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Impact of Treatment Couch On Beam Attenuation

A Khan

A Khan*, L Kim, N Yue, The Cancer Institute of New Jersey, New Brunswick, NJ

SU-E-T-47 Sunday 3:00:00 PM - 6:00:00 PM Room: Exhibit Hall

To investigate beam attenuation from the treatment couch.

A rectangular water phantom was generated using the Eclipse TPS and placed such that its centroid was at the isocenter. A couch structure was inserted with the default CT values. A 6MV photon beam with a square 10cm field size, normalized to deliver 100cGy at the isocenter was used to determine the MUs required with and without the couch. This was repeated for gantry angles between 90 and 180 degrees.
The couch structure was then exported into MATLAB and a script written to trace a ray from the gantry to the isocenter and the path length through various parts of the couch determined for a given couch shift and gantry angle. Using the CT values, the relative electron and mass densities were extracted from Eclipse and used to determine the equivalent path length in water. The beam attenuation was then determined using this length and the mass attenuation coefficient for water obtained from the NIST database.

For the same couch shift, the beam attenuation was found to be in close agreement between both methods. The peak attenuation for a couch shift of 5cm in y, occurred at a gantry angle of 110 degrees and was found to be 19.3% using either method. This was also in close agreement with ion chamber measurements.

This approach can easily be applied to different couch and beam energies. It could also have direct clinical impact by allowing therapists to simply enter the couch shift, beam angle and energy to determine if adjustments are necessary on the day of treatment. Furthermore if the couch rail position could be obtained directly by the system, this calculation could be done in real time and the beam attenuation displayed with some threshold set as an interlock.

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