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The purpose of this study was to investigate the difference between a 6 MV linear accelerator x-ray energy spectrum outside the field edge near a phantom surface, and the corresponding spectrum on the central axis. The Monte Carlo code MCNP-4A was used to calculate the spectra on the central axis and at 1, 2, 5 and 10 cm from the edge of a 4 x 4 cm2, 10 x 10 cm2 and 15 x 15 cm2 field. Compared to the spectrum on the central axis, the spectra outside the field edge showed two distinct regions: a broad peak below about 0.5 MeV, and a lower amplitude, less rapidly changing region at higher energies from 0.5 to 6 MeV. The lower energy peak was due to scattered photons, and the higher energy component was due mainly to primary photons transmitted through the jaws of the secondary collimator. The potential impact of these spectral differences on critical organ photon dosimetry was determined by calculating the ratio of the sensitivity of a Scanditronix EDD-5 diode and of a LiF:Mg:Ti thermoluminescent dosimeter (TLD) outside the field edge to their respective sensitivity at the calibration position on the central axis. The lower energy peak combined with the non-uniform energy sensitivity of each detector produced up to a two-thirds overestimate of x-ray dose outside the field by the diode, whereas the response ratio of the TLD was about unity. These results indicated that a similar evaluation was required for profile measurements of a dynamic wedged field and measurements in an intensity modulated beam with either type of detector.

Patients who receive radioiodine (iodine-131) treatment for hyperthyroidism (195-800 MBq) emit radiation and represent a potential hazard to other individuals. Critical groups amongst the public are fellow travellers on the patient's journey home from hospital and members of the patient's family, particularly young children. The dose which members of the public are allowed to receive as a result of a patient's treatment has been reduced in Europe following recently revised recommendations from ICRP. The annual public dose limit is 1 mSv, though adult members of the patient's family are allowed to receive higher doses, with the proviso that a limit of 5 mSv should not be exceeded over 5 years. Unless the doses received during out-patient administration of radioiodine can be demonstrated to comply with these new limits, hospitalisation of patients will be necessary. The radiation doses received by family members (35 adults and 87 children) of patients treated with radioiodine at five UK hospitals were measured using thermoluminescent dosimeters mounted in wrist bands. Families were given advice (according to current practice) from their treatment centre about limiting close contact with the patient for a period of time after treatment. Doses measured over 3-6 weeks were adjusted to give an estimate of values which might have been expected if the dosimeters had been worn indefinitely. Thirty-five passengers accompanying patients home after treatment also recorded the dose received during the journey using electronic (digital) personal dosimeters. For the "adjusted" doses to infinity, 97% of adults complied with a 5-mSv dose limit (range:0.2-5.8 mSv) and 89% of children with a 1-mSv limit (range: 0.2-7.2 mSv). However 6 of 17 children aged 3 years or less had an adjusted dose which exceeded this 1 mSv limit. The dose received by adults during travel was small in comparison with the total dose received. The median travel dose was 0.03 mSv for 1 h travel (range: 2 microSv-0.52 mSv for 1 h of travel time). These data suggest that hyperthyroid patients can continue to be treated with radioiodine on an out-patient basis, if given appropriate radiation protection advice. However, particular consideration needs to be given to children aged 3 years or younger. Admission to hospital is not warranted on radiation protection grounds.

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