Helical tomotherapy is an IMRT system placed into the ring gantry of a helical CT
scanner. A Siemens 6 MV linac and magnetron-powered RF system have been placed
into a General Electric (GE) CT scanner slip-ring gantry. A 64 leaf binary multileaf
collimator modulates a fan beam of radiation. The fan beam rotates continuously as the
couch translates through the 85 cm gantry bore. Megavoltage computer tomographic
(MVCT) images are acquired using a GE xenon ionization CT detector. Acceptance
testing indicates that the unit should be capable of high quality intensity modulation and
tomographic image guidance.
Tomographic images are the basis for novel radiotherapy verification processes. The
MVCT’s can be acquired just before treatment and fused at the treatment console to
determine if the patient has been properly setup. If not, the patient setup can be adjusted,
or alternatively, the leaf transitions modified to account for the offsets. At the same time
that the treatment is being delivered, the detector system acquires a signal that represents
the beam exiting from the patient. This exit signal can be used to reconstruct the dose
delivered to the patient in registration with the CT image set just acquired. Dose
reconstruction is an apples-to-apples comparison of the planned dose superimposed on
the planning CT. Deformable registration is a process to “morph” the treatment MVCT
set to the planning CT image set so that anatomic and their registered dose distributions
can be compared quantitatively. The processes of MVCT, dose reconstruction and
deformable registration enables adaptive radiotherapy whereby missing dose to the tumor
is made up on subsequent fractions with affecting as little as possible the dose to normal
tissue.
We anticipate that adaptive radiotherapy will enable patient-specific quality
assurance of the entire treatment course.