Dosimetry of small circular fields for 6-MeV electron beams

Small field electron beams used in the clinic present complex dosimetry. This investigation reports dosimetric characteristics of small 6-MeV electron field measurements and Monte Carlo calculations, and examines their impact on patient dosimetry. Radiographic film and ionization chamber were used for dosimetric measurements for a 6-MeV electron beam from a Varian 21EX linac. A set of circular cerrobend cutouts with diameters ranging from 2 to 3.0 cm was made. A clinical Monte Carlo algorithm was used to calculate dosimetric parameters. As the cutout diameter decreased, the build-up portion of the percentage depth dose (PDD) curves shifted toward the phantom surface, the depth of maximum dose, d(max,) decreased from 1.2 cm to 0.6 cm, but the practical range, R(p), remained constant at about 3.1 cm. The absorbed dose rate, D(r), decreased linearly with decreasing diameter. Profile coverage ratios CR (ie, the ratio of a given isodose line to the cutout diameter) at d(max), for 90% and 80%, CR(90), CR(80), are approximately 55% and 73%, respectively. As the cutout diameter decreased, the 90% to 10% penumbra to diameter ratio, PR, increased from 0.49 to 0.56 for 3- to 2-cm cutouts. The total 90% to 10% penumbra was about the same size as the cutout diameter. The measured output factors were in good agreement with Monte Carlo calculation within 2.2%. Accurate small electron field measurements were performed using parallel plate ion chamber and film. The data were well supported by Monte Carlo calculations. These data facilitate routine clinical treatments for small cutouts as d(max), D(r), and other data can be quickly obtained instead of performing labor-intensive individual patient measurements.