The present invention is directed to an x-ray source for irradiating a volume in accordance with a predetermined dose distribution. The source comprises a housing, an elongated tubular probe, and a target assembly. The housing encloses an electron beam source and includes elements for generating an electron beam along a beam path. The tubular probe extends along a central axis from the housing about the beam path. The target assembly extends along the central axis and is adapted for coupling to the end of the probe distal from the housing. The target assembly includes a target element, a probe tip assembly, and a variable transmission shield. The target element is positioned in the beam path. The target element is adapted to emit x-rays in response to electrons incident thereon from the beam. The probe tip assembly and associated control electronics include elements for positioning the target element in the beam path, and is substantially x-ray transparent. The variable transmission shield is positioned on an outer surface of the probe tip assembly, and is adapted to control the position of the isodose surfaces of the x-rays emitted from the target and passing through the probe tip assembly.