Measurement of fast neutrons produced by high-energy X-ray beams of medical electron accelerators

Abstract

The fast neutron contamination associated with the 25 MV X-ray beam of a clinical linear accelerator and with the 19 MV X-ray beam of a betatron has been measured a t the patient treatment location, using both fission fragment track detectors and silicon diode dosemeters. Measurements were made of the neutron fluence and dose, both in and out of the primary photon beam, at distances up to 60 cm from the cent,ral axis. Keut'ron distributions are similar for the linac and the betatron, with approximately ten times greater neutron dose in the X-ray field than at 20 cm outside it. For a linac field of 10 x 10 cm at a distance of 1 m from the target, the neutron dose in the field is 0.54 0.05yo of the X-ray dose; the corresponding value for a 7.6 x 11.4 cm betatron field at 50 cm is 0.22k0.03y0. The silicon diodes are preferred over track detectors for measurements of fast neutrons in intense highenergy photon fields.

A review of dosimetry studies on external-beam radiation treatment with respect to second cancer induction

“…In the 1970s, the transition from low-energy radiation sources such as betatrons and 60 Co machines to high-energy linear accelerators prompted early concerns about the potential neutron production when using beam energies in excess of the common 6 MV. Many of these studies from 1970s to 1990s focused on the occupational radiation safety of medical personnel (Wilenzick et al 1973, McGinley et al 1976, Rawlinson and Johns 1977, Holeman et al 1977, McCall et al 1978, McCall and Swanson 1979, Tochilin and LaRiviere 1979, Herman et al 1980, Greene et al 1983, Rogers and Van Dyke 1981, Palta et al 1984, LaRiviere 1985, Uwamino et al 1986, Agosteo et al 1995, Mao et al 1997, Kase et al 1998. Although these studies do not provide specific data on risk of radiation-induced second cancer, the early experiences in assessing neutron contamination were quite useful later on.…”

Reduction in stray radiation dose using a body‐shielding device during external radiation therapy

“…The stray radiation dose (SRD) from photon-beam radiotherapy has been studied both experimentally and theoretically for some time. [2][3][4][5][6] Stray radiation, such as phantom scatter, collimator scatter, room scatter, leakage radiation, and even the therapeutic dose near the primary field, decreases exponentially with increasing distance, but does not disappear completely after attenuation in the patient's body. There is no effective way of making phantom scatter disappear or decrease.…”

“…Therefore, when a target volume in cancer is exposed to prescription doses for a definitive or palliative goal, normal tissues and organs in patients are unavoidably exposed to stray radiation during photon‐beam radiotherapy with a linear accelerator, which increases the risk of inducing secondary cancers. The stray radiation dose (SRD) from photon‐beam radiotherapy has been studied both experimentally and theoretically for some time . Stray radiation, such as phantom scatter, collimator scatter, room scatter, leakage radiation, and even the therapeutic dose near the primary field, decreases exponentially with increasing distance, but does not disappear completely after attenuation in the patient's body.…”

Photo neutron dose equivalent rate in 15 MV X-ray beam from a Siemens Primus Linac

“…For photon energies above a threshold of approximately 7MeV (depending on the atomic number of the material being irradiated in the beam, this threshold will generally be in the range of 6.7–10.8 MeV photon energy), photonuclear reactions of the heavy metals of the accelerator head lead to production of unwanted neutrons that contaminate the photon field. [ 2 ]…”

“…Many researches have been performed regarding neutron contamination of different medical electron accelerators and various ways have been used to report the contamination including; fluence, dose equivalent, and absorbed dose measurements using various devices. [ 2 5 6 7 8 9 10 ]…”