Image-guided radiation therapy (IGRT) is a technique that uses real-time imaging during each radiation treatment session to pinpoint a tumor’s exact location before delivering the dose. Because tumors can shift slightly between sessions, or even during a single session, due to breathing, bladder filling, or normal organ movement, IGRT takes updated pictures each time you lie on the treatment table. Those images are compared to the original planning scan, and the radiation beams are adjusted to match where the tumor actually is that day.
How IGRT Works
Every course of radiation therapy starts with a planning session, where imaging (usually a CT scan) maps the tumor’s size, shape, and position. That scan becomes the reference blueprint for treatment. With IGRT, additional images are taken at each subsequent visit, sometimes right before treatment and sometimes continuously during it. Software compares the new images to the original plan and calculates any shifts needed.
If the tumor or surrounding anatomy has moved even a few millimeters, the system can reposition the patient or adjust the radiation beams before a single dose is delivered. In some setups, displacements greater than 3 mm trigger an automatic correction. Correction strategies range from simply shifting the treatment table to recalculating the entire radiation plan in real time while you’re still lying in position. Some systems also use “gating,” which means the machine only fires the beam when the target is in the right spot, pausing when you breathe in or out and the tumor drifts.
Types of Imaging Used
Several imaging technologies can be built into or paired with a radiation machine:
- Cone-beam CT: A compact CT scanner mounted on the treatment machine that produces 3D images of soft tissue right before treatment. This is the most common IGRT tool and gives a detailed view of organs like the prostate, bladder, and rectum.
- Digital X-rays: Two-dimensional images taken from different angles. These are fast but mostly useful for lining up bony landmarks rather than soft tissue.
- Ultrasound: Particularly useful for prostate treatments, where it can image the gland and compare its position to the planning scan using a coordinate system.
- MRI: Some newer machines (called MR-linacs) combine a radiation beam with a built-in MRI scanner, providing excellent soft-tissue contrast without any additional radiation dose from imaging.
- Surface tracking: Optical cameras map the skin surface in real time, flagging any positional changes during treatment. These systems don’t image the tumor itself but can catch overall body movement instantly.
Why Precision Matters for Side Effects
The core benefit of IGRT is that it lets radiation oncologists shrink the safety margin around a tumor. Without image guidance, doctors have to add a generous buffer zone to account for the possibility that the tumor has shifted since the planning scan. That extra margin means more healthy tissue gets irradiated. With IGRT, margins for prostate treatment, for example, can be reduced to as little as 4 to 5 mm in some directions, compared to larger margins used in conventional approaches.
Smaller margins translate directly into fewer side effects. In the CHHiP trial, a large study of prostate cancer patients, those treated with IGRT had a urinary toxicity rate of 3.9% at two years, compared to 8.4% for patients treated without image guidance. The phase III MIRAGE trial found a similar pattern: prostate patients treated on MRI-guided machines had significantly lower rates of acute urinary side effects compared to those treated with standard CT-based setups. These reductions matter because urinary and bowel irritation are among the most common complaints during and after pelvic radiation.
Fiducial Markers and Tumor Tracking
For some cancers, particularly prostate, tiny metal markers (called fiducial markers) are implanted in or near the tumor before treatment begins. These markers show up clearly on X-ray or CT images and serve as reference points, giving the IGRT system a reliable way to locate the tumor even when surrounding soft tissue is hard to distinguish.
Most centers place three gold markers in a triangular arrangement so the system can track position in all three dimensions, though studies have shown that two well-spaced markers can also locate the prostate accurately. The implantation procedure is similar to a biopsy: it’s done under ultrasound guidance, usually through the rectum or the perineum, with local anesthesia and prophylactic antibiotics. Mild pain, minor bleeding, and temporary inflammation can occur, but the procedure is generally well tolerated. Markers are placed carefully to avoid the urethra so they aren’t lost through urination.
Adaptive Radiation Therapy
IGRT laid the groundwork for an even more advanced approach called adaptive radiation therapy. In a standard IGRT session, the machine adjusts patient positioning based on daily images. In adaptive therapy, the entire treatment plan can be rebuilt on the spot. If the daily image shows that the tumor has shrunk, or that a nearby organ like the bladder is fuller or emptier than expected, the system generates a new plan that reshapes the radiation beams to match that day’s anatomy.
This replanning happens while you’re still lying on the table. The original image and the new image are fused together, contours around the tumor and sensitive organs are updated, and the software recalculates beam shapes and intensities. To keep the process fast, current systems typically preserve the original beam angles and only modify the beam patterns. Most adaptive sessions, including the replanning step, are completed in under 45 minutes, with roughly 80% finishing within that window. Standard sessions that don’t require full replanning are considerably shorter, with median times around 17 to 19 minutes on MRI-guided machines.
What a Treatment Session Feels Like
You’ll lie on the treatment table in the same position used during your planning scan, typically on your back with supports under your knees and feet to keep you comfortable and still. For treatments near the pelvis, you’ll follow specific preparation instructions beforehand. Some centers ask you to drink about 300 ml of water 30 minutes before treatment to fill your bladder to a consistent size. Others use an empty-bladder approach, asking you to simply void right before walking in. Rectal preparation, such as a micro-enema, may be recommended if bowel distension was an issue during planning.
Once you’re positioned, the imaging step begins. You’ll hear the machine move around you, and the scan itself takes just a few minutes. You won’t feel the imaging or the radiation. If the system detects that your position needs adjusting, the table may shift slightly. On MRI-guided machines, you’ll wear headphones for noise and may be given mirrored glasses so you can see a screen. Some systems even display the tumor’s position on a screen visible to you, along with a breathing guide, so you can help keep the target steady during delivery.
The radiation delivery portion is often the shortest part, typically a few minutes. Including setup, imaging, any plan adjustments, and delivery, most sessions last between 15 and 35 minutes depending on how much adaptation is needed.
Cancers Commonly Treated With IGRT
IGRT is especially valuable for tumors near organs that move or change shape between sessions. Prostate cancer is the most studied application, because the prostate shifts noticeably depending on bladder and rectal filling. Lung tumors move with every breath, making real-time tracking or gating essential. Head and neck cancers benefit because the tumor can shrink substantially over a weeks-long treatment course, changing the anatomy enough to warrant plan adjustments.
Beyond these, IGRT is used for cancers of the gastrointestinal tract, breast, esophagus, and brain. For brain tumors, rigid skull-based immobilization limits movement, but IGRT still adds a layer of verification. In practice, most modern radiation centers use some form of image guidance for nearly all treatments, though the complexity of imaging varies by site.
The Role of AI in IGRT
One of the most time-consuming steps in adaptive IGRT is contouring, the process of outlining the tumor and nearby organs on each day’s image so the software knows what to target and what to avoid. This has traditionally been done by hand, which is slow and varies from one clinician to another. Artificial intelligence tools now automate much of this work using pattern-recognition algorithms trained on thousands of scans. These systems can produce clinically acceptable outlines for the prostate, lungs, head and neck, and gastrointestinal structures in seconds rather than minutes.
Newer AI models can combine imaging data with clinical notes to improve accuracy further, performing better than systems that rely on images alone. As these tools mature, they’re making adaptive sessions faster and more consistent, which reduces the time you spend on the table and improves the reliability of each day’s plan.