USMLE Guide: Colorectal Cancer

Introduction

Colorectal cancer, also known as colon cancer or rectal cancer, refers to the malignant growth of cells in the colon or rectum. It is one of the most common types of cancer worldwide, and its incidence increases with age. This USMLE guide aims to provide a comprehensive overview of colorectal cancer, including its epidemiology, risk factors, clinical presentation, diagnosis, staging, treatment, and prognosis.

Epidemiology

• Colorectal cancer is the third most common cancer globally and the second leading cause of cancer-related deaths.
• Incidence rates vary geographically, with higher rates observed in developed countries.
• Age is a significant risk factor, with the majority of cases occurring in individuals over 50 years of age.

Risk Factors

• Age: As mentioned earlier, the risk of colorectal cancer increases with age.
• Family history: Individuals with a first-degree relative (parent, sibling, or child) diagnosed with colorectal cancer have an increased risk.
• Inherited genetic syndromes: Conditions such as familial adenomatous polyposis (FAP) and hereditary nonpolyposis colorectal cancer (HNPCC) contribute to a higher risk.
• Personal history: Individuals with a history of inflammatory bowel disease (e.g., ulcerative colitis, Crohn's disease) are at an increased risk.
• Lifestyle factors: Obesity, sedentary lifestyle, tobacco use, excessive alcohol consumption, and a diet high in red and processed meats have been associated with an increased risk.

Clinical Presentation

• Early stages: Often asymptomatic, but may present with nonspecific symptoms such as fatigue, weight loss, or changes in bowel habits.
• Advanced stages: Patients may experience rectal bleeding, abdominal pain, iron deficiency anemia, palpable abdominal masses, or bowel obstruction.

Diagnosis

• Colonoscopy: Gold standard for diagnosis, allowing direct visualization of the colon and rectum to identify and biopsy suspicious lesions.
• Stool tests: Fecal occult blood test (FOBT) or fecal immunochemical test (FIT) can be used for screening purposes.
• Imaging: Computed tomography (CT) scans, magnetic resonance imaging (MRI), or positron emission tomography (PET) may be used for staging and evaluation of metastasis.

Staging

• Colorectal cancer is staged using the American Joint Committee on Cancer (AJCC) TNM system.
• TNM stands for Tumor (extent of primary tumor), Node (lymph node involvement), and Metastasis (presence of distant metastasis).
• Staging helps determine prognosis and guide treatment decisions.

Treatment

• Treatment options depend on the stage, location, and overall health of the patient.
• Surgery: The primary treatment for localized colorectal cancer involves surgical resection of the tumor.
• Chemotherapy: Administered preoperatively (neoadjuvant), postoperatively (adjuvant), or for advanced/metastatic disease.
• Radiation therapy: Often used in combination with surgery or chemotherapy, especially for rectal cancer.
• Targeted therapies: Certain genetic mutations or biomarkers may guide the use of targeted therapies, such as anti-EGFR or anti-VEGF agents.

Prognosis

• Prognosis is influenced by various factors, including stage at diagnosis, tumor grade, patient's overall health, and response to treatment.
• Five-year survival rates range from over 90% for localized disease to less than 15% for metastatic disease.
• Regular surveillance and follow-up are essential to detect recurrence or metastasis.

By understanding the epidemiology, risk factors, clinical presentation, diagnosis, staging, treatment modalities, and prognosis of colorectal cancer, medical professionals can effectively manage and improve outcomes for patients affected by this disease.