The penumbra of a 6-MV x-ray beam as measured by thermoluminescent dosimetry and evaluated using an inverse square root function

Abstract

Data on the dose distribution in the penumbral region of megavoltage x rays are of importance for most radiotherapy planning systems. For medical linear accelerators the distance between the points representing 20% and 80% of the central axis dose (20/80) is typically only a few millimeters. To achieve good spatial resolution a radiation detector with small physical size has to be used for penumbra measurements. The penumbra in a 6-MV therapeutic x-ray beam was investigated for field sizes of 10 X10 cm2 and 20 x 20 cm2 at depth of maximum dose (dmax = 1.5 cm) and at 10-cm depth in a solid water phantom. In addition, the field edge of an independent jaw driven to the center of the axis of the..