Electron contamination from the lead cutout used in kilovoltage radiotherapy

In kilovoltage radiotherapy, low-energy electrons are generated on the lead cutout plate frequently used to shape the x-ray field. They can cause surface dose enhancement and affect the percentage depth dose (PDD) and cutout factor when these parameters are measured with a parallel plate chamber. Although many previous studies concerning a similar surface dose enhancement have suggested using a thin electron filter to eliminate the effect of these low-energy electrons, no details have been given regarding the adequacy of the filter. This paper describes the procedures required to test an electron filter for our Darpec 2000 superficial x-ray machine. A piece of plain paper was found to be an adequate electron filter, which could block all the low-energy electrons without affecting the PDD and cutout factor measurement. The surface dose enhancement caused by the presence of the cutout plate was studied with such a filter. The enhancement was found to increase with x-ray energy and decrease with cutout size as well as applicator size. At the highest x-ray energy in this study (120 kV, HVL 8.6 mmAl), the enhancement ranges from less than 1% for a 8 cm diameter cutout to a maximum of 14% for a 2 cm diameter cutout with a 10 cm applicator. For a 5 cm diameter applicator it ranges from about 1% to about 11%. The difference between the measured and predicted cutout factor displays similar characteristics. Since the low-energy electrons do not generate any useful treatment dose but their presence can lead to potential dosimetry problems, we suggest that a filter tested with the procedure described in this paper should be used when percentage depth dose and cutout factor for kilovoltage x-ray are measured with a parallel-plate chamber.