National Council On Radiation Protection Research Articles

Neutrons from high-energy x-ray medical accelerators: an estimate of risk to the radiotherapy patient.

The problem of neutrons produced by many of the high-energy x-ray therapy machines (10 MV and above) is reviewed, and the possible risk their presence poses to radiotherapy patients is estimated. A review of the regulatory background containing a summary of the recommendations of the U.S. Council of State Governments (USCSG), and of the International Electro-Technical Commission (IEC), as well as an indication that recommendations will be forthcoming from the National Council on Radiation Protection (NCRP) and the International Commission of Radiological Protection (ICRP) is presented. The neutrons in question are produced by high-energy photons (x rays) incident on the various materials of the target, flattening filter, collimators, and other essential components of the equipment. The neutron yield (per treatment dose) increases rapidly as the megavoltage is increased from 10 to 20 MV, but remains approximately constant above this. Measurements and calculations of the quantity, quality, and spatial distribution of these neutrons and their concomitant dose are summarized. Values of the neutron dose are presented as entrance dose, midline dose (10-cm depth), and integral dose, both within and outside of the treatment volume. These values are much less than the unavoidable photon doses which are largely responsible for treatment side effects. For typical equipment, the average neutron integral dose from accelerator-produced neutrons is about 4-7 g cGy (per treatment cGy), depending on the treatment plan. This translates into an average dose of neutrons [averaged over the body of a typical 70-kg (154 lb) patient] of 0.06-0.10 cGy for a treatment of 1000 cGy. Using these neutron doses and the best available neutron risk coefficients, it is estimated that 50 X 10(-6) fatal malignancies per year due to the neutrons may follow a typical treatment course of 5000 rads of 25-MV x rays. This is only about 1/60th of the average incidence of malignancies for the general population. Thus, the cancer risk to the radiotherapy patient from accelerator-produced neutrons poses an additional risk to the patient that is negligible in comparison.

Fortification of existing rooms used for brachytherapy patients.

Often, the radiation exposure levels around brachytherapy patient rooms exceed the limits allowed by Nuclear Regulatory Commission (NRC) regulations and National Council on Radiation Protection (NCRP) recommendations. Since a neighboring patient may be present in the radiation field from a brachytherapy patient, or patients, continuously for 7 days, the dose equivalent rate must not exceed 6 muSv/hr (0.6 mrem/hr). Several boundary conditions constrain solutions to the problem of excessive radiation levels: (1) the cost should not be recurring, (2) the rooms used for brachytherapy patients must be compatible with use for nonbrachytherapy patients, (3) free access through halls must be maintained, (4) all brachytherapy rooms must be compatible with independent use, (5) shielding must not eliminate space required for a resuscitation cart, (6) shielding should not require storage, (7) the weight of shielding must remain within the tolerance of the floor, and (8) the location of beds must match utility outlets in the wall. Possible solutions to the problem include leaving rooms empty around the patient, use of portable shields, or the addition of substantial amounts of shielding material in the walls of the brachytherapy rooms, as well as changing the radionuclides routinely used. The optimum solution for a given institution depends on the frequency of brachytherapy procedures, bed occupancy rate, and available empty space.