Your Radiation Port
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 port through which radiation enters your breast. It is essential that your radiation port be accurately aligned with the radiation machine for each treatment. Your radiation port may also be called your treatment field.
Port Films Are For Radiation Positioning
Radiation treatments are done to prevent recurrence of cancer, shrink an existing tumor, or relieve cancer symptoms. 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.
Port films are taken at the start of your treatment as well as once a week during your therapy. Portal imaging doesn't track your progress during treatment, but it does track any changes in the size, shape, or location of your treatment area.
Because of tissue healing and scarring, a lumpectomy cavity can move and change in the days and weeks following 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.
Sources:
Glossary of Radiation Therapy Terms: Port. American Cancer Society. Last Medical Review: 12/23/2008.
The radiation exposure from portal images during the course of breast radiotherapy. Wang X, Du W, Smith SA, Stovall M, Buchholz TA, Salehpour M. Am J Clin Oncol. 2008 Aug;31(4):345-51.
