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Evaluating Treatment Couch Dosimetric Effects in Spine SBRT Treatment Planning

B Winey

B Winey*, J Wolfgang, C Speier, B Crawford, Massachusetts General Hospital and Harvard Medical School, Boston, MA

SU-E-T-179 Sunday 3:00PM - 6:00PM Room: Exhibit Hall

Purpose: To evaluate the dosimetric effects of the treatment couch on skin dose and field attenuation for Spine SBRT.

Methods: Measurements were acquired with an Elekta Synergy S in a 10.4 cm x 10.4 cm open 6 MV field with a Marcus parallel plate ionization chamber for varying thickness of water equivalent plastic. Measurements were repeated with and without the Hexapod carbon fiber treatment couch. A program was written to superimpose a CT scan of the treatment couch onto the patient planning CT. Dose calculations were performed with and without the treatment couch in a solid water phantom in Raystation Treatment planning software and compared to the measured values. All doses are in reference to Dmax of the open field without couch.

Results: The measurements displayed a 69% higher dose (93% versus 24%) at the surface with the addition of the couch due to the electron contamination from the couch. Our beam model in Raystation underestimates the surface dose by 10% and agreement is achieved at 7 mm and within 4% at 4 mm depth. At depths greater than Dmax, the couch introduced an attenuation factor of 2%.

Conclusions: A carbon fiber couch significantly affects the surface dose for beams that pass through the couch. The couch introduces electron contamination that increases the dose by 69% at the surface. High doses to the skin can cause skin erythema. Couch dose enhancement combined with high dose fractions and several beams passing through the couch for spine SBRT treatments should be accurately modeled to evaluate the actual dose delivered to the skin.

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