Shielding for neutron scattered dose to the fetus in patients treated with18 MV x-ray beams

Neutrons are associated with therapeutic high energy x-ray beams as a conta minant that contributes significant unwanted dose to the patient. Measureme nt of both photon and neutron scattered dose at the position of a fetus fro m chest irradiation by a large field 18 MV x-ray beam was performed using a n ionization chamber and superheated drop detector, respectively. Shielding construction to reduce this scattered dose was investigated using both lea d sheet and berated polyethylene slabs. A 7.35 cm lead shield reduced the s cattered photon dose by 50% and the scattered neutron dose by 40%. Adding 1 0 cm of 5% berated polyethylene to this lead shield reduced the scattered n eutron dose by a factor of 7.5 from the unshielded value. When the 5% berat ed polyethylene was replaced by the same thickness of 30% berated polyethyl ene there was no significant change in the reduction of neutron scatter dos e. The most efficient shield studied reduced the neutron scatter dose by a factor of 10. The results indicate that most of the scattered neutrons pres ent at the position of the fetus produced by an 18 MV x-ray beam are of low energy and in the thermal to 0.57 MeV range since lead is almost transpare nt to neutrons with energies lower than 0.57 MeV. This article constitutes the first report of an effective shield to reduce neutron dose at the fetus when treating a pregnant woman with a high energy x-ray beam. (C) 2000 Ame rican Association of Physicists in Medicine. [S0094-2405(00)02908-4].