Colorectal Cancer: Causes, Symptoms, Diagnosis and Treatment — Overview, Diagnosis & Treatment Options | MyMedicPlus

Colorectal cancer (CRC) is the third most common cancer and the second leading cause of cancer death worldwide, with approximately 150,000 new US cases and 53,000 deaths annually. CRC encompasses adenocarcinomas of the colon and rectum, arising through the adenoma-carcinoma sequence from premalignant polyps over 10–15 years. The majority of cases are sporadic; hereditary syndromes — including Lynch syndrome (MLH1/MSH2/MSH6/PMS2 germline mutations, 3–5%) and familial adenomatous polyposis (APC mutation, 1%) — account for a minority. Approximately 25% of patients present with distant metastases at diagnosis, most commonly to the liver and lungs. CRC is highly preventable through routine screening and endoscopic polyp removal, which can halt progression before invasive cancer develops. Molecular profiling for RAS, BRAF, HER2, and MSI/MMR status now guides all systemic treatment decisions.

Sporadic CRC arises through accumulated somatic mutations: the classical adenoma-carcinoma sequence (APC loss) accounts for 70–80% of cases, while the serrated pathway accounts for 15–20%. Established risk factors include age over 45, obesity, physical inactivity, diets high in red and processed meat, low dietary fiber, heavy alcohol use, and smoking. Inflammatory bowel disease (Crohn's colitis, ulcerative colitis) significantly elevates CRC risk after 8–10 years of disease extent. Lynch syndrome (hereditary nonpolyposis CRC, HNPCC) caused by germline MMR gene mutations accounts for 3–5% of all CRC. BRAF V600E mutation is found in approximately 10% of sporadic CRC and is associated with poor prognosis. Diabetes mellitus type 2 and metabolic syndrome confer a 30–40% increased risk.

Clinical presentation varies by tumor location. Left-sided colon and rectal tumors cause bright rectal bleeding, hematochezia, altered bowel habits (constipation, diarrhea, pencil-thin stools), tenesmus (incomplete evacuation), and mucus in the stool. Right-sided tumors — particularly cecal and ascending colon — more commonly present with occult bleeding leading to iron-deficiency anemia, fatigue, and weight loss, often without overt rectal bleeding. Approximately 20% of patients present with emergencies: bowel obstruction (colicky abdominal pain, distension, vomiting), perforation, or acute hemorrhage. Constitutional symptoms including unintentional weight loss, anorexia, and progressive fatigue characterize advanced disease with systemic spread. Symptoms of Colorectal Cancer: Causes, Symptoms, Diagnosis and Treatment can range from mild to severe and may develop gradually or appear suddenly. Common presentations include pain, inflammation, or functional impairment related to the affected system. Symptoms may fluctuate over time with periods of remission and exacerbation. Consult a healthcare provider if symptoms persist or worsen, as early diagnosis improves outcomes.

Colonoscopy with biopsy remains the gold standard for diagnosis and concurrent polypectomy. Carcinoembryonic antigen (CEA) is measured at baseline for post-treatment surveillance. CT of the chest, abdomen, and pelvis provides standard staging information. MRI of the pelvis is mandatory for all rectal cancers to define the circumferential resection margin and depth of local invasion, essential for surgical planning. Endoscopic ultrasound assesses early T-stage rectal tumors. Universal MSI/MMR testing (by IHC or PCR) is performed on all CRC specimens. Comprehensive molecular profiling — KRAS exons 2/3/4, NRAS exons 2/3/4, BRAF V600E, HER2 amplification, and NTRK fusions — guides all metastatic treatment decisions. AJCC 8th edition TNM staging (I–IV) directs curative versus palliative intent.

Stage I–II: surgical resection (colectomy with adequate lymphadenectomy) is curative for most patients; adjuvant chemotherapy is considered for high-risk stage II features (T4, perforation, inadequate node harvest, high-grade). Stage III: surgery followed by adjuvant FOLFOX (folinic acid, fluorouracil, oxaliplatin) for 6 months is standard, reducing recurrence by approximately 25%. Locally advanced rectal cancer: neoadjuvant chemoradiation (capecitabine or 5-FU with 45–50.4 Gy) or short-course radiotherapy followed by total mesorectal excision (TME). Stage IV metastatic CRC: FOLFOX or FOLFIRI combined with bevacizumab (RAS-mutant) or cetuximab/panitumumab (left-sided, RAS/BRAF wild-type). MSI-H/dMMR metastatic CRC: pembrolizumab monotherapy is preferred first-line (KEYNOTE-177). HER2-amplified CRC: trastuzumab-based combinations. FOLFOXIRI plus bevacizumab is used for high-volume metastatic disease requiring rapid response.

Overall 5-year survival for all stages is approximately 65%. Stage I exceeds 90%; regional lymph node-positive disease (stage III) achieves approximately 70% with surgery plus adjuvant FOLFOX. Distant metastases (stage IV) historically carry a 14% 5-year survival, improving substantially with modern targeted therapy and immunotherapy. MSI-H metastatic CRC treated with pembrolizumab achieves an objective response rate of 44% and durable long-term responses. Oligometastatic liver or lung disease rendered resectable by chemotherapy offers potential cure in approximately 25–30% of selected patients. The prognosis for Colorectal Cancer: Causes, Symptoms, Diagnosis and Treatment varies depending on severity at diagnosis, the patient's overall health, and how promptly treatment is initiated. With early diagnosis and appropriate management, many patients achieve good outcomes and maintain quality of life. Regular follow-up with healthcare providers is essential to monitor progress, adjust treatment as needed, and detect any complications early. Adherence to prescribed treatments and lifestyle modifications significantly improves long-term prognosis.

Colorectal cancer is one of the most preventable cancers through screening. Average-risk adults should begin screening at age 45; colonoscopy every 10 years is the gold-standard, but stool-based tests (annual FIT, triennial FIT-DNA) are acceptable alternatives. First-degree relatives of CRC patients warrant colonoscopy at age 40 or 10 years before the relative's diagnosis (whichever is earlier). Lynch syndrome carriers require colonoscopy every 1–2 years from age 20–25. Modifiable risk reductions include maintaining a healthy weight, regular physical activity (reduces risk by 20–25%), limiting red meat and processed meat, increasing dietary fiber and calcium intake, avoiding smoking, and limiting alcohol. Low-dose aspirin reduces adenoma recurrence in post-polypectomy patients.

Seek urgent medical evaluation for any rectal bleeding, blood in the stool (bright red or dark/tarry), or unexplained iron-deficiency anemia. Persistent changes in bowel habits lasting more than 2–4 weeks — new-onset constipation, diarrhea, narrowed stool caliber, or a sensation of incomplete emptying — require prompt colonoscopy. Unintentional weight loss of more than 5 kg (10 lbs), persistent abdominal pain or cramping, or a palpable abdominal or rectal mass should prompt same-week evaluation. Emergency care is required for acute bowel obstruction (severe distension, inability to pass stool or gas), suspected perforation (sudden severe abdominal pain with rigidity), or heavy rectal hemorrhage. All adults aged 45 and older should engage in routine CRC screening regardless of symptoms.