Monte Carlo calculation of photo-neutron dose produced by circular cones at 18MV photon beams

Abstract

Aim

The aim of this study is to calculate neutron contamination at the presence of circular cones irradiating by 18[[ce:hsp sp="0.25"/]]MV photons using Monte Carlo code.

Background

Small photon fields are one of the most useful methods in radiotherapy. One of the techniques for shaping small photon beams is applying circular cones made of lead. Using this method in high energy photon due to neutron contamination is a crucial issue.

Materials and methods

Initially, Varian linac producing 18[[ce:hsp sp="0.25"/]]MV photons was simulated and after validating the code, various circular cones were also simulated. Then, the number of neutrons, neutron equivalent dose and absorbed dose per Gy of photon dose were calculated along the central axis.

Results

Number of neutrons per Gy of photon dose had their maximum value at depth of 2[[ce:hsp sp="0.25"/]]cm and these values for 5, 10, 15, 20 and 30[[ce:hsp sp="0.25"/]]mm circular cones were 9.02, 7.76, 7.61, 6.02 and 5.08 (n[[ce:hsp sp="0.25"/]]cm⁻²[[ce:hsp sp="0.25"/]]Gy⁻¹), respectively. Neutron equivalent doses per Gy of photon dose had their maximum at the surface of the phantom and these values for mentioned collimators were 1.48, 1.33, 1.31, 1.12 and 1.08 (mSv[[ce:hsp sp="0.25"/]]Gy⁻¹), respectively. Neutron absorbed doses had their maximum at the surface of the phantom and these values for mentioned collimators sizes were 103.74, 99.71, 95.77, 81.46 and 78.20 (μGy/Gy), respectively.

Conclusions

As the field size gets smaller, number of neutrons, equivalent and absorbed dose per Gy of photon increase. Also, neutron equivalent dose and absorbed dose are maximum at the surface of phantom and then these values will be decreased.