Comparison of neutron contamination in small photon fields of secondary collimator jaws and circular cones

Abstract 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 cm 2 field sizes was 165, 40.4, 19.78 (cm –2 .Gy -1 × 10 6 ), 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 cm 2 field size, respectively, and maximum neutron absorbed doses reached 903, 253, 131 (µGy/Gy photon dose) for 1 × 1, 2 × 2, 3 × 3 cm 2 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.