Treatment of Colorectal Cancer: Surgery-Sparing Immunotherapy, Warning Signs, and Screening

Treatment of Colorectal Cancer: Surgery-Sparing Immunotherapy, Warning Signs, and Screening

Quick Answer: Recent oncologic advances allow select patients with locally advanced, mismatch repair-deficient rectal cancer to clear tumors through targeted immunotherapy, potentially avoiding surgery, radiation, and chemotherapy. For all individuals, routine screening from age 45 and prompt clinical evaluation of persistent bowel changes remain vital for early detection and favorable outcomes.

Key Takeaways:

• The US Food and Drug Administration (FDA) granted accelerated approval on October 19, 2026, for dostarlimab to treat locally advanced mismatch repair-deficient (dMMR) or microsatellite instability-high (MSI-H) rectal cancer.
• Approximately 5% to 10% of rectal cancers exhibit dMMR/MSI-H biology, making upfront biomarker testing essential before starting any therapeutic regimen.
• Standard treatment for mismatch repair-proficient (pMMR) colorectal tumors relies on neoadjuvant therapy, precise surgical resection, and risk-adapted chemotherapy.
• Adults under age 50 face an increasing incidence of colorectal cancer. Symptoms like rectal bleeding or bowel changes require diagnostic evaluation rather than benign presumption.
• Average-risk screening begins at age 45 using optical colonoscopy every ten years or an annual non-invasive fecal immunochemical test (FIT).

Colorectal cancer refers to malignant cell growth originating in the epithelial lining of the large intestine, encompassing both the colon and the rectum. As one of the most common malignancies worldwide, it typically develops over several years through the adenoma-carcinoma sequence. This process begins as benign mucosal polyps that acquire cumulative genetic alterations. Staging ranges from localized disease confined to the bowel wall (Stage I) to regional lymph node involvement (Stage III) and distant spread (Stage IV), directly guiding the therapeutic approach.

How Dostarlimab Rectal Cancer Immunotherapy Can Spare Patients Surgery

Dostarlimab immunotherapy offers select patients with mismatch repair-deficient (dMMR) or microsatellite instability-high (MSI-H) locally advanced rectal cancer an opportunity to clear tumors through programmed death receptor-1 (PD-1) blockade alone. In clinical trials, complete clinical responses were reported in up to 68% of study participants, sparing many from radical organ resection, chemotherapy, and radiation.

This therapeutic approach follows the US Food and Drug Administration (FDA) granting accelerated approval on October 19, 2026, for dostarlimab (marketed as Jemperli) in the United States specifically for locally advanced dMMR/MSI-H rectal cancer. In mismatch repair-deficient tumor cells, the inability to correct DNA replication errors leads to thousands of somatic mutations. These hypermutated cells express high levels of abnormal surface proteins called neoantigens. The immune system identifies neoantigens as foreign. However, cancer cells evade destruction by expressing programmed death-ligand 1 (PD-L1), which binds to PD-1 receptors on cytotoxic T cells and deactivates them.

By binding selectively to the PD-1 receptor, dostarlimab blocks this inhibitory signal. This enables the patient's own immune defenses to recognize, attack, and eliminate malignant cells. In clinical trial cohorts evaluated across six months of intravenous dostarlimab infusions for rectal cancer administered every three weeks, roughly two-thirds of evaluated patients achieved a complete clinical response. Direct visual endoscopy, high-resolution pelvic magnetic resonance imaging (MRI), and digital rectal examination detected no residual tumor tissue.

For eligible individuals, this form of rectal cancer immunotherapy enables non-operative management, commonly called a "watch-and-wait" approach. Under this protocol, patients with a sustained complete clinical response can avoid the major long-term physical morbidities associated with standard pelvic surgery and radiation. These morbidities include a lifelong colostomy, chronic bowel urgency, bladder dysfunction, and impaired sexual health.

Why Every Patient Requires Biomarker Testing Before Starting Treatment of Colorectal Cancer

Every patient diagnosed with colorectal malignancy requires immediate tumor biomarker testing for mismatch repair deficiency (dMMR) and microsatellite instability (MSI-H). These genetic markers identify the 5% to 10% of candidates who can benefit from targeted immunotherapy rather than standard chemoradiotherapy.

Biomarker assessment must occur on the initial diagnostic biopsy tissue before any treatment of colorectal cancer begins. If a patient begins pelvic chemoradiation or systemic cytotoxic chemotherapy before molecular testing, the tumor tissue can become degraded or eradicated. This eliminates the opportunity to pursue upfront organ-sparing immunotherapy.

Pathologists employ two primary diagnostic techniques to establish tumor repair status:

• Immunohistochemistry (IHC): The laboratory applies specialized antibodies to evaluate the nuclear presence of four primary mismatch repair proteins: MLH1, MSH2, MSH6, and PMS2. If one or more proteins are absent, the tumor is classified as mismatch repair-deficient (dMMR).
• Polymerase Chain Reaction (PCR) or Next-Generation Sequencing (NGS): These molecular genomic assays evaluate length variations across specific DNA microsatellite markers. A high rate of instability designates the tumor as microsatellite instability-high (MSI-H).

A finding of dMMR or MSI-H also prompts evaluation for Lynch syndrome, an inherited autosomal-dominant condition predisposing families to colorectal, endometrial, and other gastrointestinal malignancies. Identifying an inherited mutation informs future surveillance strategies for both the patient and first-degree biological relatives.

Standard Pathway: When Surgery and Chemotherapy Remain Necessary

For patients with mismatch repair-proficient (pMMR) or microsatellite-stable (MSS) tumors, standard clinical practice involves neoadjuvant chemoradiation followed by total mesorectal excision (TME) surgery and risk-adapted adjuvant fluoropyrimidine-based chemotherapy.

Between 90% and 95% of colorectal cancer cases present with intact DNA mismatch repair machinery. These pMMR/MSS tumors do not generate the high burden of neoantigens seen in dMMR cases. This makes single-agent PD-1 checkpoint inhibitors largely ineffective outside specialized clinical trials. Consequently, management relies on multimodal treatment strategies established through clinical trials.

For locally advanced rectal cancer, standard care frequently uses Total Neoadjuvant Therapy (TNT). This sequence delivers systemic chemotherapy (such as FOLFOX or CAPOX) combined with either long-course chemoradiation or short-course radiation before any operative intervention takes place. Delivering medical and radiation therapy upfront aims to clear micro-metastases, shrink the primary tumor, and improve the likelihood of preserving the anal sphincter.

Following neoadjuvant therapy, patients undergo total mesorectal excision. This en-bloc surgical technique resects the affected segment of the rectum along with its surrounding fatty envelope containing regional lymph nodes and blood vessels. In colon cancer, which does not routinely require preoperative radiation, standard care entails anatomic colectomy (resection of the bowel segment containing the tumor with adjacent mesentery and lymph nodes) followed by adjuvant chemotherapy for node-positive (Stage III) or high-risk Stage II disease.

The table below contrasts the clinical pathways of these two distinct molecular subtypes:

Diagnostic & Clinical Feature	dMMR / MSI-H Pathway (5%-10% of Cases)	pMMR / MSS Pathway (90%-95% of Cases)
Primary First-Line Therapy	PD-1 targeted immunotherapy (e.g., dostarlimab)	Neoadjuvant chemoradiation or TNT
Primary Goal of Initial Therapy	Organ preservation and complete clinical response	Tumor downstaging and local recurrence control
Surgical Resection	Deferred if complete response is maintained	Standard procedure (Total Mesorectal Excision)
Response to PD-1 Monotherapy	High clinical response rate (up to 68% complete)	Ineffective in standard clinical settings
Surveillance Protocol	Intensive pelvic MRI, endoscopy, and ctDNA	Standard postoperative imaging and CEA tracking
Germline Implications	Requires screening for Lynch syndrome	Primarily sporadic; germline screening as indicated

Recognizing the Warning Signs and Symptoms of Colorectal Cancer

The hallmark warning signs of colorectal cancer include visible rectal bleeding, persistent changes in bowel frequency or stool caliber lasting beyond a few days, unexplained iron-deficiency anemia, and persistent abdominal cramping.

Because early-stage colorectal tumors frequently develop without noticeable symptoms, clinical signs often indicate that a lesion has grown large enough to bleed intermittently, obstruct the bowel lumen, or alter gastrointestinal motility. Recognizing these signs early allows patients to seek definitive diagnostic testing before disease progression occurs.

Patients experiencing the symptoms of colorectal cancer typically describe one or more of the following clinical features:

• Visible Rectal Bleeding: Blood can present as bright red streaks on toilet paper or mixed into the stool (hematochezia). Darker, maroon-colored stools or black tarry stools (melena) generally signify bleeding from higher points within the colon.
• Persistent Alteration in Bowel Habits: New-onset constipation, persistent diarrhea, or an alternating pattern of both lasting for several consecutive weeks.
• Changes in Stool Caliber: A distinct narrowing of the stool, often described as pencil-thin or ribbon-like, caused by a partially obstructing mass in the descending colon or rectum.
• Tenesmus: A distressing sensation of incomplete bowel evacuation, where the rectal vault feels full due to the physical presence of a mass.
• Unexplained Iron-Deficiency Anemia: Chronic, microscopic blood loss from a proximal colon tumor often causes no visible stool changes but leads to fatigue, weakness, pale skin, and shortness of breath.
• Unintended Weight Loss and Persistent Discomfort: Unexplained weight loss alongside chronic abdominal pain, cramping, bloating, or excessive gas.

Why Colorectal Cancer in Young Adults Under 50 Is Rising Rapidly

Colorectal cancer in adults under age 50 has increased by approximately 1% to 2% each year over recent decades. It often presents at later stages because early signs like intermittent rectal bleeding are frequently misattributed to benign hemorrhoids or irritable bowel syndrome.

This epidemiological trend, clinically termed early-onset colorectal cancer (EOCRC), represents a significant shift in gastrointestinal oncology. While total disease incidence among adults aged 65 and older has steadily declined, largely due to widespread screening colonoscopy adoption, younger demographics face an elevated risk. Colorectal cancer has become a leading cause of cancer-related mortality in individuals aged 20 to 49.

Researchers attribute this increase to a convergence of environmental, dietary, and biological factors:

• Dietary Composition: Diets high in ultra-processed foods, refined carbohydrates, and red or processed meats, coupled with low dietary fiber intake, alter colonic transit time and inflammatory profiles.
• Microbiome Dysbiosis: Early exposure to systemic antibiotics and dietary additives can alter intestinal bacterial populations, promoting pro-inflammatory microbes that produce carcinogenic metabolites.
• Sedentary Habits and Metabolic Changes: Increasing rates of metabolic syndrome, obesity, and prolonged sitting correlate with systemic, low-grade mucosal inflammation.
• Diagnostic Delay: The median duration between the onset of initial symptoms and formal tissue diagnosis in colorectal cancer in young adults often spans four to six months. Young individuals and primary clinicians frequently assume rectal bleeding stems from hemorrhoids, anal fissures, or dietary intolerances, delaying endoscopic evaluation until the malignancy has progressed to Stage III or IV.

Colorectal Cancer Screening Options: Colonoscopy and At-Home Stool Tests

Routine screening for average-risk individuals begins at age 45 and centers on optical colonoscopy every ten years, which can simultaneously detect and remove precancerous polyps, or annual non-invasive stool evaluations using a high-sensitivity fecal immunochemical test (FIT).

Lowering the recommended screening initiation age from 50 to 45 reflects major clinical guidelines designed to counter the rising incidence in younger adults. Average-risk individuals, defined as those without a personal or family history of colorectal polyps, inflammatory bowel disease, or known hereditary syndromes, have two primary testing pathways to consider.

Optical colonoscopy for colorectal cancer remains the reference standard in colorectal cancer screening because it evaluates the entire mucosal surface of the rectum and colon up to the cecum. Its primary advantage is dual action: during the examination, the gastroenterologist uses electrosurgical snares or biopsy forceps to remove precancerous adenomas, halting their potential progression into invasive cancer. A normal exam requires repetition only once every ten years. However, colonoscopy requires clear liquid dietary preparation the day prior, bowel-cleansing laxatives, and conscious sedation during the procedure.

A non-invasive option is the FIT colorectal cancer test, which uses specific antibodies to detect human globin protein in a small stool sample collected at home. Because lower gastrointestinal lesions bleed microscopically into the fecal stream, a positive FIT indicates potential ulceration or vascular fragility. The FIT requires no dietary restrictions, is cost-effective, and must be completed once every 12 months.

If a FIT, fecal occult blood test, or stool DNA test yields an abnormal or positive result, the patient must undergo a timely follow-up colonoscopy to locate and manage the source of bleeding.

Navigating a Second Opinion and Colorectal Cancer Surgery Planning

Securing a formal second opinion from a multidisciplinary gastrointestinal oncology team prior to surgical intervention helps ensure accurate molecular profiling, re-evaluates eligibility for organ-sparing clinical protocols, and confirms the optimal surgical technique.

A comprehensive second opinion convenes a multidisciplinary tumor board comprising surgical oncologists, medical oncologists, radiation oncologists, gastrointestinal pathologists, and diagnostic radiologists. Independent review of initial biopsy specimens can identify overlooked dMMR or MSI-H status, which can reorient treatment away from radical surgery toward targeted immunotherapy.

When colorectal cancer surgery remains clinically necessary, the surgical team plans the approach based on tumor location, clinical stage, and proximity to pelvic structures:

• Approach Selection: Minimally invasive methods, including robotic-assisted and laparoscopic surgery, yield equivalent oncologic outcomes to traditional open laparotomy while reducing postoperative pain, hospital length of stay, and recovery time.
• Sphincter Preservation: For distal rectal tumors, specialized surgeons evaluate whether a low anterior resection (LAR) with coloanal anastomosis can preserve the anal sphincter complex, avoiding an abdominoperineal resection (APR) that requires a lifelong colostomy.
• Lymphadenectomy Standards: Surgical teams aim to achieve adequate regional lymph node clearance, a minimum of 12 harvested nodes is the accepted oncologic benchmark, to stage the disease accurately and determine the need for postoperative chemotherapy.
• Circulating Tumor DNA (ctDNA): Postoperative management increasingly uses blood-based liquid biopsies to detect minimal residual disease (MRD). Identifying fragments of tumor-derived DNA after surgery helps guide whether adjuvant chemotherapy can be omitted or intensified.

Risks and Treatment Complications: What Patients Should Anticipate

Colorectal cancer treatments carry specific therapeutic risks ranging from immune-mediated inflammatory reactions during immunotherapy to anastomotic leakage, bowel dysfunction, or surgical infection following resection.

While dostarlimab and related PD-1 inhibitors avoid the toxicities of cytotoxic chemotherapy and radiation, they can provoke immune-related adverse events (irAEs). These occur when the stimulated immune system targets healthy organ systems. Common irAEs include skin rashes, thyroiditis, and fatigue. More serious complications, reported in a smaller subset of patients, include immune-mediated colitis (severe diarrhea and intestinal inflammation), pneumonitis (lung inflammation causing cough and shortness of breath), and autoimmune hepatitis. These conditions require treatment interruption and systemic corticosteroids to resolve safely.

Conventional surgical interventions carry risks common to major abdominal and pelvic operations:

• Anastomotic Leakage: A breakdown of the surgical connection where two ends of the bowel are joined together. Reported in approximately 3% to 10% of low pelvic anastomoses, an anastomotic leak allows intestinal contents to enter the abdominal cavity, requiring urgent intervention or a temporary diverting ostomy.
• Low Anterior Resection Syndrome (LARS): Patients undergoing significant rectal resections frequently experience variable degrees of bowel dysfunction, including stool clustering, fecal urgency, incontinence, and frequent bowel movements. Symptoms can persist for months or years.
• Infection and Bleeding: Wound infections, deep pelvic abscesses, and post-procedural hemorrhages require standard medical, antibiotic, or radiologic drainage management.
• Chemotherapy Toxicity: Oxaliplatin-based chemotherapy carries a risk of acute cold sensitivity and chronic peripheral sensory neuropathy, while fluoropyrimidines can induce stomatitis, diarrhea, and myelosuppression (neutropenia).

Patients must seek emergency care if they develop signs of acute peritonitis or sepsis: severe abdominal rigidity, unmanageable pain, high fever with chills, persistent vomiting preventing fluid retention, or sudden, massive rectal hemorrhage.

Frequently Asked Questions

What are the symptoms of colorectal cancer that demand immediate medical evaluation?

Sudden, large-volume rectal bleeding accompanied by dizziness, fainting, or marked weakness requires immediate emergency medical evaluation to manage acute blood loss. Additionally, persistent vomiting paired with an inability to pass stool or gas signals a potentially complete mechanical bowel obstruction, which demands urgent hospital assessment to prevent intestinal perforation.

How long do patients take dostarlimab rectal cancer immunotherapy during treatment?

In the clinical trials establishing its organ-preserving efficacy, patients received intravenous dostarlimab once every three weeks over a six-month duration, totaling nine cycles. If diagnostic restaging confirms a complete clinical response at the conclusion of this period, the medical team initiates strict watch-and-wait surveillance rather than continuing indefinite immunotherapy.

Can mismatch repair testing be performed on a standard colonoscopy biopsy?

Pathologists routinely perform immunohistochemistry (IHC) and microsatellite instability (MSI) testing directly on standard formalin-fixed paraffin-embedded tissue obtained during a routine diagnostic colonoscopy. If the initial biopsy specimen contains insufficient tumor tissue, a repeat biopsy or evaluation of surgical specimens confirms the tumor's genetic status.

What happens if a rectal tumor recurs after avoiding surgery with immunotherapy?

If non-operative watch-and-wait surveillance identifies local tumor recurrence, the patient remains eligible for standard salvage therapy using pelvic chemoradiation and total mesorectal excision surgery. Because close monitoring uses frequent digital examinations, serial endoscopies, and pelvic MRIs, recurrences are typically identified early while the malignancy remains surgically resectable.