Understanding Radiation Therapy Port Films

Importance and Role of Portal Imaging

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.

Port films are checked in frequent intervals, 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 healthy tissues nearby such as skin, the lungs, or the heart.

Reza Estakhrian / Stone / Getty Images

Understanding Radiation for Cancer

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

  • Lowering the risk of recurrence after surgery (called adjuvant therapy)
  • Decreasing the size of a tumor before surgery (called neoadjuvant therapy)
  • Relieving the symptoms of cancer, such as bone pain from bone metastases

In external beam radiation, a machine directs high-energy beams into a tumor. It's administered over many weeks in an outpatient center. Radiation therapy is not painful and takes only a few minutes—but the actual treatment sessions can last 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 to ensure good positioning for the radiation beam.

What is a Radiation Port?

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 before surgery. It is also often referred to as simply the "treatment field."

Only a small beam of sunlight can come through a small hole, 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 small hole access through which radiation enters your body.

Definition of Port Film

A port film is an x-ray taken at the beginning of radiation treatment, and once a week during your therapy to ensure proper radiation positioning. 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. The port films do not 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. 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.

Your radiation port must be accurately aligned with the radiation machine for each treatment. The radiation beam must travel to the target area, but this location can change throughout the treatment process as the tumor area changes size.

Respiratory Gating

Port films are particularly important in women who have left-sided breast cancer. Heart disease is one of the potential long-term side effects of radiation. Radiation therapy after a lumpectomy or mastectomy also increases the risk of several types of heart disease.

Respiratory gating was developed to reduce the amount of radiation reaching the heart. To move the heart out of the way, 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 So Often?

A lumpectomy cavity can move and change in the days and weeks following breast surgery. As your breast tissue begins to heal it will shrink in response to the ionizing energy from the radiation.

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

Limiting Your Radiation Exposure

Port films may expose the breast that did not have surgery to radiation. Additionally, 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.


If you're undergoing radiation therapy for cancer, your radiologist may have multiple port films or x-rays taken of your tumor site throughout the treatment process. Port films have a critical role in determining the appropriate direction of radiation at the tumor location because the tumor area changes shape in response to the radiation over time.

While port films do not detect cancer, they can help radiologists identify the appropriate location for radiation treatment over time.

A Word From Verywell

The need for repeated radiation port films can be annoying due to the extra time it takes during radiation. However, it 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.

Frequently Asked Questions

  • Is a port used for chemo or radiation?

    A chemotherapy port can be placed into the chest under the skin to allow chemotherapy medications to be given without repeated needle sticks. This device is typically called a Port-a-cath and can be used to give intravenous fluids, medications, and chemotherapy drugs into a large vein.

    Radiation is energy directed through a beam and does not require port access. A port film is a term used to describe a precise location using x-ray guidance to deliver radiation.

  • How often are port films taken?

    Port films are taken weekly to see how the treatment area is responding to frequent doses of radiation.

5 Sources
Verywell Health uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read our editorial process to learn more about how we fact-check and keep our content accurate, reliable, and trustworthy.
  1. Barnett GC, West CM, Dunning AM, et al. Normal tissue reactions to radiotherapy: towards tailoring treatment dose by genotypeNat Rev Cancer. 2009;9(2):134–142. doi:10.1038/nrc2587

  2. Boerma M, Sridharan V, Mao XW, et al. Effects of ionizing radiation on the heartMutat Res. 2016;770(Pt B):319–327. doi:10.1016/j.mrrev.2016.07.003

  3. Bennion NR, Baine M, Granatowicz A, Wahl AO. Accelerated partial breast radiotherapy: a review of the literature and future directionsGland Surg. 2018;7(6):596–610. doi:10.21037/gs.2018.11.05

  4. National Cancer Institute. Definition of a port.

  5. American College of Radiology. Systematic port film measurement deviations in breast cancer patients treated with radiotherapy.

Additional Reading

By Blyss Splane
Blyss Splane is a certified operating room nurse working as a freelance content writer and former travel nurse. She works as a freelance content writer for healthcare blogs when she's not spending time with her husband and dog.

Originally written by Pam Stephan
Pam Stephan is a breast cancer survivor.
Learn about our editorial process