Understanding Radiation Therapy Port Films

Question and Answers on Port Films and How They Keep You Safe

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During breast radiation, your radiation oncologist will want to ensure that the radiation is targeting cancer cells, so proper positioning is critical — and this is the purpose of port films. 

Let's learn more about port films, how often they are taken, and how they affect your overall radiation exposure in this question-answer style format. 

What is Radiation?

Radiation treatments are done to prevent recurrence of cancer, shrink an existing tumor, or relieve cancer symptoms.

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.

During your first treatment session, and then weekly following 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?

Your radiation port may also be called your treatment fieldor just a port. It's the part of the body through which the external beam of radiation is directed to reach the cancer cells, or tumor. 

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. 

What is a 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 don't track your progress during treatment — meaning they do not show if any cancer is present. The port films do track any changes in the size, shape, or location of your radiation treatment area.

Why Are Port Films Important?

The ionizing energy of radiation therapy will affect the target area, as well as a margin of tissue around the target. In order to avoid exposing healthy tissue to radiation, accurate positioning is essential. Radiation technicians use skin markings and port films to ensure that the treatments will be aimed accurately.

Why are Port Films Checked 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 affects 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.

Limiting Your Radiation Exposure

When taking a port film of your breast, you may get a little radiation in the breast that is opposite to the one being treated. 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.


American Cancer Society. (2015). Glossary of Radiation Therapy Terms: Port. Retrieved December 23rd 2008 and October 19th 2015.

Wang X et al. The radiation exposure from portal images during the course of breast radiotherapy. Am J Clin Oncol. 2008 Aug;31(4):345-51.

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