Results

Different parts of simulated Varian Clinac 2100 C/D linac producing 18 MV photon beams are depicted in Figure 2.

Figure 2.

Simulated 18 MV Varian Clinac 2100 C/D linac

PDD and beam profile obtained by Semiflex ion chamber and MCNPX calculations for 10 × 10 cm² field size are shown in Figure 3. Additionally, Table 1 reports the results of validation and error calculation of this comparison.

Figure 3.

Comparison of Semiflex ion chamber measurements and MC calculations for 10X10 cm². a: PDD curve, b: beam profile

Table 1.

Simulation validation parameters: ion chamber measurements versus MC simulation

All results of MCNP are calculated per source initial particles (number of electrons prior to the target as number of histories). Thus, analysing these results in radiotherapy situation, normalisation of photoneutron production per Gy of delivered photon absorbed dose at each field size was required. Therefore, number of neutron, neutron equivalent dose, and neutron absorbed dose were normalised to the 1 Gy of photon dose in the maximum depth dose (d_max). ^{Reference Martínez-Ovalle, Barquero, Gómez-Ros and Lallena14,Reference Ghorbani, Vejdani Noghreiyan, Tabatabaei, Pakravan and Davenport15} With such method of calculation, the results of this study are comparable to the treatment situation. Table 2 shows the maximum photon dose at (d_max) for small field sizes shaped by secondary collimator jaws and circular cones.

Table 2.

Photon absorbed dose at (d _max) for small field sizes shaped by secondary collimator jaws and circular cones ^{Reference Hosseinzadeh, Banaee and Nedaie7}

Figure 4 shows the neutron fluence per Gy of photon dose along the central axis for different field sizes shaped by jaws and circular cones. The number of neutrons per Gy of photon dose in secondary collimator for small fields is more than those of circular field. There are two reasons that number of neutrons per Gy of photon is less for circular cones:

1. (a) Circular cones and secondary collimators are made of lead and tungsten, respectively. Therefore, photons are more attenuated by lead (circular cone) compared to tungsten (secondary collimator). Attenuated photons produce less number of neutrons per Gy of photon. Therefore, neutrons per Gy of photon dose in secondary collimator for small fields are more than those of circular field.

2. (b) The comparison of number of neutrons per Gy of photon is done for nominally same field sizes (e.g., 1 cm² cone versus 1x1 cm² collimator). However, the area of these field sizes is not same and secondary collimator creates bigger field size compared to corresponding cone field. And as the field size increases, number of neutron per Gy of photon decreases which is consistent with previous publications. ^{Reference Hosseinzadeh, Banaee and Nedaie7}

Figure 4.

Number of neutrons per Gy of photon dose for different field sizes along the central axis

Table 3 indicates neutron fluence for small field sizes shaped by secondary collimator jaws and circular cones. As it is shown, neutron fluence increased to a specific depth of phantom and then decreased monotonically by increasing depth. The largest value of neutron fluence appeared at depth of 2 cm in all field sizes, and as the field size decreases, neutron fluence increases. According to the neutron fluence in Figure 4, number of neutron per Gy of photon dose in secondary collimator small fields is more than those of circular cones.

Table 3.

Number of neutrons per Gy of photon dose for different field sizes ^{Reference Hosseinzadeh, Banaee and Nedaie7}

Figure 5 indicates the neutron equivalent dose along the central axis for different field sizes shaped by collimator jaws and circular cones. Table 4 shows the comparison of neutron equivalent doses for various field sizes. These results show that neutron equivalent dose has been decreased by increasing depth of phantom, and also the smallest field size has the largest value of neutron equivalent dose. Additionally, neutron equivalent dose in secondary collimator is more than circular cone fields. It shows that the cross-section of photoneutron interaction with high Z materials of cone and secondary collimator increases at smaller fields. Although the intensity of photons decreases in small fields, due to the absorption of low-energy photons inside the collimators, beam hardening occurs. Through inelastic scattering of neutrons with the internal wall of the collimator, more photons are produced and these cause the increase of neutron equivalent dose for smaller field sizes. Also as mentioned before, the comparison of neutron parameters is done for nominally same field sizes, and it should be noted that the area of these field sizes is not same and secondary collimator creates bigger field size compared to the corresponding cone field. Therefore, these reasons lead to larger equivalent dose with the secondary collimators than circular fields.

Figure 5.

Comparison of neutron equivalent doses for various field sizes along the central axis

Table 4.

Neutron equivalent dose for different field sizes ^{Reference Hosseinzadeh, Banaee and Nedaie7}

Figure 6 shows the neutron absorbed doses for different field sizes along the central axis. As it is indicated, neutron absorbed dose similar to neutron equivalent dose reaches its maximum value at the surface of the phantom and then decreases monotonically by increasing depth. Similar to equivalent dose and neutron fluence, absorbed dose in secondary collimator is more than those of circular cones. Table 5 compares neutron absorbed dose quantitatively for different field sizes. Therefore, similar to neutron equivalent dose, the photoneutron interactions increase in smaller fields and consequently the neutron absorbed dose increases in smaller fields.

Figure 6.

Neutron absorbed dose along the central axis for different field sizes

Table 5.

Comparison of neutron absorbed doses for different field sizes ^{Reference Hosseinzadeh, Banaee and Nedaie7}

As it is shown in Tables 3, 4, and 5, all neutron parameters are bigger at central axis of beam created by secondary collimator than those of circular cones. In secondary collimator and circular cone fields, photoneutron production per Gy of photon dose increases by decreasing field size. It seems that circular cones reduce the amount of neutron in central axis.