The present disclosure generally relates to an applicator for radiotherapy and a method for determining a thickness of a scattering foil and modulator therein. According to one embodiment, an applicator for radiotherapy may comprise a housing having a hollow structure with an opening, a scattering foil disposed at an opening of the hollow structure and configured to receive a first radiation and convert a portion of the first radiation into a second radiation while scattering the first radiation, and a modulator disposed inside the hollow structure and configured to modulate an intensity of mixed radiation including the first radiation and the second radiation.