Intensity-modulated Radiation Therapy (IMRT)

Intensity-modulated Radiation Therapy (IMRT) leverages a combination of multiple radiation beams with dynamic shaping of the radiation intensity to uniquely match radiation to the size and shape of the tumor. This technique allows our physicians to deliver a higher dose of radiation to the tumor with less damage to nearby healthy tissue. IMRT can shape the radiation dose such that it avoids as much of the normal organs as possible while delivering a large dose to the tumor. 

IMRT is commonly used in cancers of the head and neck where many critical structures that may be near the tumor, such as the spinal cord and the salivary glands, must be avoided. It has also been shown to be a beneficial treatment for prostate cancer because it significantly decreases the risk of rectal bleeding and provides the opportunity for dose escalation to decrease the likelihood of developing metastatic prostate cancer. 

One type of IMRT commonly used by our department is called volumetric modulated arc therapy (VMAT). In this method, the linear accelerator rotates around the patient while simultaneously delivering the radiation, thus increasing the number of angles and decreasing the high dose radiation to normal tissues. This approach is typically used for head and neck cancers, brain tumors, GI cancers, prostate cancers, and lung cancers. 

Technical Details

For those seeking a more detailed technical description of IMRT modalities, the following describes techniques commonly performed at our institution with regard to prostate and head and neck cancers: 

• As with stereotactic therapies, multiple beams are positioned at various points around the patient to optimally deliver the radiation. However, in IMRT, these beams are divided into a grid-like pattern, separating the one big beam into numerous smaller "beamlets." Special software is used to determine the best pattern of beamlets to use from each larger beam, in order to deliver the optimal amount of radiation to the tumor while sparing normal organs as much as possible.
• IMRT improves coverage of the prostate gland and head and neck cancers while reducing the radiation dose to organs at risk, such as the rectum, bladder and salivary glands.
• We now have randomized controlled trials comparing IMRT to the previous standard, which was 3-dimensional (3D) conformal radiation therapy (CRT). IMRT results in significantly less rectal and bladder side effect, as well as less changes to the salivary glands in patients with head and neck cancers.
• Because the prostate cannot be seen on the x-ray imaging typically used before each treatment to ensure accurate targeting, we implant gold seeds as fiducial markers that can be seen within the prostate. This allows very precise targeting of the tumor allowing us to spare more healthy tissue. This also allows us to safely increase the dose of radiation to the prostate which studies show improves outcomes. We also obtain a weekly computerized tomographic (CT) scan using cone beam CT to assess whether the patient’s bladder is full and rectum empty, which further reduces the radiation doses to these structures.