Two self-referencing methods for the measurement of beam spot position.

PURPOSE: Two quantitative methods of measuring electron beam spot position with respect to the collimator axis of rotation (CAOR) are described. METHODS: Method 1 uses a cylindrical ion chamber (IC) mounted on a jig corotational with the collimator making the relationship among the chamber, jaws, and CAOR fixed and independent of collimator angle. A jaw parallel to the IC axis is set to zero and the IC position adjusted so that the IC signal is approximately 50% of the open field value, providing a large dose gradient in the region of the IC. The cGy∕MU value is measured as a function of collimator rotation, e.g., every 30°. If the beam spot does not lie on the CAOR, the signal from the ion chamber will vary with collimator rotation. Based on a measured spatial sensitivity, the distance of the beam spot from the CAOR can be calculated from the IC signal variation with rotation. The 2nd method is image based. Two stainless steel rods, 3 mm in diameter, are mounted to a jig attached to the Linac collimator. The rods, offset from the CAOR, lay in different planes normal to the CAOR, one at 158 cm SSD and the other at 70 cm SSD. As the collimator rotates the rods move tangent along an envelope circle, the centers of which are on the CAOR in their respective planes. Three images, each at a different collimator rotation, containing the shadows of both rods, are acquired on the Linac EPID. At each angle the shadow of the rods on the EPID defines lines tangent to the projection of the envelope circles. From these the authors determine the projected centers of the two circles at different heights. From the distance of these two points using the two heights and the source to EPID distance, the authors calculate the distance of the beam spot from the CAOR. Measurements with all two techniques were performed on an Elekta Linac. Measurements were performed with the beam spot in nominal clinical position and in a deliberately offset position. Measurements were also performed using the Flexmap image registration∕ball-bearing test. RESULTS: Within their uncertainties, both methods report the same beam spot displacement. In clinical use, a total of 203 monthly beam spot measurements on 14 different beams showed an average displacement of 0.11 mm (σ = 0.07 mm) in-plane and 0.10 mm (σ = 0.07 mm) cross-plane with maximum displacement of 0.37 mm in-plane and 0.34 mm cross-plane. CONCLUSIONS: The methods described provide a quantitative measure of beam spot position, are easy to use, and provide another tool for Linac setup and quality assurance. Fundamental to the techniques is that they are self-referencing-i.e., they do not require the user to independently define the CAOR.

Motivating general practitioners to change their prescribing: the incentive of working together.

OBJECTIVE: To determine the extent to which GPs were motivated to change their prescribing upon joining a Primary Care Commissioning Group (PCCG) and how effective certain interventions planned by the PCCG might be as a means to change prescribing. To define the characteristics of GPs less motivated to change their prescribing. DESIGN: A cross-sectional survey of participating general practitioners linked with current prescribing information derived from PACT data. SETTING: General practice covering a geographical locality within inner-city south London. SUBJECTS: All 72 general practitioners who had joined a GP Commissioning Group. MAIN OUTCOME MEASURES: questionnaire responses. RESULTS: 93% of GPs entering the GP Commissioning Group expected their prescribing to change but none expected substantial change. There was no difference between fundholders, singlehanders nor training practices in their expectation of change. GPs in practices with the lowest quality prescribing, as measured by a quality index, were least likely to expect change (Spearman's r = 0.25, P = 0.04). Those in practices with higher prescribing costs were not more likely to expect their prescribing to change, whereas expensive prescribers who were unaware of their practices' prescribing costs were associated with a reduced expectation of prescribing change (P = 0.05). Educational interventions were thought to be the most effective means by which prescribing could be changed, whereas formularies and financial factors were perceived as weaker influences. CONCLUSION: Acceptance of a cash-limited prescribing budget by GPs is accompanied by the expectation of personal prescribing change. The motivation to change prescribing may be related to a strongly developed collectivist perspective amongst GPs who are prepared to consider the prescribing implications for their fellow GPs. It is ironic that those with the least expectation of change should have the lowest quality prescribing, or be unaware of their high cost prescribing. Engendering greater commitment to the professional group may be one way of changing their prescribing.