A radiation modulator is used with a low energy source for breast brachytherapy using a balloon-like catheter. The clinical evaluation of the patient determines the modulator's optimal configuration so as to produce a non-uniform dose distribution to avoid healthy tissue. This modulation may range from a few percent to many tens of percent. A modulating member having predetermined dimensions and materials, i.e., length, width, thickness, is attached to a catheter in close proximity to the intended location of the radioactive source. The modulating member, for example, may be shaped as a rectangular plate, an oval plate or such and be attachable to a catheter.