Rectum
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Etiology & Risk Factors
Rectal cancer arises through a multi-step progression from adenomatous polyps to adenocarcinoma, driven by specific genetic alterations and environmental stressors.
1. Genetic Predispositions
- Chromosomal Instability (CIN) Pathway: Found in ~85% of cases, involving mutations in the APC, TP53, and KRAS genes.
- Lynch Syndrome (HNPCC): Germline mutations in DNA mismatch repair (MMR) genes (MLH1, MSH2, MSH6, PMS2).
- Familial Adenomatous Polyposis (FAP): Caused by germline mutations in the APC gene, leading to hundreds of polyps.
2. Environmental & Lifestyle Triggers
- Dietary Factors: High consumption of processed and red meats; low dietary fiber intake.
- Modifiable Factors: Obesity and Type 2 Diabetes are strongly correlated with increased risk.
- Inflammatory Bowel Disease (IBD): Long-standing Ulcerative Colitis or Crohn’s Colitis significantly elevates risk.
- The "Early-Onset" Trend: There is a notable global increase in rectal cancer among adults under age 50, likely linked to changes in the gut microbiome and sedentary lifestyles.
Clinical Manifestations
Symptoms often depend on the tumor’s proximity to the anal verge and the degree of luminal obstruction.
Early-Stage Symptoms
- Hematochezia: Bright red blood per rectum (often mistaken for hemorrhoids).
- Change in Bowel Habits: Particularly a new onset of constipation or "pencil-thin" stools.
- Tenesmus: The distressing sensation of incomplete evacuation.
Advanced-Stage Symptoms
- Pelvic Pain: May indicate invasion into the sacral plexus or adjacent pelvic organs.
- Unintentional Weight Loss: Often associated with systemic spread.
- Intestinal Obstruction: Nausea, vomiting, and abdominal distension.
- Paraneoplastic Syndromes: Though rarer than in lung or renal cancers, rectal cancer can be associated with Dermatomyositis or Trousseau Syndrome (migratory thrombophlebitis).
Diagnostic Workup
A definitive diagnosis requires a combination of physical examination and histopathological confirmation.
- Digital Rectal Exam (DRE): A critical initial step to assess tumor height, fixation, and distance from the sphincter complex.
- Colonoscopy / Proctosigmoidoscopy: To accurately measure the distance from the anal verge and obtain biopsies, also to look for any synchronous lesions in proximal colon.
- Biomarker & Molecular Testing: Mandatory for all advanced or metastatic cases, including:
- MSI/MMR Status: To identify Lynch syndrome and eligibility for immunotherapy.
- RAS (KRAS/NRAS) and BRAF V600E mutations: To guide targeted therapy selection.
- HER2 Amplification: An emerging target in metastatic colorectal cancer.
- Liquid Biopsy (ctDNA): Increasingly used for monitoring Minimal Residual Disease (MRD) post-treatment.
Common Investigations & Staging
Accuracy in staging is paramount, as it determines the feasibility of organ preservation.
Imaging Modalities
- Pelvic MRI (High-Resolution): The gold standard for local staging. It is essential for evaluating the Circumferential Resection Margin (CRM) and the Extramural Venous Invasion (EMVI).
- CT Chest/Abdomen/Pelvis: Standard for detecting distant metastasis, particularly to the liver and lungs.
- Endorectal Ultrasound (ERUS): Highly effective for distinguishing T1 from T2 tumors; often used when local excision is considered.
- PET-CT: Generally reserved for clarifying indeterminate findings on CT or evaluating recurrent disease.
Treatment Overview
Current management emphasizes Total Neoadjuvant Therapy (TNT)—delivering all chemotherapy and radiation prior to surgery.
Surgery
- Total Mesorectal Excision (TME): The surgical gold standard, involving the sharp dissection of the fatty tissue envelope surrounding the rectum.
- Sphincter-Sparing Surgery: Low Anterior Resection (LAR) with coloanal anastomosis.
- Abdominoperineal Resection (APR): Required when the tumor involves the external sphincter, resulting in a permanent colostomy.
- Minimally Invasive Surgery (MIS): Robotic-assisted TME is increasingly preferred over laparoscopy for better visualization in the narrow male pelvis.
Radiotherapy
- Short-Course Radiation (SCRT): 25 Gy delivered in 5 fractions over one week, often followed by immediate surgery or consolidation chemotherapy.
- Long-Course Chemoradiation (LCRT): 45–50 Gy with concurrent 5-FU or Capecitabine over 5–6 weeks.
- IMRT/SBRT: Used to limit dose to the small bowel and bladder.
Systemic Therapy
- Chemotherapy: FOLFOX (5-FU, Leucovorin, Oxaliplatin) or CAPOX remain the backbone of treatment.
- Immunotherapy: Dostarlimab or Pembrolizumab are now first-line considerations for patients with dMMR/MSI-H rectal cancer, showing unprecedented clinical complete response rates.
- Targeted Therapy: Anti-EGFR agents (Cetuximab) for RAS wild-type; Anti-VEGF (Bevacizumab) for metastatic disease.
Emerging Therapies & Strategies
Organ Preservation (The "Watch & Wait" Protocol)
For patients who achieve a Clinical Complete Response (cCR) after neoadjuvant therapy (no visible tumor on DRE, endoscopy, and MRI), surgery may be omitted in Favor of strict “Watch & Wait” surveillance. This avoids the morbidity of a permanent stoma or “Low Anterior Resection Syndrome” (LARS).
Targeted Neo-Adjuvant Approaches
- Selective Elimination of Radiation: Trials (like PROSPECT) have shown that for certain mid-to-high rectal cancers, neoadjuvant chemotherapy alone may be sufficient, sparing patients the long-term toxicities of pelvic radiation.
- ctDNA-Guided Therapy: Utilizing liquid biopsies to escalate or de-escalate treatment based on the presence of circulating tumor DNA.
Medical Disclaimer
This clinical guide is for professional educational purposes and reflects the standard of care as of 2026.
It should not replace clinical judgment or official institutional protocols.
Treatment must be individualized.
Please consult the latest NCCN or ESMO guidelines for specific dosing and therapeutic updates.