High-energy linacs produce secondary particles such as neutrons (photoneutron production). The neutrons have the important role during treatment with high energy photons in terms of protection and dose escalation. In this work, neutron dose equivalents of 18 MV Varian and Elekta accelerators are measured by thermoluminescent dosimeter (TLD) 600 and TLD700 detectors and compared with the Monte Carlo calculations. For neutron and photon dose discrimination, first TLDs were calibrated separately by gamma and neutron doses. Gamma calibration was carried out in two procedures; by standard 60Co source and by 18 MV linac photon beam. For neutron calibration by 241Am-Be source, irradiations were performed in several different time intervals. The Varian and Elekta linac heads and the phantom were simulated by the MCNPX code (v. 2.5). Neutron dose equivalent was calculated in the central axis, on the phantom surface and depths of 1, 2, 3.3, 4, 5, and 6 cm. The maximum photoneutron dose equivalents which calculated by the MCNPX code were 7.06 and 2.37 mSv.Gy-1 for Varian and Elekta accelerators, respectively, in comparison with 50 and 44 mSv.Gy-1 achieved by TLDs. All the results showed more photoneutron production in Varian accelerator compared to Elekta. According to the results, it seems that TLD600 and TLD700 pairs are not suitable dosimeters for neutron dosimetry inside the linac field due to high photon flux, while MCNPX code is an appropriate alternative for studying photoneutron production.

Medical linear accelerators, besides the clinically high energy electron and photon beams, produce other secondary particles such as neutrons which escalate the delivered dose. In this study the neutron dose at 10 and 18MV Elekta linac was obtained by using TLD600 and TLD700 as well as Monte Carlo simulation. For neutron dose assessment in 2020 cm2 field, TLDs were calibrated at first. Gamma calibration was performed with 10 and 18 MV linac and neutron calibration was done with 241Am-Be neutron source. For simulation, MCNPX code was used then calculated neutron dose equivalent was compared with measurement data. Neutron dose equivalent at 18 MV was measured by using TLDs on the phantom surface and depths of 1, 2, 3.3, 4, 5 and 6 cm. Neutron dose at depths of less than 3.3cm was zero and maximized at the depth of 4 cm (44.39 mSvGy-1), whereas calculation resulted  in the maximum of 2.32 mSvGy-1 at the same depth. Neutron dose at 10 MV was measured by using TLDs on the phantom surface and depths of 1, 2, 2.5, 3.3, 4 and 5 cm. No photoneutron dose was observed at depths of less than 3.3cm and the maximum was at 4cm equal to 5.44mSvGy-1, however, the calculated data showed the maximum of 0.077mSvGy-1 at the same depth. The comparison between measured photo neutron dose and calculated data along the beam axis in different depths, shows that the measurement data were much more than the calculated data, so it seems that TLD600 and TLD700 pairs are not suitable dosimeters for neutron dosimetry in linac central axis due to high photon flux, whereas MCNPX Monte Carlo techniques still remain a valuable tool for photonuclear dose studies.

Bremsstrahlung  X-rays  from  high-energy  linear    accelerators  (Linacs)  produce  neutrons  as  a  result  of  photonuclear  reactions  mainly  with  the  different  materials  constituting  the  accelerator  head.  The  neutrons  produced  during  high  energy  radiotherapy  should  be  considered  in  terms  of  protection  and  ose  escalation.  Due  to  the  very  intense  photon  component  in  the  Linac  field  causing  pulse  pile-up  and  dead-time  effects  in  detectors,  measurement  of  the  corresponding  neutron  dose  by  active  dosimeters  is  extremely  troublesome.  In  this  study,  the  neutron  probe  LB  6411,  which  active  detector  of  3He  proportional  counter  tube  was  replaced  with  the  passive  detectors  of  TLD600  and  TLD700,  has  been  used  to  perform  neutron  measurements  at  four  points  around  the  18  MV  Varian  2100C  Linac  facility.  The  neutron  dose  equivalent  at  the  distance  of  1  and  2  m  from  the  isocentre  on  the  patient  couch  was  obtained  2.2  and  0.75  mSv.Gy-1  respectively.  According  to  the  results,  the  dose  equivalent  from  emitted  photoneutrons  is  not  negligible  and  therefore  treatment  conditions  should  be  optimized.  The  results  of  this  study  emphasized  that  TLD600  and  TLD700  dosimeters  inside  the  polyethylene  sphere  of  neutron  probe  LB  6411  is  an  appropriate  choice  for  studying  photoneutron  production  in  the  vicinity  of  the  accelerator.