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Clinical Context of Neutron Dose Contribution From 10MV FFF Vs 15MV Photon Beams

J Vickress1,2*, G Zeng1, R Sankreacha1, (1) Trillium Health Partners/the Credit Valley Hospita, Mississauga, ON, CA (2) University of Toronto Department of Radiation Oncology, Toronto, Ontario, CA

Presentations

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

Room: AAPM ePoster Library

Purpose:
Neutron generation in the treatment head of a linear accelerators starts to occur with 10 MV photon beams. Neutron dose is important because it can be deposited outside of the treatment field, and up to 20 times more damaging to healthy tissue than MV photons. With flattening filter free 10 MV (10 MV FFF) being commissioned for highly modulated SBRT treatment, we measured the neutron dose per MU of 10FFF compared to 15MV to justify the clinical application.

Methods:
Neutron measurements were performed using a Ludlum Neutron meter (model 12-4) and two BD-PND bubble detectors from Bubble Technology Industries. All measurements were performed on a Varian TrueBeam. The jaws were set to 0.5cm x 0.5cm, dose rate 400 MU/min and gantry at 0 degrees. To test the neutron dose within the maze the neutron meter was placed at the inner entrance of the maze and measured the neutron dose per MU. Within the bunker the neutron dose was measured using bubble detectors at 2, 20 and 100 cm from iso along the treatment couch at 0 degrees.

Results:
The neutron dose per MU from the inner entrance of the treatment maze was 0.019 uSv/MU for 10 MV FFF vs. 0.33 uSv/MU for 15MV. The results within the treatment room were 0.63, 0.48, 0.4 uSv/MU for 10 MV FFF at 2, 20, 100 cm from iso respectively vs. 10.96, 7.3, 6.87 uSv/MU for 15MV.

Conclusion:
We have shown 10 MV FFF produces 17 times less neutrons per MU than 15 MV. Clinically, since 10 MV FFF is widely used in SBRT treatment, one can infer that the neutron dose in one fraction SBRT treatment with 4000MU of 10MV FFF is about the same as that of conventional treatment with 250MU of 15MV.

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Keywords

Radiation Protection, Radiation Dosimetry, Thermal Neutrons

Taxonomy

TH- External Beam- Photons: radiation protection and shielding

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