Shielding for neutron scattered dose to the fetus in patients treated with 18 MV x-ray beams.

Neutrons are associated with therapeutic high energy x-ray beams as a contaminant that contributes significant unwanted dose to the patient. Measurement of both photon and neutron scattered dose at the position of a fetus from chest irradiation by a large field 18 MV x-ray beam was performed using an ionization chamber and superheated drop detector, respectively. Shielding construction to reduce this scattered dose was investigated using both lead sheet and borated polyethylene slabs. A 7.35 cm lead shield reduced the scattered photon dose by 50% and the scattered neutron dose by 40%. Adding 10 cm of 5% borated polyethylene to this lead shield reduced the scattered neutron dose by a factor of 7.5 from the unshielded value. When the 5% borated polyethylene was replaced by the same thickness of 30% borated polyethylene there was no significant change in the reduction of neutron scatter dose. The most efficient shield studied reduced the neutron scatter dose by a factor of 10. The results indicate that most of the scattered neutrons present 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 transparent 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.

Shielding design studies for a neutron irradiator system based on a 252Cf source.

This study aims to investigate a shielding design against neutrons and gamma rays from a source of 252Cf, using Monte Carlo simulation. The shielding materials studied were borated polyethylene, borated-lead polyethylene and stainless steel. The Monte Carlo code MCNP4B was used to design shielding for 252Cf based neutron irradiator systems. By normalising the dose equivalent rate values presented to the neutron production rate of the source, the resulting calculations are independent of the intensity of the actual 252Cf source. The results show that the total dose equivalent rates were reduced significantly by the shielding system optimisation.

Delayed radiation necrosis of brainstem related to fast neutron beam irradiation: case report and literature review.

A 42-year-old man developed necrosis of the brainstem 10 months following fast neutron irradiation of a recurrent adenocystic carcinoma of the right submandibular salivary gland. The neoplasm had been diagnosed 15 years earlier. Neck dissection and several local excisions failed to control local extension, so that 7000 rad 4.8 MeV x-rays were administered. The right mandible and part of the tongue and palate were resected for recurrence and chemotherapy and transfer factor were given. Because of cranial neuropathy and erosion of the skull base, fast neutron radiation (2.080 rads) was administered five and one-half years after the x-ray therapy, but to a nonoverlapping field. Ten months later, rapidly progressive symptoms and signs of multiple cranial nerve palsies on the right side and left hemihypesthesia and hemiplegia appeared. Clinical manifestations of brainstem necrosis lasted 4 months. Postmortem examination demonstrated radionecrosis of pons and upper medulla, predominantly on the right side. This case illustrates the efficacy of neutron radiation in eradicating local carcinoma and also the serious complications of this therapeutic modality.