Difference Between IMRT and IGRT
IMRT (Intensity-Modulated Radiation Therapy) and IGRT (Image-Guided Radiation Therapy) are fundamentally different technologies that serve complementary but distinct purposes: IMRT is a radiation delivery technique that shapes and modulates the radiation beam intensity to conform dose to the tumor while sparing normal tissues, whereas IGRT is an imaging verification system that ensures accurate patient positioning and accounts for daily anatomical changes during treatment delivery. 1, 2, 3
IMRT: Radiation Delivery Technology
IMRT is an advanced form of 3D conformal radiation therapy that uses inverse treatment planning with beams from multiple angles, with intensity adjusted throughout treatment to optimize dose distribution. 2, 3
Key Technical Features of IMRT:
- Uses differential "dose painting" to deliver varying radiation intensities within the treatment field, allowing simultaneous delivery of 66-74 Gy to gross disease and 50-60 Gy to subclinical disease in each fraction. 1, 3
- Creates steep dose gradients between target tissues and surrounding normal structures, improving the therapeutic ratio. 4
- Can be delivered using Simultaneous Integrated Boost (SIB) or Sequential (SEQ) techniques, each with distinct fractionation approaches. 1, 3
Clinical Benefits of IMRT:
- Reduces long-term toxicity in oropharyngeal, paranasal sinus, and nasopharyngeal cancers by decreasing dose to salivary glands, temporal lobes, auditory structures, and optic structures. 1, 3
- Enhances disease control and survival in patients with nasopharyngeal carcinoma, as demonstrated in randomized controlled trials and meta-analyses. 1
- Reduces acute grade 2 or higher gastrointestinal toxicities compared to conventional techniques. 5
IGRT: Image Verification Technology
IGRT is an imaging system that verifies patient positioning and target anatomy during treatment delivery, using radiological imaging devices to control alignment and account for daily changes in target position. 1, 6
Key Technical Features of IGRT:
- Uses cone-beam CT scanning, X-ray imaging, or other modalities to match the position of the tumor during treatment delivery with the treatment-planning CT scan. 1, 7, 8
- Minimizes interfractional setup variation during high-precision radiotherapy and enables customized margins for planning target volumes. 1
- Allows monitoring of geometric and dosimetric changes during the planned course of radiation therapy, including tumor growth, shrinkage, or movement. 1, 8
Clinical Benefits of IGRT:
- Considered central to the safe and effective delivery of hypofractionated regimens, with strong recommendation for use in prostate cancer treatment. 1
- Permits smaller treatment margins (1-3 mm reduction) without sacrificing tumor control when using prostate-based IGRT compared to bone-based IGRT. 7, 5
- Successfully reduces acute rectal toxicity by allowing more precise treatment delivery. 5
The Complementary Relationship
IMRT and IGRT work synergistically: IMRT is relatively intolerant to setup uncertainties, making IGRT a natural and necessary corollary to IMRT delivery. 4
Why Both Are Needed:
- IMRT creates highly conformal dose distributions with steep dose gradients, but these tight margins require precise daily positioning that only IGRT can provide. 4, 8
- When moderately or ultrahypofractionated external beam radiation therapy is undertaken, meticulous attention to technical aspects requires both IGRT use and IMRT techniques, with strong recommendation to avoid non-modulated 3D-CRT. 1
- Daily image guidance should be implemented to minimize interfractional setup variation during high-precision radiotherapy like IMRT. 1
Common Pitfalls to Avoid
Inadequate image guidance when using IMRT can lead to geographic miss and treatment failure, as the tight margins created by IMRT demand precise daily positioning. 9
Both IMRT and IGRT increase the volume of tissue exposed to low-dose radiation, which may increase the risk of secondary malignancies, though this risk must be balanced against improved tumor control and reduced high-dose toxicity. 6, 4
Not accounting for cumulative doses to organs at risk when combining advanced techniques can result in unexpected toxicity. 9