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Measurement of Secondary Neutrons Produced in the (gamma,n) Reaction From the TrueBeam Linac Head

M Harvey

M Harvey1*, J Pollard2, Z Wen3, S Gao4, (1) Texas Southern University, Houston, Texas, (2) The University of Texas M. D. Anderson Cancer, Houston, Texas, (3) The University of Texas M. D. Anderson Cancer, Houston, Texas, (4) MD Anderson Cancer Ctr., Houston, TX

SU-E-T-265 Sunday 3:00PM - 6:00PM Room: Exhibit Hall

Purpose: The purpose of this study was to measure secondary neutrons produced via the (γ,n) reaction in the linac head of the Varian TrueBeam system for 10 MV and 15 MV x-ray beams, respectively. Of note is that there is currently no measured data available in the literature on secondary neutron production from the TrueBeam treatment head.

Methods: The linac was operated in flattening filter-free (FFF) mode for the 10 MV beam and flattening filter (FF) mode for the 15 MV beam with the jaws and multileaf collimators in the fully closed position. A nominal setting of monitor units per minute (MU/min) were delivered for each beam energy and the secondary neutron dose rate was measured in triplicate using a LudLum neutron survey meter (model 12-4) positioned at multiple locations including isocenter, longitudinal (gun-target) and lateral to the isocenter.

Results: For the 15 MV x-ray beam (FF), 600, 300 and 100 MU/min were delivered. The secondary neutron dose rate peaked at a distance of 20 cm longitudinal to the isocenter at 600 MU/min and decreased with distance from the isocenter. Fluctuations in the longitudinal neutron dose rate were negligible; however, the lateral neutron dose rate varied up to nearly a factor of 2 away from the isocenter with decreasing MU/min. For the 10 MV x-ray beam (FFF), 2400, 1200 and 400 MU/min were delivered and the secondary neutron dose rate was again observed to peak around 20 cm longitudinal to the isocenter with fluctuations varying by about 50% with decreasing MU/min in positions lateral to the isocenter.

Conclusion: Our preliminary results suggest a secondary neutron dose rate on the order of a 45 mSv/hr or less for all measurements considered in this study indicating a relatively low contribution of produced secondary neutrons to the primary therapeutic beam.

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