Chemoradiotherapy and Radiotherapy for Colorectal Cancer: What Patients Need to Know
When you hear the words radiation therapy or chemoradiotherapy, it can be difficult to know what they mean, why they are being recommended, and what treatment will actually involve.
Radiation therapy is an important part of treatment for some people with rectal cancer, particularly when the cancer has grown into or through the rectal wall or has spread to nearby lymph nodes. It is used much less often for colon cancer because of differences in where these cancers are located and how they behave.
New research is also changing the conversation about radiation treatment for some people with early-stage rectal cancer. Results from the STAR-TREC trial, published in The Lancet Oncology in August 2026, suggest that carefully selected people with early or intermediate-stage rectal cancer may be able to avoid major rectal surgery after treatment with chemoradiotherapy and close monitoring.
Here is what patients and families should know.
What is radiation therapy?
Radiation therapy uses high-energy radiation to damage cancer cells and stop them from growing. For rectal cancer, radiation is usually delivered from a machine outside the body. The radiation is carefully aimed at the tumor and nearby areas where cancer cells might be present.
Radiation can be used at different points during cancer treatment. For rectal cancer, it is often given before surgery, where it may shrink the tumor and reduce the risk that the cancer will come back in the pelvis. Radiation can also be used to treat symptoms caused by cancer or, in certain situations, to treat cancer that has returned.
What is chemoradiotherapy?
Chemoradiotherapy combines radiation therapy with chemotherapy.
The chemotherapy used alongside radiation is often given in a lower dose than chemotherapy that is intended to treat cancer throughout the body. Its role during radiation is to make the cancer cells more sensitive to the radiation.
For example, people receiving long-course chemoradiotherapy for rectal cancer may receive the chemotherapy drug capecitabine or 5-fluorouracil (5-FU) while receiving radiation. Chemotherapy may also be given separately before or after radiation as part of a larger treatment plan.
Can radiation help someone avoid surgery?
This is one of the most important areas of current research in rectal cancer.
For decades, surgery to remove the rectum and surrounding tissue, known as total mesorectal excision (TME), has been an important treatment for many people with rectal cancer. TME can be very effective at removing the cancer. But rectal surgery can also have lasting effects on bowel function, urinary or sexual function, and quality of life. Some people may need a temporary or permanent ostomy.
Because of these potential effects, researchers have been studying whether some people can safely preserve their rectum if their cancer responds extremely well to treatment. This approach is sometimes called organ preservation.
The idea is relatively straightforward: treat the cancer first, carefully evaluate how well it responds and, if the cancer has disappeared or shrunk enough, monitor the person closely rather than immediately performing major surgery.
If the cancer does not respond adequately or later comes back, surgery may still be needed.
What did the STAR-TREC study find?
The STAR-TREC trial looked specifically at whether organ preservation could be an option for carefully selected people with early or intermediate-stage rectal cancer. The trial was conducted at 37 centers in five European countries.
Participants had relatively small, node-negative rectal tumors identified by MRI. The trial included people with tumors staged from T1 through T3b, with tumors no larger than 40 mm.
People who wanted to pursue organ preservation were randomly assigned to one of two approaches:
- Long-course chemoradiotherapy: radiation over five weeks combined with capecitabine chemotherapy.
- Short-course radiotherapy: five radiation treatments over one week.
After treatment, patients were carefully evaluated. Depending on how well the tumor responded, they could enter a watch-and-wait approach, have a less extensive procedure to remove remaining tumor, or proceed to major rectal surgery.
What happened?
Among the patients pursuing organ preservation, 78.5% of those who received long-course chemoradiotherapy had avoided total mesorectal excision at 12 months, compared with 60.6% of those who received short-course radiotherapy.
The difference was largely related to how completely the tumors responded to treatment. A complete clinical response was seen in about 64% of people receiving long-course chemoradiotherapy compared with 36% receiving short-course radiotherapy.
In other words, for the carefully selected patients in this study, long-course chemoradiotherapy was more likely than short-course radiation to produce a response that allowed the patient to avoid major rectal surgery at one year.
This is an important finding because avoiding major rectal surgery may help some patients preserve bowel function and avoid the need for an ostomy.
Does STAR-TREC mean everyone with rectal cancer can avoid surgery?
No.
This is an important distinction.
STAR-TREC studied a specific group of people with relatively small, early or intermediate-stage, node-negative rectal cancers. The results should not be applied to every person with rectal cancer. The study also does not mean that radiation can replace surgery for everyone. For people whose tumors do not respond completely, whose cancer comes back, or whose cancer has characteristics that make organ preservation inappropriate, surgery may still be the best option.
And although the 12-month results are encouraging, the trial's main endpoint is organ preservation at 30 months. Longer follow-up is needed to understand how often cancer returns after organ-preserving treatment and what the long-term effects are on cancer control, bowel function, and quality of life.
That makes STAR-TREC an important step forward, but not the final word.
The bottom line
Radiation therapy and chemoradiotherapy are important treatments for many people with rectal cancer, but they are not one-size-fits-all treatments.
Recent research is expanding the possibilities. The STAR-TREC trial found that, among carefully selected people with early and intermediate-stage rectal cancer pursuing organ preservation, long-course chemoradiotherapy resulted in a higher rate of avoiding major rectal surgery at 12 months than short-course radiotherapy.
The results are encouraging, particularly for people who want to preserve their rectum and avoid the potential long-term effects of major surgery. But longer follow-up is still needed, and organ preservation requires careful monitoring.
If you are facing a rectal cancer diagnosis, talk to your healthcare team about your treatment options, your goals, and your priorities.
A note for readers: This article is for educational purposes and is not a substitute for medical advice. Treatment decisions for colorectal cancer should be made with your cancer care team based on your individual diagnosis, tumor characteristics, overall health, and preferences.
References
Bach SP, Sebag-Montefiore D, Homer V, et al; STAR-TREC Collaborative. Chemoradiotherapy versus short-course radiotherapy for response-adapted organ preservation in early-stage and intermediate-stage rectal cancer (STAR-TREC): 12-month results of an international, multicentre, open-label, parallel-group, randomised, phase 2/3 trial. Lancet Oncol. 2026;27(9):1120-1132. doi:10.1016/S1470-2045(26)00228-7. PMID: 42633754.
Bach SP, et al. Can we save the rectum by watchful waiting or transanal surgery following (chemo)radiotherapy versus total mesorectal excision for early rectal Cancer (STAR-TREC)? Protocol for the international, multicentre, rolling phase II/III partially randomized patient preference trial. BMC Cancer. 2022. PMID: 35114057.
Schrag D, et al. Preoperative Treatment of Locally Advanced Rectal Cancer. N Engl J Med. 2023;389:322-334. doi:10.1056/NEJMoa2303269. PMID: 37272534
Bahadoer RR, Dijkstra EA, van Etten B, et al. Short-course radiotherapy followed by chemotherapy before total mesorectal excision (TME) versus preoperative chemoradiotherapy, TME, and optional adjuvant chemotherapy in locally advanced rectal cancer (RAPIDO): a randomised, open-label, phase 3 trial. Lancet Oncol. 2021;22(1):29-42. doi:10.1016/S1470-2045(20)30555-6. PMID: 33301740.
Sauer R, Becker H, Hohenberger W, et al. Preoperative versus postoperative chemoradiotherapy for rectal cancer. N Engl J Med. 2004;351:1731-1740. doi:10.1056/NEJMoa040694.
NCI. Rectal Cancer Treatment (PDQ®). National Cancer Institute. Updated information on radiation therapy, chemoradiotherapy, surgery, active surveillance and treatment options for stage II and III rectal cancer.