gantry in posterior position-180° and 5 various angles with the photon beam passing through the couch. An evaluation following AAPM TG-176 guidelines for dosimetry of couch tops was made to assess modeled support structure within the TPS versus measurements acquired in this study.
Results: The transmission-factors(Tr) were obtained by taking a ratio of charges measured with the beam passing through couch over those without couch. The attenuation-factor(A) was derived from A = 1–Tr in percentage. The attenuation-factor was found as high as 7.00% in H3-location (thin portion of the couch) at 110° gantry angle with 6 MV-FFF beam. For other beams, the attenuation-factor were found to be 5.84% for 6 MV, 5.20% for 10 FFF MV, 4.53% for 10 MV, and 3.71% for 15 MV at 110°-gantry-angle, respectively. The transmission and attenuation were calculated on 6DoF(IGRT) couch provided by AAA algorithm, in which an excellent agreement in calculation to measurement was achieved with less than 0.65% difference found in Tr.
Conclusion: Transmission measurements on 6DoF-couch are conducted for all photon beams on TrueBeam. The attenuation-factor can reach 7.00% at gantry of 110° for 6 MV-FFF beam. Reliable values of Tr are obtained from both measurement and calculation. The couch structure should be included in a treatment planning and QA, especially in SRS and SBRT special procedures.

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Keywords

Transmission Measurements, Treatment Planning

Taxonomy

TH- External Beam- Photons: Quality Assurance - Linear accelerator

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