Radiotherapy for rectal cancer

Radiotherapy uses high-energy X-ray beams to damage the DNA of cancer cells. Cancer cells, because they divide quickly and repair poorly, are more vulnerable to this damage than the normal cells around them. Over the course of a planned treatment, the cancer shrinks and many of its cells die — while the normal tissue, given time between sessions, recovers.

For rectal cancer, radiotherapy is delivered from outside the body — called external beam radiotherapy. A machine called a linear accelerator (or "linac") rotates around you and aims X-ray beams from several angles into the pelvis, all crossing through the tumour. Each beam alone is mild; the cumulative dose where the beams cross is the treatment.

This is fundamentally different from chemotherapy. Chemotherapy is medicine delivered into the bloodstream that affects cells throughout the body; radiotherapy is a focused, local treatment. They are often used together — but they do different things.

This is a question patients often ask, especially if they have a relative who had bowel cancer treated with surgery alone. The answer comes down to anatomy.

The colon sits inside the abdomen on a long, mobile sheet of tissue called the mesentery. When a colon cancer is removed, the surgeon can take a wide margin of normal bowel and lymph nodes around it. The clearance is generous, and radiotherapy adds little.

The rectum sits deep in the pelvis, fixed in place, surrounded on all sides by the bony pelvic walls, the bladder, the prostate or uterus, and the muscles that control the back passage. The room to cut wide around a rectal cancer is much smaller. Shrinking the cancer first with radiotherapy makes the surgery safer, improves the chance of getting a clear margin, and reduces the chance of the cancer coming back in the same area (called local recurrence).

There is more than one way to deliver radiotherapy to the rectum. The two main regimens used in Australia are:

Both are well-established. The choice depends on the size, position and features of your specific cancer, and is made at a multidisciplinary team (MDT) meeting after MRI of the pelvis and CT staging.

Before any radiotherapy is delivered, a planning appointment is arranged. This usually involves:

• A planning CT scan — taken with you lying in the position you will be in for every treatment session. The radiation oncologist uses this to map out where the cancer is and where the surrounding healthy tissue lies, so the beams can be aimed away from the bowel, bladder and skin where possible.
• Small skin marks (or tiny tattoos) — placed on your skin to help the radiotherapy team line you up the same way at every session. The marks are small permanent dots; some centres now use peel-off stickers instead.
• Positioning aids — usually a moulded foam cushion or a board to keep you in a comfortable, repeatable position with your hands above your head or by your sides.
• Bladder and bowel instructions — many centres ask you to come with a comfortably full bladder (which pushes the small bowel away from the radiation field) and an empty rectum (which keeps the treatment area consistent). You will be told exactly what is needed.

Between planning and the first session there is usually a 1–2 week gap while the radiation oncologist, medical physicists and dosimetrists work out the exact beam angles and doses.

Each treatment session takes about 10–20 minutes from when you enter the room — most of that is positioning. The radiation itself takes only a few minutes. A typical visit looks like this:

1. You arrive, change into a gown if needed, and follow any bladder-filling instructions.
2. You lie on the treatment couch in the position established at planning.
3. The radiotherapy team lines you up using your skin marks and the room's positioning lasers. They then leave the room — they are watching and listening on cameras and microphones throughout.
4. The linear accelerator rotates around you. From each angle, the beam is on for less than a minute.
5. You stay still, breathing normally. You feel nothing during the radiation — no heat, no tingling, no pain.
6. The team comes back in, lowers the couch, and you go home.

For short-course radiotherapy, this happens 5 times in 1 week. For long-course, it happens once a day, Monday to Friday, for 5–6 weeks. Weekends are usually rest days. Most people drive themselves to and from treatment, particularly in the first few weeks; in the second half of long-course treatment, fatigue may make passenger or taxi transport easier.

Acute side effects build through the course of treatment and usually peak shortly after it finishes. They settle in the following 4–6 weeks. Common acute effects include:

• Fatigue — the most pervasive symptom. It is real, and it is not the same as being out of shape. Light daily activity (walking, gentle exercise) actually helps; pushing too hard makes it worse.
• Bowel changes — looser stools, urgency, more frequent motions, mucus, and sometimes mild rectal bleeding. Usually settles within weeks.
• Bladder changes — more frequent need to pass urine, mild burning, urgency. Usually settles.
• Skin changes — redness or pink discolouration in the treatment area (lower back, buttock cleft, perineum). The skin can feel like mild sunburn. Gentle moisturising cream and avoiding friction helps; your radiotherapy team will recommend products.
• Loss of pubic hair — hair in the treated area usually thins or falls out. The hair on your head is not affected. Pubic hair usually grows back.
• Nausea — uncommon with pelvic radiotherapy alone, but can occur with the chemotherapy tablets given alongside long-course treatment. Anti-nausea medication is provided.
• Hand-and-foot redness, mouth ulcers — possible with capecitabine, used during long-course chemoradiotherapy.

Most people focus understandably on getting through the treatment itself, but it is worth understanding the longer-term effects too — because some are best discussed and prepared for before treatment starts.

Long-term effects worth knowing

• Bowel function changes. More frequent or urgent bowel motions are common after combined radiotherapy + surgery — this is part of what is called low anterior resection syndrome (LAR syndrome). Most people improve over the first year, though some changes can be lasting. Pelvic floor physiotherapy is helpful for many.
• Bladder. Some long-term irritability and reduced bladder capacity can occur. Bothersome lower urinary tract symptoms affect a minority of patients.
• Sexual function. Women: vaginal dryness, narrowing, and discomfort with intercourse are common. Vaginal dilator use during and after treatment helps reduce this. Men: erectile dysfunction and reduced ejaculation are possible. Effective treatments are available — please raise it with your team.
• Fertility. Pelvic radiotherapy reliably causes infertility — ovaries and testes are sensitive to even low doses. Fertility preservation (sperm storage for men; egg or embryo storage for women) should be discussed before treatment starts. Hormonal effects (early menopause, low testosterone) are also common.
• Pelvic bone thinning. Radiotherapy can cause local osteoporosis and a higher risk of pelvic insufficiency fractures, especially in postmenopausal women.
• Healing of subsequent surgery. If you have surgery after radiotherapy, the perineal wound (in the buttock crease) heals more slowly than it otherwise would — typically 3–6 months rather than 4–6 weeks for the equivalent unirradiated wound.
• Secondary cancers. A small additional risk of new cancers in the treated area many years later — rare but real, and one reason radiotherapy is not used for cancers that do not need it.

None of these are reasons to refuse treatment if it is recommended — locally advanced rectal cancer is the more pressing risk. But they are reasons to plan around them: fertility preservation booked early, vaginal dilator use planned, partner conversations had in advance, pelvic floor physiotherapy lined up for after treatment.

Radiotherapy is one part of a wider plan. Where it fits depends on your situation:

• Before surgery (neoadjuvant) — the most common use. Radiotherapy shrinks the cancer, then surgery removes what is left. Surgery is typically anterior resection or, for very low cancers, abdominoperineal resection.
• As part of total neoadjuvant therapy (TNT) — radiotherapy plus several months of chemotherapy, all given before surgery. See total neoadjuvant therapy (TNT) for rectal cancer.
• With a view to Watch and Wait — for some patients, the response to radiotherapy and chemotherapy is so complete that no tumour can be detected afterwards. In that situation, surgery may be deferred and you may enter a Watch and Wait pathway.
• After surgery (adjuvant) — uncommon for rectal cancer now, but occasionally used when the operation revealed unexpected features (such as positive surgical margins).
• For palliation — radiotherapy can also be used to shrink a cancer to relieve symptoms (bleeding, pain, obstruction) when cure is not the goal. Different doses, shorter schedule.