Proton Therapy: What to Expect Day by Day

Standard radiation uses photons, which are X-rays. Proton therapy uses protons, which are particles carrying a positive charge.

The biology is the same. RadiologyInfo states there is no significant difference in the biological effects of protons versus photons. The difference is where the dose stops.

Photons pass through the body and keep depositing dose past the tumor. Protons release most of their energy inside the tumor region and deliver only a minimal dose beyond the tumor boundary. For smaller tumors, the dose can hug the target more tightly and spare more healthy tissue.

That has a practical consequence. The radiation oncologist can potentially give a higher dose to the tumor while keeping side effects down. It matters most in children, where protons reduce radiation to tissue that is still growing.

Sites where proton therapy is used, per RadiologyInfo, include the lung, prostate, brain, liver, breast, esophagus and rectum, plus skull base sarcomas, pediatric brain tumors, head and neck cancers, and eye melanomas.

Proton therapy needs a particle accelerator. The two common types are a cyclotron and a synchrotron. They accelerate protons to speeds up to 60 percent of the speed of light, reaching energies up to 250 million electron volts.

Magnets steer that beam into the treatment room and then to the exact part of the body being treated. Older machines used extra hardware to shape the beam. Newer centers instead fine-tune the beam energy and the magnetic fields, a method called pencil beam scanning.

This is why proton centers are rare and why treatment may mean travel. Plan for that before the first appointment.

Nothing is delivered on your first visit. The first step is called simulation, and it defines everything after it.

You are fitted into an immobilization device. What it is depends on where the tumor is. For treatment above the neck, that means a custom mask around the face, or a custom tray that attaches temporarily to the teeth. For treatment below the neck, custom molds are shaped under or around the body.

Some people need a fiducial marker first. That is a small metallic marker placed in or near the tumor to guide the beam. When one is needed, it is usually placed a few days before simulation.

Once the device is made, you have a CT scan in exactly the treatment position. That scan builds a virtual 3-D model of the tumor and the normal tissue around it. Additional imaging, such as MRI or PET/CT, is often added to define the edges of the tumor.

The radiation oncologist then draws the tumor and the critical structures on that model. Physicists work out how to deliver the dose. That planning is why there is a gap between simulation and the first treatment.

Proton therapy runs on a team, and knowing the roles saves you asking the wrong person.

• The radiation oncologist is the physician. They decide the therapy, the treatment area and the dose.
• The medical physicist and the dosimetrist do the detailed calculations that make the plan deliverable.
• Radiation therapists run the daily treatments, position you, and watch you during the beam.
• Radiation nurses handle day-to-day concerns and manage side effects.
• A diagnostic radiologist reads the imaging that the plan depends on.

Specialized engineers maintain the accelerator itself. They are the reason a machine fault can push a day's appointments.

Proton therapy is typically an outpatient treatment. For most tumor sites the course runs two to eight weeks, with treatment five days a week.

The beam itself lasts only a few minutes. The visit is longer. Expect about 15 to 30 minutes in the treatment room, most of it spent on positioning and equipment settings.

The daily sequence is the same each time. You enter the room. Your own immobilization device goes on. Laser lights position you to within a few millimeters, checked against the images from simulation. In some cases a CT system images the target before the beam. That alignment is repeated before every single treatment.

You should not feel pain or discomfort during the treatment. There is no sensation from the beam itself.

Side effects come from radiation damage to healthy cells in the treated area. Their number and severity depend on the dose, the type of radiation and the part of the body being treated.

Early side effects appear during treatment or shortly after, and are typically gone within a few weeks. The most common are fatigue and skin changes in the beam path. Skin may become sensitive, red, irritated or swollen, and may become dry, itchy, peeling or blistered. Temporary hair loss occurs in the treated area.

Other early effects depend on the site. RadiologyInfo lists mouth problems and difficulty swallowing, eating and digestion problems, diarrhea, nausea and vomiting, headaches, soreness and swelling in the treated area, and urinary and bladder changes.

Fatigue is worse when a large area is treated, and worse again when chemotherapy is given at the same time. If both are planned, ask how they will be sequenced across the week.

Late side effects can appear months or years later. They are rare, and often permanent. The list includes changes to the brain, spinal cord, lung, kidney, colon and rectum, along with infertility, joint changes, lymphedema, mouth changes and second cancers.

On that last item, RadiologyInfo is specific. There is a slight risk of developing a cancer from radiation therapy, and that risk is lower for protons than for conventional X-ray treatment. Your radiation oncologist checks for complications and for new or recurrent cancers at follow-up.

• How many sessions, over how many weeks, and on which days?
• Which immobilization device will I need, and can I see it before simulation?
• Do I need a fiducial marker placed, and when?
• Will I have chemotherapy at the same time?
• Which side effects are expected for my treatment site specifically?
• How far is the center, and is lodging or transport help available?
• Who do I call about a side effect at night or on a weekend?

If fertility is a concern, raise it before simulation rather than after. Options narrow once treatment begins, and our page on fertility preservation before treatment covers what is available and how quickly it must be arranged.

• Call 911 or go to an emergency department if you cannot catch your breath, your chest hurts, or you become confused or hard to rouse.
• Contact your care team straight away, at any hour, if your temperature reaches 100.4°F (38°C) or higher while chemotherapy is running alongside your proton course. The CDC describes fever during chemotherapy as a medical emergency, because it may be the only sign of an infection your blood counts cannot fight. If you cannot reach them quickly, go to an emergency department and say as you arrive that you are on cancer treatment.
• Call your care team the same day if skin in the beam path blisters, or turns wet and raw. Peeling wet skin can become infected, and your nurse may need to start a dressing or an antibiotic. Call the same day for a fever of 100.4°F (38°C) or higher on proton therapy alone, with no chemotherapy in the plan.
• Call your care team within a day or two if mouth soreness, trouble swallowing, diarrhea or nausea is keeping you from eating and drinking as usual. These effects are easier to settle early than late.

• NCI: skin and nail changes during cancer treatment
• NCI: infection and neutropenia during cancer treatment
• RadiologyInfo: proton therapy
• NCI: radiation therapy for cancer
• CDC: fever during cancer treatment