The basics of IMRT: an overview

Intensity modulated radiation therapy (IMRT) is one of the workhorse delivery methods in current radiation therapy. In many ways, it is a significant step up over the previous standard technique of 3D conformal radiation therapy (3DCRT). I'm going to start talking about IMRT with a very basic overview of how it works.

IMRT is a technique to plan and deliver MeV range x-ray therapy. The main advance with IMRT over 3DCRT is the algorithmic optimization of dose from all delivery angles at the same time to meet a pre-defined set of objectives, so-called inverse planning. This is accomplished by using multileaf collimators, which can create arbitrary aperture shapes (within reason), thus modulating the dose to the targets. In turn, this gives the dose optimization algorithm a large number of parameters to work with to achieve the desired dose shape.


Key steps in IMRT planning and delivery:

• Acquisition of patient geometry (via CT, MRI, etc).
• Delineation of targets and avoidance volumes.
• Beam angle and energy selection.
• Optimization of fluences to desired prescription and avoidance objectives.
• Assessment of DVH's.
• QA via secondary calculations and field measurements.

Diagram of a multileaf collimator.

Further reading:

• Bortfeld, IMRT: a review and preview, Phys. Med. Biol. 51 R363, 2006 doi:10.1088/0031-9155/51/13/R21
• KY Cheung, Intensity modulated radiotherapy: advantages, limitations and future developments, Biomed Imaging Interv J 2006;2(1):e19 (open access)

Top image by Taheri-Kadkhoda et al. Radiation Oncology 2008 3:4 doi:10.1186/1748-717X-3-4 licensed under CC licensing.
Bottom image by ZEEs and licensed under CC licensing.