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The effect of external wedge on the photoneutron dose equivalent at a high energy medical linac

Hashemi , S. M., Raisali G., Taheri M., Majdabadi A., Ghafoori M.

Nukleonika

2011

Vol. 56, No. 1

49-51

Medical linacs used in radiotherapy produce bremsstrahlung spectra. In the energy range from 8 to 25 MV medical linacs produce, besides the clinically useful electron and photon beams, secondary neutrons. The aim of this study was to investigate the effect of an external wedge filter on the photoneutron dose equivalent produced by a medical linac at patient plane. Polycarbonate (PC) films were used for the determination of photoneutron dose equivalent produced by a Varian 2100 C/D linac working at 18 MV photon mode. Neutron dose equivalent was measured at distances 0, 10, 20 and 50 cm from the center of the X-ray beam for open field and after inserting a wedge filter. It was noted that by inserting the external wedge in the path of the X-ray beam, the photoneutron dose equivalent was increased compared to open field. It can be concluded that an external wedge, made from heavy materials may act like the other components of linac head, producing undesired photoneutrons and thus increasing patient dose.

A study of the photoneutron dose equivalent resulting from a Saturne 20 medical linac using Monte Carlo method

Hashemi F., Hashemi-Malayeri B., Raisali G., Shokrani P., Sharafi A.

Nukleonika

2007

Vol. 52, nr 1

39-43

High energy linacs have several advantages including lower skin dose and higher dose rate at deep sighted tumors. But, at higher energies photonuclear reactions produce neutron contamination. Photoneutron contamination has been investigated from the early days of modern linacs. However, more studies have become possible using Monte Carlo codes developed in recent years. The aim of this study was to investigate the photoneutron spectrum and dose equivalent produced by an 18 MV Saturne linac at different points of a treatment room and its maze. The MCNP4C code was used to simulate the transport of photoneutrons produced by a typical 18 MV Saturne linac. The treatment room of a radiotherapy facility in which a Saturne 20 linac is installed was modeled. Neutron dose equivalent was calculated and its variations at various distances from the center of the X-ray beam was studied. It was noted that by increasing the distance from the center of the beam, fast neutrons decrease rapidly, but thermal neutrons do not change significantly. In addition, the photoneutron dose equivalent was lower for smaller fields. The fast photoneutrons were not recorded in the maze. It can be concluded that the fast photoneutrons are highly attenuated by concrete barrier, while the slow photoneutrons are increased. In addition, increasing the X-ray field size increases the photoneutron dose equivalent around the treatment room and maze. It seems that the walls play an effective role in increasing the photoneutron dose equivalent.