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The influence of field size and off-axis distance on photoneutron spectra of 18 mMV Siemens Oncor linear accelerator beam

Introduction At present, high energy electron linear accelerators (LINACS) producing photons with energies higher than 10 MeV have a wide use in radiotherapy (RT). However, in those beams fast neutrons could be generated which results in undesired contamination of the therapeutic beams (1, 2). These neutrons affect the shielding requirements in RT rooms (1) and also increase the out-of-field radiation dose to patients (2). Neutron flux becomes even more important when high numbers of monitor units (MU) are used as in the intensity modulated radiotherapy (IMRT) (2). Herein, to evaluate the exposure of patients and medical personnel, it is important to determine the full radiation field correctly. Methods and materials A model of the dual photon beam medical linear accelerator (LINAC), Siemens ONCOR, used at the University hospital Osijek was built using the MCNP611 code (3). We tuned the model according to measured photon percentage depth dose (PDD) curves and profiles. Only 18 MV photon beams were modeled and neutron contamination in patient plane simulated. Since the main interest of our study was the neutron contamination, the accuracy of the modeled photon PDD curves and beam profiles was not essential, so a 3% discrepancy was accepted in the high dose region and 20% in the low dose region. The dependence of neutron dose equivalent and energy spectrum on field size and off-axis distance in the patient plane was analyzed. Results The neutron source strength (Q) defined as a number of neutrons coming from the head of the treatment unit per x-ray dose (Gy) delivered at the isocenter was calculated and found to be 1.12·10 12 neutrons per photon Gy at isocenter. This is comparable with results found in references (4). The simulation showed that the neutron flux increases with increasing field size but field size has almost no effect on the shape of neutron dose profiles. The calculated neutron dose equivalent of different field sizes was between 1 and 3 mSv per photon Gy at isocenter. The mean energy changed from 0, 24 MeV to 0, 44 MeV with collimator opening from 0x0 cm 2 to 40x40 cm 2 . At 50 cm off-axis the change was less pronounced. Discussion According to the results, it is reasonable to conclude that the neutron dose equivalent to the patient is proportional to the photon beam-on time as suggested (2). Since the beam-on time is much higher when advanced radiotherapy techniques are used to fulfill high conformity demands, this makes the neutron flux determination even more important. We also showed that the neutron energy in the patient plane significantly changes with field size. This can introduce significant uncertainty in dosimetry of neutrons due to strong dependence of the neutron detector response on the neutron energy in the interval 0.1-5 MeV

Radiotherapy ; neutron contamination ; Monte Carlo

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