They are not the same. Some manufacturers, such as Siemens, confuse the issue more by using some of the terms interchangeably. 3D-CRT (conformal radiotherapy) is an older, less advanced technology than IMRT. The following article below discusses the fine differences between the two:

"Intensity-modulated radiation therapy (IMRT). Under computer guidance, "dynamic" multileaf collimators can change their shape during treatment, thereby giving rise to intensity-modulated radiation therapy (IMRT). IMRT can deliver radiation more conformally than 3D-CRT can, which allows for the delivery of higher doses. IMRT differs from 3D-CRT in that each x-ray beam is broken up into many "beamlets," and the intensity of each beamlet can be adjusted individually. As a result, IMRT has a significant advantage over 3D-CRT in inverse planning, which is the process by which a physicist, using a computer program, starts with the dose distribution desired by the radiation oncologist and works backward to determine the intensity of every beamlet necessary to achieve that distribution. By contrast, treatment planning with 3D-CRT is an exercise in trial and error."+


+Source - Recent advances in radiotherapy for head and neck cancers.
Ear, Nose & Throat Journal, Oct, 2001, by Rachel H. Chou, Richard B. Wilder, Michael S. Wong, Kenneth M. Forster