Radiation therapy for anal cancer

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Radiation therapy uses high-energy rays or particles to destroy cancer cells.

Most people with anal cancer have radiation therapy. Your healthcare team will use what they know about the cancer and about your health to plan the type and amount of radiation, and when and how it is given.

You may have radiation therapy:

  • combined with chemotherapy as the main treatment for early-stage anal cancer (stages 1, 2 and 3)
  • to treat anal cancer that could not be removed by surgery
  • to treat anal cancer that has spread to the lymph nodes in the groin
  • to relieve pain or control the symptoms of advanced anal cancer (called palliative therapy)
Radiation therapy is most often given along with chemotherapy during the same time period to treat anal cancer. This is called chemoradiation. Some chemotherapy drugs can help make radiation work better by making cancer cells more sensitive to radiation. Combining chemotherapy and radiation therapy can be more effective than either treatment on its own.

Types of radiation therapy

The following types of radiation therapy are used to treat anal cancer.

External radiation therapy

During external radiation therapy, a machine directs radiation through the skin to the tumour and some of the tissue around it, including lymph nodes. External radiation therapy is also called external beam radiation therapy. It is usually given as part of chemoradiation to treat anal cancer.

Doctors may use the following external radiation techniques to accurately target the area to be treated and spare as much surrounding normal tissue as possible.

3D conformal radiation therapy (3D-CRT) has many beams of radiation directed at the tumour. The radiation oncologist uses a CT scan or an MRI to map the exact location and shape of the anal tumour. The radiation beams are then shaped and aimed at the tumour from different directions to treat the tumour from all angles. Each individual beam is fairly weak and less likely to damage normal tissue. A higher dose of radiation is delivered where the beams meet at the anal tumour.

Intensity-modulated radiation therapy (IMRT) is similar to 3D-CRT in that it delivers radiation from many different angles to treat the entire anal tumour. In addition to shaping and aiming the radiation beams, IMRT allows the radiation oncologist to adjust the strength (intensity) of the individual beams. This reduces the dose of radiation reaching nearby normal tissue while allowing a higher dose to be delivered to the tumour. It is useful for treating tumours in hard-to-reach areas.

External radiation therapy is usually given once a day, 5 days a week, for 5 to 6 weeks. An extra boost of radiation may be given if the anal tumour is large or if the cancer has spread to lymph nodes.


Brachytherapy is a type of internal radiation therapy. A sealed container of a radioactive substance, called an implant, gets placed right into the area where the anal tumour is found. The radiation kills the cancer cells over time.

Brachytherapy may be used after external beam radiation therapy to give a boost of higher-dose radiation to the tumour and area around the tumour.

Radiation oncologists need special equipment to give brachytherapy. As a result, it isn't available in all treatment centres and it isn't a standard treatment for anal cancer.

Side effects of radiation therapy

During radiation therapy, your healthcare team protects healthy cells in the treatment area as much as possible. Side effects of radiation therapy will depend mainly on the size of the area being treated, the specific area or organs being treated, the total does of radiation and the treatment schedule.

Tell your healthcare team if you have side effects that you think are from radiation therapy. The sooner you tell them of any problems, the sooner they can suggest ways to help you deal with them.

The common side effects of radiation therapy used for anal cancer are:

Find out more about radiation therapy

Find out more about radiation therapy and side effects of radiation therapy. To make the decisions that are right for you, ask your healthcare team questions about radiation therapy.

Expert review and references

  • Shahid Ahmed , MD, FRCPC, PhD, FACP
  • American Cancer Society. Treating Anal Cancer. 2020: https://www.cancer.org.
  • Ahmed S, Eng C, Messick CA. Squamous cell carcinoma of the anal canal. Yalcin S, Phillip PA (eds.). Textbook of Gastrointestinal Oncology. Springer Nature Switzerland; 2019: 10:171–180.
  • Cancer Research UK. Radiotherapy for Anal Cancer. 2019: https://www.cancerresearchuk.org/.
  • Cancer Research UK. Side Effects of Radiotherapy for Anal Cancer. 2019: https://www.cancerresearchuk.org/.
  • Czito BG, Ahmed S, Kalady MF, Eng C. Cancer of the anal region. DeVita VT Jr., Lawrence TS, Rosenberg SA, eds. DeVita, Hellman, and Rosenberg's Cancer: Principles & Practice of Oncology. 11th ed. Philadelphia, PA: Wolters Kluwer; 2019: 64:997–1013.
  • Gotfrit J, Goodwin R, Asmis T, et al. Eastern Canadian Gastrointestinal Cancer Consensus Conference 2019. Current Oncology. 2021: 28:1988–2006.
  • Gondal TA, Chaudhary N, Bajwa H, Rauf A, Duc L, Ahmed S. Anal cancer: the past, present and future. Current Oncology. 2023: 30(3):3232–3250.
  • National Comprehensive Cancer Network . NCCN Clinical Practice Guidelines in Oncology: Anal Carcinoma Version 1.2022 . 2022.
  • Hosni A, Elamir A. Princess Margaret Cancer Center Clinical Practice Guidelines: Gastrointestinal: Anal. University Health Network; 2019.
  • Rao S, Guren MG, Khan K, et al. Anal cancer: ESMO Clinical Practice Guidelines for diagnosis, treatment and follow-up. Annals of Oncology. 2021: 32(9):1087–1100.

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