Understanding Radiation Therapy Port Films

Importance and Role of Portal Imaging

radiologist reading a radiation port film prior to therapy
Reza Estakhrian/Stone/Getty Images

A radiation port film or portal image is an X-ray that is taken to ensure that radiation targets a tumor or tumor cavity while minimizing exposure to healthy tissues. During breast radiation, your radiation oncologist will check a radiation port film (also called a treatment field or treatment port) before your first session, and often at a regular interval such as weekly in the case of a lumpectomy. Some people are concerned about having these X-ray studies repeated, but proper positioning is critical for the radiation to do its job while not harming other tissues such as nearby skin, the lungs, or the heart.

Understanding Radiation for Cancer

Radiation treatments may be done with a number of different goals in mind depending on the stage and location of your cancer. Goals may include:

  • To lower the risk of recurrence after surgery (called adjuvant therapy)
  • To decrease the size of a tumor before surgery (called neoadjuvant therapy)
  • To relieve the symptoms of cancer, such as bone pain from bone metastases

In external beam radiation, a machine directs high-energy beams into a person's tumor. It's administered over a course of many weeks and is done in an outpatient center. Radiation therapy is not painful and takes only a few minutes—but the actual treatment sessions take 15 to 45 minutes, due to the radiation therapist needing to ensure you are positioned properly.

Before your first treatment session (and sometimes at regular intervals after that), port films or x-rays of your tumor site will need to be taken—this helps ensure good positioning for the radiation beam.

What is a Radiation Port?

The term radiation port can be confusing, especially to people who have a chemotherapy port in place. Unlike a chemo port, a radiation port is not a device, but is rather a term used to describe the part of the body through which external beam radiation will be directed to reach your tumor or the region where your tumor once was prior to surgery. It is also often referred to as simply the "treatment field."

Your radiation port is something like a porthole in a ship's cabin. Only a small beam of sunlight can come through a porthole, and only a specific amount of radiation will be beamed at a targeted area of your body. The skin over your tumor site is the porthole through which radiation enters your breast. It is essential that your radiation port be accurately aligned with the radiation machine for each treatment. 

Definition of Port Film

A port film is an x-ray taken at the beginning of a radiation treatment, and once a week during your therapy to ensure proper radiation positioning. Port films are done to make sure that you and the radiation machine are properly aligned to each other. These port films ensure your safety and help your radiation technician stay on target with your radiation therapy.

Port films will determine if there have been any changes in the size, shape, or location of your radiation treatment field to ensure that the therapy is both effective and safe. That said, they don't track your progress during treatment—meaning they do not show if any cancer is present.

Importance of Port Films

The ionizing energy of radiation therapy will affect the target area, as well as a margin of normal tissue around the target. In order to avoid exposing healthy tissue to radiation, accurate positioning is essential. Radiation technicians use skin markings (radiation tattoos) and port films to ensure that the treatments will be aimed accurately. Even though radiation therapy has improved greatly in recent years so that there is less damage to healthy tissue, this is dependent on the radiation reaching its target, something that can change.

Breath Hold (Respiratory Gating) During Radiation

Portal films are particularly important in women who have left-sided breast cancer. One of the potential long-term side effects of radiation to this area is heart disease, and people who receive radiation to the breasts after a lumpectomy or the chest wall after a mastectomy have an increased risk of several types of heart disease. The technique of respiratory gating was developed to reduce the amount of radiation reaching the heart. In order to move the heart out of place, a person takes a deep breath and holds it while the radiation is given. Careful portal images are essential to properly positioning the radiation beam to take advantage of the shape of the chest during the breath hold.

Why Are Port Films Done Regularly, Such as Once a Week?

Because of tissue healing and scarring, a lumpectomy cavity can move and change in the days and weeks following breast surgery. Tissues will shrink and change in response to the ionizing energy from the radiation. Every day that you go in for radiation, you won't be able to get into the exact same position on the table, and that can affect your lumpectomy site as well. In order to get the right radiation dose to the right tissues every time, your radiation team will check your port films, and reposition you whenever needed.

With accelerated breast radiation, port films may be done before each session. Accelerated radiation involves fewer sessions but higher doses of radiation at each session, leaving more potential harm if positioning is incorrect.

Limiting Your Radiation Exposure

When taking a port film of your breast, you may have some radiation exposure to your breast that did not have surgery. In addition, the breast that is receiving radiation takes in a small dose of X-ray energy during the portal imaging procedure. This dose of radiation can be calculated into your total prescribed dose, so your treatments won't exceed the amount of radiation that is needed for treatment. If you have a radiation dosimeter implanted at your treatment site, your radiation technician can verify the actual dose that your tissue received at each session.

A Word From Verywell

The need for repeated radiation port films can be annoying due to the extra time it takes during radiation, but is essential to make sure the radiation reaches its intended target while avoiding healthy tissues and organs. Some people have been concerned about the extra doses of radiation related to the films, but the benefits greatly outweigh the potential risks.

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Article Sources

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