I scarfed this off the University of Wisconsin-Madison's website, edited by me for brievity:

"Tomotherapy, literally "slice" therapy, is a new form of cancer radiation therapy that combines the precision of a CT (computerized tomography) scan with the potency of radiation treatment to selectively destroy cancerous tumors while avoiding surrounding tissue.

Unlike traditional radiation therapy systems which have beams projecting onto the tumor from a few different directions, tomotherapy rotates the beam source around the patient, thus allowing the beam to enter the patient from many different angles in succession. The advantage is that instead of having, for example, six beams, each with 1/6 the dosage necessary to irradiate the tumor, the tomotherapy beam has only 1/72 the dosage and is projected into the tumor from 72 different positions as it rotates. Thus the tumor is more precisely targeted and the healthy tissue surrounding the tumor is subjected to much lower dosages of radiation.

The intensity of the beam is modulated through the use of a multi-leaf collimator system, thus further improving the precision of the treatment. By moving the radiation-blocking leaves in and out of the beam path with speed and precision, the location and intensity of the radiation entering the patient is accurately controlled. Also aiding in the precision targeting offered by tomotherapy is the inclusion of CT imaging technology within the tomotherapy device itself. This allows technicians to precisely locate the tumor before and during treatment.

...unlike other precision radiotherapy devices being tested, the patient is continuously moved through the device while the beam source rotates thus creating a spiral scan pattern. This continuous helical delivery pattern is faster, more accurate and avoids "seams" between scan slices that can occur with some other methods."

This seems to be an advanced version or variation of IMRT.

-Brett