Abstract: Three-dimensional conformal radiotherapy is a mostly-used external radiotherapy for some tumors, e.g. prostate carcinoma, head and neck cancer, breast cancer and lung cancer ect. Compared with the conventional radiotherapy, it can improve the dose to the target area in the tumor and reduce the ones to the proximal normal tissues and in return, decreases the probabilities of irradiation induced acute and long-term effects. Developed on the technology basis of conventional conformal radiotherapy, intensity modulated radiotherapy (IMRT) modulates the intensity spatial distribution in the incident beam, obtaining much better therapeutic effect.