Dostarlimab Reframes Organ Preservation in Rectal Cancer

An approval that changes the first conversation

The decision moves dostarlimab (Jemperli), a programmed death receptor-1 inhibitor, into initial treatment planning for previously untreated stage II or III locally advanced rectal cancer that is microsatellite instability-high (MSI-H) or mismatch-repair-deficient (dMMR). For an eligible patient, the first multidisciplinary discussion is no longer limited to how chemotherapy, radiation, and surgery should be sequenced. It can also address whether immunotherapy followed by structured observation could preserve the rectum if a clinical complete response is achieved.

That possibility is consequential. Standard multimodality treatment can control localized rectal cancer, but pelvic radiation and rectal surgery may produce lasting bowel, urinary, sexual, reproductive, and ostomy-related effects. Avoiding those treatments could preserve function as well as anatomy. The trade-off is that organ preservation depends on the accuracy of biomarker testing, response assessment, and long-term surveillance.

The indication is narrow. It does not apply to all rectal cancers, to metastatic disease by extension, or to tumors presumed to be dMMR without adequate testing. Nor does approval mean that every treated patient can safely omit established local therapy. The decision point comes after response assessment: patients without a clinical complete response still require prompt multidisciplinary planning for standard treatment.

What supports the organ-preservation strategy

The pivotal clinical concept came from a prospective, single-group phase 2 study of dostarlimab in patients with previously untreated stage II or III dMMR rectal adenocarcinoma. Treatment was planned before chemoradiotherapy and surgery. Patients achieving a clinical complete response could proceed to nonoperative follow-up, while those with residual disease were expected to receive standard therapy.

In the initial peer-reviewed report, 12 patients had completed treatment and at least six months of follow-up. All 12 had a clinical complete response based on the study’s combined assessment, which included imaging, endoscopy, digital rectal examination, and biopsy. None had received chemoradiotherapy or undergone surgery at publication, and no progression or recurrence was reported during follow-up ranging from six to 25 months. No grade 3 or higher adverse events were reported in that small cohort.

These findings were striking but preliminary. A 12-patient, single-center, nonrandomized cohort cannot establish that dostarlimab provides equivalent long-term cancer control to established multimodality therapy. There was no concurrent comparator, confidence intervals around a small denominator would be wide, and early clinical response is not the same outcome as long-term disease-free or overall survival.

The FDA’s accelerated approval reflects a larger regulatory judgment that the observed response outcome is reasonably likely to predict clinical benefit in this biomarker-defined setting. Accelerated approval does not lower the legal standard for evidence, but it permits earlier authorization using a surrogate or intermediate endpoint. Continued approval may depend on verification and description of benefit in required follow-up evidence.

How initial planning changes

MSI or mismatch-repair testing becomes a treatment-defining result that should be available early, rather than an ancillary finding reviewed after a conventional plan has already been set. The result must be interpreted alongside high-quality pelvic staging, assessment for distant disease, tumor location, baseline function, comorbidities, and the patient’s ability to participate in intensive follow-up.

A multidisciplinary team remains central. Medical oncology determines whether the patient fits the approved population and can receive immune-checkpoint blockade. Colorectal surgery, radiation oncology, radiology, pathology, gastroenterology, and genetics each remain relevant even when the goal is to avoid surgery and radiation. Surgical and radiation options need to remain available if response is incomplete or disease later regrows.

Counseling should separate three ideas that can otherwise blur together. A clinical complete response means that no residual tumor is detected with the examinations used; it is not a pathologic complete response, because there is no resected specimen to examine. Organ preservation means deferring surgery under surveillance, not declaring the patient cured. Accelerated approval means the FDA found sufficient evidence for access in a defined population while additional evidence is developed; it does not resolve every question about durability.

The discussion should also include immune-related toxicity. Programmed death receptor-1 blockade can cause inflammatory adverse effects involving organs such as the thyroid, lungs, liver, gastrointestinal tract, kidneys, skin, and endocrine glands. The absence of severe toxicity in the first 12 published patients does not exclude uncommon or delayed harms. Current prescribing information, rather than the early cohort alone, should guide safety screening, monitoring, and management.

For patients who achieve a clinical complete response, surveillance is an active treatment strategy. It requires repeated clinical examination, endoscopic assessment, pelvic imaging, and systemic evaluation according to the treating program and evolving evidence. Access matters: an organ-preservation approach may be less safe if a patient cannot reliably obtain specialist reassessment or timely salvage treatment.

Uncertainties after accelerated approval

The largest uncertainty is durability. Rectal preservation at one or two years is important, but it does not by itself establish long-term freedom from local regrowth, distant metastasis, or cancer-related death. The proportion of patients who can undergo successful salvage treatment after regrowth also needs mature follow-up.

Generalizability is another limitation. Patients enrolled in a specialist-center trial may have had unusually consistent biomarker confirmation, imaging, endoscopy, response adjudication, and follow-up. Outcomes may differ when the strategy is used across community settings or among patients with more extensive local disease, significant autoimmune conditions, prior transplantation, or other characteristics underrepresented in the evidence base.

There is also no randomized comparison showing whether starting with dostarlimab improves survival, quality of life, or net harms relative to contemporary total neoadjuvant therapy and selective nonoperative management. Randomization may be difficult after very high response rates, but comparative evidence and patient-reported outcomes remain important for policy, coverage, and shared decision-making.

Finally, dMMR can signal Lynch syndrome. Tumor testing that establishes treatment eligibility should therefore connect, when appropriate, to genetic counseling and germline evaluation. That has implications beyond the immediate rectal cancer plan, including surveillance for other cancers and risk assessment for relatives.

Questions clinicians ask

Does approval mean an eligible patient should automatically receive dostarlimab first?

No. The evidence supports presenting dostarlimab as an initial option for the approved biomarker-defined population. Suitability still depends on confirmed MSI-H or dMMR status, accurate stage II or III staging, contraindications or added risks from immunotherapy, patient preferences, and access to multidisciplinary response assessment and surveillance.

When can chemotherapy, radiation, and surgery be omitted?

Omission is tied to a documented clinical complete response, not simply completion of immunotherapy. Assessment should integrate examination, endoscopy, biopsy when used, and appropriate imaging. Residual or equivocal disease requires multidisciplinary review, and standard chemoradiotherapy or surgery must remain available rather than being delayed without a clear surveillance or salvage plan.

Is a clinical complete response equivalent to cure?

No. It means that the available clinical tests do not detect residual tumor at that time. Microscopic disease may remain, and local regrowth or distant recurrence can occur later. Patients choosing organ preservation therefore need prolonged, structured surveillance even when repeated early assessments remain negative.

How should accelerated approval be explained?

It provides lawful access based on an endpoint the FDA considers reasonably likely to predict clinical benefit, while additional evidence is required to confirm and characterize that benefit. Patients should understand both parts: the response data support a meaningful chance of organ preservation, but long-term comparative cancer-control evidence remains less mature than for established treatment pathways.

References

1. FDA Approves Immunotherapy That May Allow Certain Patients With Rectal Cancer to Avoid Chemotherapy, Radiation and Surgery — US Food and Drug Administration, 2026 2. PD-1 Blockade in Mismatch Repair–Deficient, Locally Advanced Rectal Cancer — The New England Journal of Medicine, 2022 3. Study of Dostarlimab in Untreated Mismatch Repair Deficient Locally Advanced Rectal Cancer — ClinicalTrials.gov, 2019 4. Accelerated Approval Program — US Food and Drug Administration, n.d.