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Commissioning Experience for the New Gamma Beam 500 Co-60 TBI Irradiator

J Bredfeldt

J Bredfeldt*, R Zhang , E Huynh , W Cai , C Williams , Y Lyatskaya , M Czerminska , R Cormack , Brigham and Women's Hospital, Boston, MA


SU-I-GPD-T-498 (Sunday, July 30, 2017) 3:00 PM - 6:00 PM Room: Exhibit Hall

Purpose: To report our commissioning experience and results for a dedicated commercially available Co-60 total body irradiation (TBI) system.

Methods: The Gamma Beam 500 is a Co-60 irradiator (Best Theratronics, Ottawa, Ontario) designed for large field treatments. The source can be positioned 2.5m above the floor. Strict TG-51 dose calibration cannot be performed due to fixed field size. We therefore follow AAPM report 17 by performing dose calibration at mid-plane in a 30X34X20cm 1D-scanning water phantom with corrections for scatter from the scanner and tank walls. To calculate dose to patient mid-plane, a body separation correction is used which relates dose for the current patient separation to the separation of the reference condition. Body separation and body size factors were measured in solid water and water. Scatter from the floor and the head of the machine was evaluated using in-air measurements. Transmission through lead lung block sheets was measured in solid water at 10cm depth. Backscatter from the couch was evaluated for 30X30cm and 60X60cm phantoms.

Results: Beam area with 5% flatness at 217 cm from the source in air was measured to be 180X64cm. Beam symmetry under similar circumstances along the cardinal axes of the beam was <1.5%. The body separation factor varied from 1.14 to 0.73 over a separation range of 8 to 40cm. Body size correction varied approximately linearly from 1.0 to 1.027 for equivalent square phantom sizes of 30 to 60cm. Tank wall and scanner corrections were 1.2% and 0.25% respectively. Measurements deviated from inverse-square by <3% for SSDs within 145-225cm. Lead transmission was 0.85 for a 1/8” lead lung block.

Conclusion: We have commissioned a dedicated Co-60 TBI system for clinical use and developed a method for TBI dose calculation that accommodates a wide range of patient sizes.

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