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Neutron Dose Measurements in the Elekta MR-Linac Using Bubble Detectors

B Keller1,2*, H Nusrat3 , S Ahmad1,2 , S Al-Ward1 ,A Sahgal1,2, A Sarfehnia1,2 , S Breen1,2 , G Pang1,2 , (1) Sunnybrook Health Sciences Centre, Toronto, Canada (2) University of Toronto, Department of Radiation Oncology, Toronto, Canada (3) Ryerson University, Toronto, Canada


(Sunday, 7/29/2018) 1:00 PM - 1:55 PM

Room: Davidson Ballroom A

Purpose: A commercial MR-linac (MRL) treatment machine (Elekta AB,Stockholm,Sweden) is in production to provide real time MRI-guided adaptive radiotherapy. The purpose of this work is to measure both the in-field and out-of-field neutron contamination and compare this with a conventional linac operating in the 6 to 18 MV energy range.

Methods: Two types of bubble detectors (Bubble Tech Industries,Chalk River,ON) were used to measure neutron doses: BD-PND (fast neutrons) and BDT (thermal neutrons). Profile and depth dose measurements were done in the bore of the MRL (7.2 MV) with no magnetic field and on the tabletop of an Elekta Versa HD for 6 and 18MV using solid water and field sizes=10x10 and 2.5x2.5cm². Some irradiations were done using an Elekta Gamma Knife (Co60) to ensure bubble formation was due only to neutrons. Measurements were normalized per Gy of photon dose under reference conditions.

Results: The total neutron dose at the isocentre of the MRL for a 10x10cm² field size at the surface, in mSv/Gy, was 0.202±0.041 (0.185 fast and 0.017 thermal) compared with 5.0±0.45 (18MV) and 0.086±0.010 (6MV). MRL neutron doses outside of the primary field were of the order of 0.001 mSv/Gy. The neutron doses on cax at depth 10 cm were: 0.096±0.01 (MRL), 0.035±0.004 (6MV), and 1.615±0.162 (18MV) in mSv/Gy. For the 2.5x2.5cm² FS on cax, the MRL neutron dose was reduced by 17%. Exposure of the detectors in the Gamma Knife yielded no neutron dose.

Conclusion: The measured in-field neutron dose for the MRL is about 25 times less compared to an 18 MV x-ray beam and well within safe limits. The thermal neutron dose in the primary beam of the MRL contributes about 7% of the total neutron dose. The MRL neutron doses outside of the primary beam, but within the MRL bore, were negligible.


Radiation Protection


TH- External beam- photons: Contaminant neutron/out of field dosimetry

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