The measurement of photoneutron dose around an 18 MV Varian linear accelerator by TLD600 and TLD700 dosimeters inside the polyethylene sphere of neutron probe LB 6411
Introduction:
Advanced treatment modalities involve applying small fields which might be shaped by collimators or circular cones. In these techniques, high-energy photons produce unwanted neutrons. Therefore, it is necessary to know neutron parameters in these techniques.
Materials and methods:
Different parts of Varian linac were simulated by MCNPX, and different neutron parameters were calculated. The results were then compared to photoneutron production in the same nominal fields created by circular cones.
Results:
Maximum neutron fluence for 1 × 1, 2 × 2, 3 × 3 cm2 field sizes was 165, 40.4, 19.78 (cm–2.Gy-1 × 106), respectively. The maximum values of neutron equivalent doses were 17.1, 4.65, 2.44 (mSv/Gy of photon dose) for 1 × 1, 2 × 2, 3 × 3 cm2 field size, respectively, and maximum neutron absorbed doses reached 903, 253, 131 (µGy/Gy photon dose) for 1 × 1, 2 × 2, 3 × 3 cm2 field sizes, respectively.
Conclusion:
Comparing the results with those in the presence of circular cones showed that circular cones reduce photoneutron production for the same nominal field sizes.
Introduction:
Advanced treatment modalities involve applying small fields which might be shaped by collimators or circular cones. In these techniques, high-energy photons produce unwanted neutrons. Therefore, it is necessary to know neutron parameters in these techniques.
Materials and methods:
Different parts of Varian linac were simulated by MCNPX, and different neutron parameters were calculated. The results were then compared to photoneutron production in the same nominal fields created by circular cones.
Results:
Maximum neutron fluence for 1 × 1, 2 × 2, 3 × 3 cm2 field sizes was 165, 40.4, 19.78 (cm–2.Gy-1 × 106), respectively. The maximum values of neutron equivalent doses were 17.1, 4.65, 2.44 (mSv/Gy of photon dose) for 1 × 1, 2 × 2, 3 × 3 cm2 field size, respectively, and maximum neutron absorbed doses reached 903, 253, 131 (µGy/Gy photon dose) for 1 × 1, 2 × 2, 3 × 3 cm2 field sizes, respectively.
Conclusion:
Comparing the results with those in the presence of circular cones showed that circular cones reduce photoneutron production for the same nominal field sizes.
Using high-energy photon beams is one of the most practical methods in radiotherapy treatment of cases in deep site located tumors. In such treatments, neutron contamination induced through photoneutron interaction of high energy photons (>8 MeV) with high Z materials of LINAC structures is the most crucial issue which should be considered. Generated neutrons will affect shielding calculations and cause extra doses to the patient and the probability of increase induced secondary cancer risks. In this study, different parameters of neutron production in radiotherapy processes will be reviewed.
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