Rectal Cancer

Rectal cancer is malignancy arising within the rectum and differs from colon cancer because local pelvic recurrence, circumferential resection margin, sphincter preservation and pelvic autonomic function strongly influence treatment.

Accurate preoperative pelvic staging determines whether surgery should occur first or after neoadjuvant treatment.

Total mesorectal excision is the oncological foundation for most mid and low rectal cancers.

Common features are rectal bleeding, altered bowel habit, tenesmus, urgency, incomplete evacuation and narrowing of stool calibre.

Low tumours may be palpable on digital rectal examination.

Advanced disease can cause pelvic pain, urinary symptoms, fistulation, obstruction or fixation to surrounding pelvic organs.

Every rectal cancer examination should document tumour height, mobility, circumferential involvement, relation to the sphincter complex and baseline continence.

Colonoscopy with biopsy: establishes histology and excludes synchronous proximal disease.

High resolution pelvic MRI: key local staging investigation; define T stage, mesorectal nodes, mesorectal fascia, extramural vascular invasion and relationship to the sphincter complex.

CT chest, abdomen and pelvis: stage distant disease.

Obtain baseline carcinoembryonic antigen and mismatch repair or microsatellite instability testing.

Early rectal cancer

Carefully selected favourable T1 N0 cancers can undergo full thickness transanal local excision when complete en bloc excision with adequate margins is achievable.

Poor differentiation, lymphovascular invasion, significant tumour budding, deep submucosal invasion or an involved margin after local excision generally requires radical rectal resection because nodal disease cannot be treated by local excision alone.

Locally advanced disease

Most stage II and III mid or low rectal cancers are treated with total neoadjuvant therapy, delivering systemic chemotherapy and pelvic radiation before definitive surgery.

A standard modified FOLFOX6 cycle is:

• Oxaliplatin 85 mg/m² IV
• Leucovorin 400 mg/m² IV
• Fluorouracil 400 mg/m² IV bolus
• Fluorouracil 2400 mg/m² over 46 hours

every 14 days.

Long course chemoradiation commonly uses approximately 50 to 50.4 Gy with:

Capecitabine 825 mg/m² orally twice daily on radiotherapy days.

Short course radiotherapy uses 25 Gy in 5 fractions and can be incorporated into a total neoadjuvant strategy.

Selected lower risk locally advanced cancers suitable for sphincter preserving surgery can receive neoadjuvant FOLFOX with selective use of radiation if response is inadequate. Six cycles of FOLFOX was noninferior to routine chemoradiation in this carefully selected population.

Surgery

Low anterior resection with total mesorectal excision is preferred when an oncologically safe distal margin and functional sphincter can be preserved.

Abdominoperineal resection is required when the tumour directly involves the sphincter or levator complex or when a safe functional anastomosis cannot be achieved.

A diverting loop ileostomy is commonly used after a very low pelvic anastomosis when consequences of leakage would be substantial.

Complete clinical response

A patient who achieves a rigorously documented complete clinical response after neoadjuvant therapy can be considered for a structured nonoperative surveillance strategy in an experienced rectal cancer programme. This is not casual observation. It requires intensive digital examination, endoscopy, pelvic MRI and systemic surveillance.

Mismatch repair deficient locally advanced rectal cancer can show profound response to PD1 blockade. Dostarlimab has been studied at 500 mg IV every 3 weeks for approximately 6 months in this setting, with organ preservation possible in complete responders under specialist surveillance.