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.

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.

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.

Comparing the results with those in the presence of circular cones showed that circular cones reduce photoneutron production for the same nominal field sizes.

Spatially fractionated Grid radiation therapy (SFGRT) in an effective technique for bulky and radio-sensitive tumours. SFGRT using a constructed block has been used to evaluate the photon and photo-neutron (PN) dose measurement in 18-MV photon beam energy.

A mounted Grid block on to a Varian Clinac 2100c linear accelerator was used to perform photon dosimetry. The percentage depth dose, in-plane and cross-plane beam profile and output factor was measured by ionization chamber in water. The PN contamination was measured after photon dosimetry using the combination of thermoluminescence dosimetry types 600 and 700, and Polycarbonate Film dosimeters on the surface and in the maximum depth dose (dmax) of solid water™ slabs.

The valley-to-peak ration for 6 and 18 MV photon beams obtained from the beam profiles was ~35 and 72%, respectively. Fast and thermal PN equivalent dose decreased in the Grid field compared to an open field (without Grid).

The Grid therapy dosimetry compared to the conventional radiotherapy (without the grid) the production of fast and thermal neutrons were reduced. Using of a Grid block in high-energy photon beams for a long period of the treatment continuously might be a new source of contamination due to the interaction of photon beam resulting the activation of the Grid block