MR-Linac Radiation Therapy for Prostate Cancer

By: David Grew MD MPH

"MR-Linac treatment uses daily MRI imaging to help adapt radiation to how your anatomy looks that day."

If your doctor has recommended MR-Linac radiation therapy for prostate cancer, you may be wondering how it differs from other types of radiation treatment.

An MR-Linac combines two technologies in one machine: magnetic resonance imaging (MRI) and a linear accelerator, the machine that delivers radiation.

The MRI allows your radiation team to see the prostate and nearby organs clearly on the day of each treatment. That matters because the prostate, bladder, rectum, and bowel do not look exactly the same from one day to the next. With MR-Linac treatment, your team can use that day's images to adjust the radiation plan before treatment is delivered.

The goal is to treat the prostate accurately while limiting radiation exposure to nearby healthy tissue.

What is an MR-Linac?

An MR-Linac is a radiation treatment machine with an MRI scanner built into it.

A traditional linear accelerator delivers radiation using imaging to help position the patient accurately. An MR-Linac adds high-quality MRI imaging directly into the treatment process.

MRI is particularly useful in prostate cancer because it provides excellent images of soft tissues, including the:

• prostate
• bladder
• rectum
• nearby bowel and other structures

Your care team can therefore see the anatomy they are treating immediately before radiation is delivered.

Why does the prostate need to be imaged every day?

The prostate does not stay in exactly the same position every day.

Changes in the amount of urine in the bladder or stool and gas in the rectum can slightly shift the prostate and nearby organs. Even relatively small changes matter when radiation is being delivered very precisely.

With MR-Linac treatment, an MRI is taken during each treatment visit.

Your radiation team can compare what your anatomy looks like that day with the original treatment plan and, when needed, create an updated plan that better matches your current anatomy.

This process is called adaptive radiation therapy.

What does "adaptive radiation therapy" mean?

Traditional radiation treatment is carefully planned before treatment begins. Imaging during treatment is then used to make sure the patient and target are positioned correctly.

With daily adaptive radiation therapy, the treatment plan itself can be adjusted based on what the team sees that day.

For example, the bladder may be somewhat larger than it was during the original planning scan, or the rectum may be in a slightly different position. Instead of simply treating according to the original anatomy, the team can account for these changes before delivering that day's radiation.

The goal is to continue covering the prostate with the intended radiation dose while avoiding nearby healthy tissue as much as possible.

How should I prepare for an MR-Linac treatment?

Your treatment center will give you specific instructions, and these can differ from one program to another.

Because the prostate's position is influenced by the bladder and rectum, you may be asked to arrive with:

• a comfortably full bladder
• a relatively empty rectum

Some programs use bowel medications or an enema before treatment to help reduce stool and gas in the rectum. You may also be asked to drink a specific amount of water before your appointment.

Follow the instructions from your own treatment team rather than using a general preparation routine, since the timing and requirements can vary.

Why are there MRI safety checks?

Because an MR-Linac contains an MRI scanner, the same safety precautions used for other MRI examinations are important.

MRI uses a powerful magnetic field. Before entering the treatment area, your team will ask about things such as:

• implanted medical devices
• previous surgeries
• metal implants or fragments
• hearing aids
• jewelry
• watches
• removable dental or medical devices
• other metal objects

You may be asked to change into a hospital gown and remove anything that could be unsafe in the MRI environment.

These questions may be reviewed more than once. That repetition is intentional and is part of keeping you safe.

What happens when I enter the treatment room?

Your radiation therapists will help position you on the MR-Linac treatment table.

You may be given earplugs or headphones because MRI machines can be noisy. Depending on the treatment center, you may be able to listen to music or view calming images while you are on the table.

An MRI receiver, sometimes called a coil, is positioned near your body so the machine can create detailed images.

You will also have a way to communicate with your treatment team. Even after the staff leave the room, they can see and hear you throughout the treatment. If you become uncomfortable or need help, you can let them know.

What happens before the radiation starts?

Once you are positioned, the treatment table moves into the MR-Linac and MRI images are taken. This is where MR-Linac treatment differs significantly from a conventional radiation appointment.

Your radiation team reviews the images and looks at the prostate and nearby organs as they appear that day.

If your anatomy has changed enough to affect the planned radiation dose, the treatment plan can be adjusted. Your physician and radiation team review the updated plan before radiation is delivered.

Much of the time you spend on the treatment table is therefore not the radiation itself. It is imaging, planning, checking, and making sure the treatment is accurate.

How long does an MR-Linac treatment take?

MR-Linac appointments are often longer than conventional radiation treatments because daily imaging and treatment adaptation take time.

At some centers, patients may be on the treatment table for roughly 45 minutes to an hour, although the exact time varies based on the treatment plan and whether adaptation is needed.

The actual radiation delivery accounts for only part of that appointment.

It is a good idea to ask your treatment team approximately how long your visits will take so you know what to expect.

What happens while the radiation is being delivered?

Once the treatment plan has been reviewed and approved, the linear accelerator delivers the radiation. You will not see or feel the radiation.

MRI imaging can continue during treatment, allowing the system and treatment team to monitor your anatomy. Some MR-guided radiation systems can pause the radiation if the target moves outside a predefined treatment area, allowing the team to make sure the prostate is still where it should be before continuing.

This combination of imaging and radiation delivery gives the care team another way to account for movement during treatment.

Will I be able to communicate with my team?

Yes. Although the radiation therapists and other team members leave the treatment room while the radiation is being delivered, they continue to monitor you using cameras, microphones, and the MR-Linac system.

You will generally be given a way to alert the team if you need assistance.

If you feel uncomfortable, anxious, or unable to remain in position, let them know. The team can communicate with you and stop treatment if necessary.

Is MR-Linac treatment the same as an MRI scan?

Not exactly. The imaging portion uses MRI technology, so some parts of the experience will feel similar to having a diagnostic MRI. You will lie inside the opening of the machine, hear MRI noises, and need to remain still.

But an MR-Linac is also delivering radiation therapy.

The MRI is being used to help plan, guide, and monitor that radiation rather than simply to diagnose or evaluate a medical condition.

What are the potential advantages of MR-guided radiation?

The main advantage is the ability to see the prostate and surrounding soft tissues clearly during each treatment and account for changes in anatomy.

This may allow doctors to:

• adjust the treatment plan based on the day's anatomy
• more precisely account for changes in the bladder and rectum
• monitor the prostate during treatment
• limit radiation exposure to nearby healthy tissues

Clinical research has also compared MRI-guided and CT-guided prostate SBRT. In the randomized MIRAGE trial, MRI guidance with smaller treatment margins was associated with fewer short-term urinary and bowel side effects than CT-guided SBRT.

That does not mean MR-Linac treatment is necessary or better for every patient. Modern prostate radiation can be delivered very effectively using several different technologies, and your radiation oncologist can explain why a particular approach is being recommended for you.

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MR-Linac radiation therapy combines MRI imaging and radiation treatment in the same machine.

Its defining feature is the ability to look at the prostate and surrounding organs each day and, when appropriate, adapt the radiation plan to match how your body looks at that visit.

That means an MR-Linac appointment may take longer than a conventional radiation visit, but much of that additional time is being used for imaging, planning, and safety checks before the radiation is delivered.

If MR-Linac treatment has been recommended for you, ask your radiation team about your treatment schedule, bowel and bladder preparation, MRI safety requirements, and how long you should expect each appointment to take.

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To learn more, watch this video we made on MR-Linac Radiation Therapy for Prostate Cancer.

To learn more about prostate cancer, browse our dedicated prostate cancer education library for more information.

Hire PRIMR to create custom video content for your clinical trial or medical product today.

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FAQs:

Do all prostate cancer patients need MR-Linac radiation therapy?

No. Many patients are treated successfully using other modern radiation machines and image-guidance techniques. MR-Linac is one option that may be useful when daily MRI guidance and treatment adaptation are valuable. Your radiation oncologist can explain why MR-Linac or another radiation approach is most appropriate for your cancer and anatomy.

Does MR-Linac treatment hurt?

No. You cannot feel the MRI or the radiation itself. The main challenge for some patients is lying still inside the machine for a longer period of time. If you have back pain, anxiety, claustrophobia, or difficulty remaining in one position, tell your care team before treatment so they can help make the experience more comfortable.

Can I have MR-Linac treatment if I have metal or an implanted medical device?

Sometimes, but not always. Because the machine contains an MRI scanner, certain implants or metal can create safety concerns. Many modern medical implants are MRI-compatible or MRI-conditional, but they still need to be reviewed before treatment. Tell your team about every implant, device, prior surgery, or possible metal exposure so they can determine whether MR-Linac treatment is safe for you.

Am I radioactive after MR-Linac treatment?

No. MR-Linac prostate treatment uses external beam radiation. The radiation is produced by the machine and does not remain in your body afterward. You can safely be around family members, children, and pets after your appointment.

Does the daily MRI show whether my prostate cancer is shrinking?

No. The daily MRI is primarily used to guide treatment by showing the prostate and nearby organs and helping the team adapt the radiation plan. It is not being performed as a daily test of whether the cancer is responding. Your care team will monitor treatment response during follow-up using measures such as PSA and other testing when appropriate.