No abstract

A dosimetric comparison between therapeutic kilovoltage (kV) equipment in the South West of the UK was carried out. The aim was to detect dosimetric errors and examine consistency across the region. This work also investigated the use of a single ionization chamber to minimize errors introduced through multicentre auditing and assessed the potential impact of the "Addendum to the IPEMB Code of Practice" for the determination of absorbed dose for X-rays below 300 kV. Comparisons were carried out at low and medium energies, with additional measurements made "in-air" at medium energy in accordance with the addendum. The measurement of output with a non-standard applicator was also assessed by calculating the ratio of the two outputs or applicator factor. The mean ratio of visitor's measured dose to that measured locally was 0.994 (standard deviation, 0.006) for nine low-energy beams and 0.991 (standard deviation, 0.011) for four medium-energy beams. The mean ratio of visitor's measured dose in-air and in-phantom at medium energy was 0.981 (standard deviation, 0.024). Applicator factors were within +/-4% for low and medium energies. In conclusion, the use of a single ionization chamber and single centre to carry out the audit gives a smaller deviation between measured values, although there are both advantages and disadvantages to the method. There are differences between doses calculated from the in-air and in-phantom protocols for medium energy, although these could be within quoted uncertainties.

A phantom was developed as a reproducible means of measuring the irradiance in an occupied ultraviolet cabin, by placing the phantom, or replica person, in the cabin, obviating the need for human exposure. The contributions to the patient irradiance measured in the cabin were investigated, looking in particular at the effect of the reflectors. Radiation undergoing single reflection was seen to contribute to a greater extent than multiple reflections. Placing an object in the cabin reduces the measured irradiance due to the blocking of multiple reflections, but variation in the exact shape and size of the object has less effect, which is useful as patients are of all shapes and sizes and a representative phantom was to be developed. The phantom was made of expanded polystyrene blocks with an embedded probe. Measurements were made to verify the equivalence of human and phantom cabin occupancy. It was found that the irradiance measured with the phantom in the cabin lies within the values measured with human occupancy.

Three dosimetric methods for commissioning the dynamic wedges on a Varian 600C linear accelerator operating at 4.7 MV were compared. The techniques involved the use of ionization chambers, x-ray verification film and solid state detectors. Ionization chambers gave the most accurate results, as expected, but data acquisition was very time consuming. Film dosimetry was found to have good spatial resolution for beam profiles, but problems were encountered with the acquisition of depth doses. Solid state detectors generally showed reasonable agreement with the ionization chambers.

The conventional aluminium filtration of a general purpose X-ray tube in a district general hospital was replaced by an erbium filter for a 2-month period. The resulting reduction in skin dose was measured for a number of examinations using thermoluminescent dosimetry. The change in effective dose equivalent was calculated from these measurements and compared with previous theoretical estimates. The use of the erbium filter had no effect on radiograph quality. It was concluded that the use of an erbium filter resulted in a worthwhile reduction in radiation dose, and was a cost-effective method of dose reduction.