IMRT and VMAT

History of Technical Developments in IMRT and VMAT

Prior to 1994 optimal treatment plans were developed using beam aperture, collimator angle, gantry angle, beam energy, beam weighting, and modifiers such as wedges or compensators. Some basic modulation may have been accomplished using a field-in-field approach. The planner would manually adjust the various parameters until an acceptable, not necessarily optimal, result was achieved. With the development of the multi-leaf collimator (MLC) and inverse planning, the adjustments moved from mechanical parameters to target and organ-at-risk (OAR) parameters. The objectives and dose constraints are modified to produce an acceptable, but still not necessarily optimal, result. This is achieved through the automatic adjustment of the various mechanical parameters by the optimization algorithm in sequential trials to meet the specified goals and constraints. The first versions only modified the beam intensity throughout a beam aperture. This was called intensity-modulated radiation therapy. More modern systems can optimize gantry angle and collimator angle as well.

The first commercially available IMRT system (Peacock, NOMOS Corp.) became available in 1994 and used a binary collimator as an add-on device for a standard linear accelerator (linac) in an arc delivery. The beam intensity was modulated throughout the arc by the opening and closing of the collimator leaves. Since the binary collimator leaves are either open or closed the intensity of a given arc segment was determined by the amount of time the leaf was open during that segment. Once an arc was completed, the couch was indexed a precise amount to treat the next volume or “slice” of the target. This was repeated until the entire target was covered. Because of this slice-by-slice approach, it is referred to as serial or axial TomoTherapy. This original system led to the development of helical TomoTherapy as implemented in the TomoTherapy Hi-Art system (Accuray Systems Inc.). The implementations that followed on traditional linacs used the standard MLC to modulate the beam at fixed gantry and collimator angles as mentioned above. This can be done by delivering the dose at multiple fixed segments where the beam is turned off while the MLC is changing shape, called step and shoot, or by having the MLC continuously change shape as the beam is constantly on, called sliding window delivery or dynamic MLC (dMLC). For sliding window delivery the dose rate may be fixed or variable. The next development was to use the conventional linac and MLC and to perform the dynamic delivery throughout an arc, which is called VMAT. This is sometimes referred to by its original development name of intensity-modulated arc radiotherapy (IMAT) or by vendor-specific names such as RapidArc (Varian Inc.) or SmartArc (Philips Inc.).

In terms of the usage of IMRT and VMAT, a 2004 survey of U.S. practices found that IMRT was used by 73% of respondents. Given that 91% of nonusers at the time planned to adopt IMRT, the availability in U.S. clinics is now likely close to 100%. Studies of Medicare billing patterns show that IMRT was used in 54% of cases in 2010. A 2010 Canadian survey demonstrated that IMRT was available in 87% of centers, up from 37% in 2006.