Radiation Physics
10/24/14
Intensity-Modulated Radiation Therapy Intensity-modulated radiation therapy is a high-tech precise way to deliver prescribed cancer treatment. It is more effective in killing cancer with far less chance for reoccurrence than conventional radiation therapy. The advantages of intensity modulated radiation therapy out-way the disadvantages. As IMRT technology advances and spreads more cancer patients will have the benefit of the technology.
The theory behind IMRT has been around for many years but not until the year two thousand did we have the ability and technology to create a working treatment machine with IMRT capabilities. The theory behind IMRT is largely attributed to Brahme A. in 1982. The image above is an illustration of the IMRT principle from Brahme (1988). A number of intensity modulated beams (5 in this case) with their intensity profiles are shown. The schematic shows an axial cut through the patient’s body where the hatched area symbolizes the target volume. The intensities are typically reduced in those regions where the rays pass through critical structures and increased where the rays ‘see’ primarily the target volume. (Sprmn) The concept behind IMRT is to modulate the intensity of the radiation beam at different angles to lower the dose of critical structures and organs within the patient’s body. With this same concept IMRT technology increases the intensity of the beam when critical structures are not compromised to apply a more lethal dose of radiation to the cancer. This allows the radiation dose to more evenly conform to the 3D shape of the tumor allowing for higher cancer lethality and fewer patient side effects compared to conventional therapy. The precision of IMRT is amplified when used in conjunction with multileaf collimation. Multi-leaf Collimation (or the MLC) uses movable leaves, which can block some fractions of the radiation beam. Typical MLCs have 40 to 120 leaves, arranged in pairs. By moving and controlling a large number of narrow, closely abutting individual leaves, one can generate almost any desired field shape. The advantages of MLCs are lower therapy expenses because individual shielding blocks are not needed, thus eliminating the need to handle the toxic alloy that the blocks are made out of. Other advantages are constant control and continuous adjusting of the field shape during irradiation in advanced conformal radiotherapy. The multi-leaf collimator’s main dis-advantages are a stepping edge effect, radiation leakage between leaves, wider penumbra, and problems with generating some complex field shapes. Planning IMRT treatments is accomplished with the use of 3D CT, MRI and sometimes even PET-CT images. These images are uploaded to a treatment planning system where dosimetry carefully plans out the best angles and beam intensities to deliver the safest treatment to the patient with no chance of cancer survival. Planning out IMRT treatments does take longer and is more involved than conventional therapy because of this; treatment with IMRT usually begins a week after simulation. Many treatment plans are derived using a Dose Volume Histogram (or DVH). The basic data in a DVH is generated by binning the dose values from each voxel in the volume. A voxel is a 3D form of a pixel. By finding and configuring the dose values from each voxel the dosimitrist (using treatment planning software) figures out the exact amount of radiation and the direction in which the radiation should enter the body to best kill the cancer in each voxel. Below is a picture of tumor outline using treatment planning software and a typical cumulative dose-volume histogram chart. (medphys)

The treatment machine is a linear accelerator that has IMRT capabilities. During treatment there is a highly trained radiation therapist that usually specializes or is trained in IMRT treatments to set up and deliver treatment. One of the downfalls to IMRT treatments is that they usually take longer than conventional therapy treatments because of the many different angles the treatment machine has to align to. Treatment times can differ but they usually take fifteen or more minutes. This is not only a disadvantage to the patient but it also further limits the amount of patients a department can treat in a day. (radiologyinfo) IMRT is usually used to treat the spine, lung, prostate, breast, liver, tongue, kidney, sinus, larynx, and pancreas. The brain is treated with IMRT when one-session radiosurgery is not appropriate or unavailable. (Irsa) IMRT has recently been the subject of debate because of its high costs. According to Medicare claims, reimbursement for IMRT rose tenfold, from 0.9% of all breast cancer patients in 2001 to 11.2% of claims in 2005. The average cost of radiation within the first year of diagnosis was $7179 without IMRT and $15,230 with IMRT. The basis of the debate is that oncologists depending on where they work might be influenced in prescribing IMRT to bring in more reimbursement for their department. Studies show that in regions of the country where local Medicare carriers covered IMRT, billing for IMRT was more frequent than in regions where it was not covered. (hayesinc) Intensity-modulated radiation therapy is superior to conformal radiation therapy in reducing recurrence and significant side effects in men with localized prostate cancer, according to a comparative effectiveness study based on the Surveillance, Epidemiology, and End Results (SEER)-Medicare database. The study also found that proton beam therapy, the most expensive radiation modality available, did not improve outcomes as compared with IMRT. (onclive) Intensity modulated radiation therapy is a technology that will only grow and be more available in the future, its advantages far out way its disadvantages. As the technology with IMRT increases and becomes more wide spread more patients will have the benefit of its advantages.

Works Cited
(hayesinc): http://www.hayesinc.com/hayes/media_center/news-service/the-high-cost-of-intensity-modulated-radiation-therapy/
(Irsa): http://www.irsa.org/imrt.html
(medphys): http://medphys365.blogspot.com/2012/05/dose-volume-histogram-basics.html onclive (Radiologyinfo): http://www.radiologyinfo.org/en/info.cfm?pg=imrt
(Sprmn): http://www.sprmn.pt/pdf/pmb6_13_r21_IMRT_a_review&preview_(TBortfeld).pdf