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Rectal Cancer Diagnosis

Screening — early detection

Many countries offer systematic screening programs for people over 50, aimed at detecting colorectal polyps and early-stage cancer. The reasons are: early colorectal cancer often produces no symptoms, polyps are well-defined precancerous lesions, and age is an important risk factor.

A typical screening program includes:

  • Fecal occult blood test (FOBT) — detects traces of blood invisible to the naked eye, which may indicate a polyp or tumor. Recommended every 1-2 years
  • Colonoscopy — used to confirm cases with a positive FOBT, or as a primary screening method

Recommendation: Screening is recommended for men and women aged 50 and older. If you have a family history of colorectal cancer, discuss with your doctor about starting screening earlier.

Diagnostic investigations

The diagnosis of rectal cancer requires a series of complementary investigations that provide complete information about the tumor, local extent, and presence of any metastases.

Colonoscopy with biopsy

Colonoscopy is the primary investigation, allowing for:

  • Direct visualization of the tumor
  • Biopsy sampling for histopathological confirmation
  • Exclusion of synchronous lesions in the rest of the colon

The histopathology result confirms the type of cancer (most commonly adenocarcinoma) and the degree of tumor differentiation.

Pelvic MRI (Magnetic Resonance Imaging)

Pelvic MRI is the essential investigation for rectal cancer — it is not routinely used in colon cancer. It provides precise detail about:

  • Depth of tumor invasion in the rectal wall (T stage)
  • Relationship to the mesorectal fascia (CRM — circumferential resection margin)
  • Involvement of local lymph nodes (N stage)
  • Invasion of adjacent structures (urinary bladder, uterus, prostate, sacrum)
  • Presence of extramural vascular invasion (EMVI)

MRI helps decide whether neoadjuvant chemoradiotherapy (before surgery) is needed. After neoadjuvant treatment, it is repeated for re-staging and assessment of treatment response.

Endorectal ultrasound

Endorectal ultrasound assesses the depth of tumor invasion in the rectal wall. It is particularly useful for early-stage tumors (T1-T2), offering excellent resolution of the rectal wall layers.

CT chest-abdomen-pelvis

Computed tomography (CT) is required for:

  • Assessment of distant metastases (liver, lungs)
  • Visualization of regional lymph nodes
  • Planning of complete treatment

TNM staging

Rectal cancer staging follows the TNM (Tumor — Nodes — Metastases) system:

T stage (Tumor)

  • T1 — tumor invades the submucosa
  • T2 — tumor invades the muscularis propria
  • T3 — tumor crosses the muscularis propria into the perirectal tissue (mesorectum)
  • T4 — tumor invades adjacent organs or visceral peritoneum

N stage (Lymph nodes)

  • N0 — no nodal involvement
  • N1 — 1-3 lymph nodes involved
  • N2 — 4 or more lymph nodes involved

M stage (Metastases)

  • M0 — no distant metastases
  • M1 — presence of metastases (liver, lungs, peritoneum)

Importance of accurate staging

Precise staging by MRI is essential because it determines treatment strategy:

  • Early stages (T1-T2, N0) → primary surgery
  • Locally advanced stages (T3-T4 or N+) → neoadjuvant chemoradiotherapy followed by surgery
  • Metastatic disease (M1) → systemic treatment, with surgery possible in selected cases

Discussion of every case in a multidisciplinary oncology board (MDT — Multidisciplinary Team) including surgeon, oncologist, radiotherapist, radiologist and pathologist is the modern standard of care.

Histopathological examination

Histopathological examination is the laboratory analysis of tumor tissue under a microscope. It is performed on the biopsy or polyp obtained by endoscopy, and after surgery — on the surgical specimen, removed lymph nodes and any invaded organs.

Results include:

  • Histological type — most rectal cancers are adenocarcinomas. Subtypes (mucinous, signet ring cell) may influence prognosis
  • Degree of differentiation — from grade 1 (cells similar to normal, better prognosis) to grade 4 (very abnormal cells, more reserved prognosis). Classified as low grade (1-2) and high grade (3-4)
  • Resection margins — checking that the tumor was completely removed. A minimum of 12 lymph nodes must be examined for accurate staging
  • Vascular and perineural invasion — presence of tumor cells in blood vessels or nerves, an indicator of aggressiveness

Molecular profiling and biomarker testing

Molecular profiling analyzes the genetic alterations of the tumor and is essential for choosing the optimal treatment. According to NCCN 2025 guidelines, all rectal cancers must be tested for certain biomarkers.

Required testing

  • MMR/MSI testing — All rectal cancers must be tested for DNA mismatch repair deficiency (dMMR) or high microsatellite instability (MSI-H). Approximately 1 in 6 colorectal cancers shows this biomarker. The result fundamentally influences treatment strategy: dMMR/MSI-H tumors may benefit from immunotherapy as first-line treatment
  • RAS mutations (KRAS and NRAS) — about 50% of metastatic colorectal cancers have this mutation. Its presence makes anti-EGFR therapies (cetuximab, panitumumab) ineffective
  • BRAF mutation — present in 5-10% of cases. The BRAF V600E mutation has specific targeted therapy (encorafenib + cetuximab)
  • HER2 amplification — a newer biomarker for which targeted therapies exist (trastuzumab + pertuzumab)
  • PIK3CA testing — for stage II or III cancers before surgery. Tumors with PIK3CA mutation respond better to aspirin for recurrence prevention. Aspirin for 3 years after surgery is recommended in these patients
  • DPYD testing — 1 in 15 people inherits a variant of the DPYD gene that causes DPD deficiency. These individuals cannot properly metabolize fluoropyrimidine chemotherapy (5-FU, capecitabine), risking severe and potentially fatal adverse effects. Testing is recommended before starting chemotherapy
  • Next-generation sequencing (NGS) — can identify rare biomarkers for which targeted treatments exist: NTRK fusions, RET fusions, KRAS G12C mutations, POLE/POLD1 mutations

Circulating tumor DNA (ctDNA)

ctDNA testing (also called “liquid biopsy”) looks for small fragments of tumor DNA released into the blood. It can detect microscopic residual disease after surgery. At this time, ctDNA testing is not recommended as standard of care, but is under active research.

These analyses help doctors personalize treatment, understand the tumor’s metastatic potential, and the likelihood of recurrence.