Measuring linac photon beam energy through EPID image analysis of physically wedged fields.

PURPOSE: Electronic portal imaging devices (EPIDs) have proven to be useful tools for measuring several parameters of interest in linac quality assurance (QA). However, a method for measuring linac photon beam energy using EPIDs has not previously been reported. In this report, such a method is devised and tested, based on fitting a second order polynomial to the profiles of physically wedged beams, where the metric of interest is the second order coefficient α. The relationship between α and the beam quality index [percentage depth dose at 10 cm depth (PDD10)] is examined to produce a suitable calibration curve between these two parameters. METHODS: Measurements were taken in a water-tank for beams with a range of energies representative of the local QA tolerances about the nominal value 6 MV. In each case, the beam quality was found in terms of PDD10 for 100 × 100 mm(2) square fields. EPID images of 200 × 200 mm(2) wedged fields were then taken for each beam and the wedge profile was fitted in MATLAB 2010b (The MathWorks, Inc., Natick, MA). α was then plotted against PDD10 and fitted with a linear relation to produce the calibration curve. The uncertainty in α was evaluated by taking five repeat EPID images of the wedged field for a beam of 6 MV nominal energy. The consistency of measuring α was found by taking repeat measurements on a single linac over a three month period. The method was also tested at 10 MV by repeating the water-tank crosscalibration for a range of energies centered approximately about a 10 MV nominal value. Finally, the calibration curve from the test linac and that from a separate clinical machine were compared to test consistency of the method across machines in a matched fleet. RESULTS: The relationship between α and PDD10 was found to be strongly linear (R(2) = 0.979) while the uncertainty in α was found to be negligible compared to that associated with measuring PDD10 in the water-tank (± 0.3%). The repeat measurements over a three month period showed the method to be reasonably consistent (i.e., well within the limits defined by local QA tolerances). The measurements were repeated on a matched machine and the same linear relationship between α and PDD10 was observed. The results for both machines were found to be indistinguishable across the energy range of interest (i.e., across and close to the thresholds defined by local QA tolerances), hence a single relation could be established across a matched fleet. Finally, the experiment was repeated on both linacs at 10 MV, where the linear relationship between α and PDD10 was again observed. CONCLUSIONS: The authors conclude that EPID image analysis of physically wedged beam profiles can be used to measure linac photon beam energy. The uncertainty in such a measurement is dominated by that associated with measuring PDD10 in the water-tank; hence, the accuracies of these two methods are directly comparable. This method provides a useful technique for quickly performing energy constancy measurements while saving significant clinical downtime for QA.

Factors influencing tissue nitrate concentration in field-grown wild rocket (Diplotaxis tenuifolia) in southern England.

Wild rocket (Diplotaxis tenuifolia) is a leafy vegetable known for its high tissue nitrate concentration (TNC) which can exceed the limits set in the relevant European legislation designed to protect human health. The aim of this work was to understand the factors influencing TNC and to develop best practice guidelines to growers. Commercial crops of field-grown wild rocket were studied over two seasons. In 2010, ten separate crops were sampled representing a range of soil types and time periods during the summer. Two fields sampled using a 'W'- or 'X'-shaped sampling pattern demonstrated that 10 incremental samples bulked to make 1 kg of fresh material could be used to provide an adequate sample for determination of TNC in the wild rocket crop, as is the case for other leafy vegetables. Of eight commercial crops sampled in 2010 with an average nitrogen (N) fertiliser application of 104 kg N ha(-1), two exceeded the limit of 6000 mg NO3(-) kg(-1) set in the legislation. In 2011, six N response experiments were carried out, and only two sites showed a significant yield response to N fertiliser. The reason for the lack of response at the other sites was principally due to high levels of soil mineral N prior to drilling, meaning the crops' requirement for N was satisfied without additional fertiliser N. In the experimental situation at an N fertiliser application rate of 120 kg N ha(-1), 50% of crops would have exceeded the 6000 mg NO3(-) kg(-1) limit. In both seasons, low radiation levels in the 5 days prior to harvest were shown to increase TNC, although the relationship was also influenced by N supply. Strategies for optimising N nutrition of field-grown wild rocket are discussed.

Experimental reactivation of equid herpesvirus 1 (EHV 1) following the administration of corticosteroids.

Eight ponies were experimentally infected with equid herpesvirus 1 (EHV 1) (subtype 1). All animals showed clinical and serological evidence of infection and virus was isolated from nasal swabs and leucocytes. These ponies were kept in isolation for a further three months during which time complement fixing antibody decreased at least four-fold. Following immunosuppression with dexamethasone and prednisolone subtype 1 virus was recovered from six of the eight animals within 14 days. Five of these six ponies were viraemic and three of them shed virus in nasal secretions; only four displayed significant rises in complement fixing antibody and only two in neutralising antibody. Clinical abnormalities were not detected during reactivation.