| Abstrakt: |
Three Philips 250/15, 250 kV X-ray units are in use in the Radiotherapy Department at Groote Schuur Hospital, operating at 15 mA and 50 cm FSD. The beam is filtered by 1 mm of tin, 0·25 mm of copper and 1 mm of aluminium, giving a half value layer of 3·5 mm copper. Checks of skin dosage by means of thermoluminescent dosimetry (TLD) revealed unexpectedly low doses towards the edge of a wide field (25 × 8 cm) on the long axis. In order to investigate this further, isodose curves were plotted using a Toshiba MRA-301 automatic isodose curve plotter on each of the three X-ray units. With each unit, a marked asymmetry of the radiation beam was found, the beam being more intense on the anode side than the cathode side. This effect is illustrated in Fig. 1 for a 25 × 10 cm field. The dotted lines on the cathode side are the curves from the anode side reflected about the central axis to facilitate comparison. An isodose curve plotted without an applicator exhibited the same asymmetry, eliminating the applicator as a possible cause of the effect. Note that in Fig. 1 the purpose is to demonstrate beam asymmetry and no attempt has been made to give accurate percentage depth dose values.In Fig. 1, the plot is in the plane containing the cathode-anode axis and the central axis of the tube. The beam was found to be reasonably uniform in the plane at right angles to this. |
