Cancer is a group of diseases characterized by uncontrolled cell proliferation, resistance to cell death, and the ability to invade adjacent tissues and spread to distant sites. It is the second leading cause of death globally, responsible for approximately 10 million deaths annually. However, advances in early detection, molecular targeting, and immunotherapy have steadily improved outcomes for many cancer types.
The Biology of Cancer
Hallmarks of Cancer
Cancer cells acquire distinct capabilities during their evolution from normal cells. These hallmarks include sustaining proliferative signaling, evading growth suppressors, resisting cell death, enabling replicative immortality, inducing angiogenesis, and activating invasion and metastasis. Two enabling characteristics — genome instability and inflammation — facilitate the acquisition of these hallmarks. Tumor-promoting inflammation and evasion of immune destruction have been added as additional hallmarks.
Genetic and Epigenetic Basis
Cancer arises from the accumulation of genetic mutations in oncogenes, tumor suppressor genes, and DNA repair genes. Oncogenes (such as RAS, MYC, and EGFR) are activated by gain-of-function mutations that promote cell growth. Tumor suppressor genes (such as TP53, RB1, and APC) are inactivated by loss-of-function mutations that remove growth constraints. Epigenetic alterations — abnormal DNA methylation, histone modifications, and chromatin remodeling — also silence tumor suppressor genes and activate oncogenes without changing the DNA sequence.
Carcinogenesis
The process of cancer development occurs over years to decades. Initiation is the first genetic alteration in a cell. Promotion involves the expansion of initiated cells through repeated exposure to promoting factors (hormones, growth factors, inflammation). Progression is the acquisition of additional mutations that lead to invasive and metastatic behavior.
Diagnosis and Staging
Diagnostic Modalities
Screening. Screening tests detect cancer at an early, treatable stage in asymptomatic individuals. Examples include mammography (breast), Pap smear (cervix), colonoscopy (colorectal), low-dose CT (lung in high-risk individuals), and PSA testing (prostate). Each test balances sensitivity, specificity, and the risk of overdiagnosis.
Biopsy and pathology. Definitive diagnosis requires tissue sampling. Pathological evaluation determines the tumor type, grade (degree of differentiation), and molecular markers. Immunohistochemistry identifies cell surface proteins (e.g., ER/PR in breast cancer, CD20 in lymphoma). Genetic profiling identifies driver mutations actionable with targeted therapies.
Imaging. CT, MRI, PET-CT, and ultrasound are used for initial staging, assessment of treatment response, and surveillance for recurrence. PET-CT with FDG (a glucose analog) exploits the increased glucose uptake of cancer cells.
Staging Systems
TNM staging. The most widely used system: T describes the primary tumor size and extent (T0–T4), N describes regional lymph node involvement (N0–N3), and M describes distant metastasis (M0 or M1). The TNM combination assigns a stage (0, I, II, III, IV) that guides prognosis and treatment.
Stage grouping. Stage 0 is carcinoma in situ (non-invasive). Stage I is early, localized cancer. Stage II and III are locally advanced with regional spread. Stage IV is metastatic disease. Five-year survival rates vary dramatically by stage at diagnosis.
Major Cancer Types
Lung Cancer
The leading cause of cancer death worldwide. Non-small cell lung cancer (NSCLC; 85%) includes adenocarcinoma, squamous cell carcinoma, and large cell carcinoma. Small cell lung cancer (SCLC; 15%) is highly aggressive and strongly associated with smoking. Molecular testing for EGFR mutations, ALK rearrangements, and PD-L1 expression guides targeted therapy and immunotherapy. The USPSTF recommends annual low-dose CT screening for high-risk individuals aged 50–80 with a 20 pack-year smoking history.
Breast Cancer
The most common cancer in women worldwide. Risk factors include age, BRCA1/BRCA2 mutations, early menarche, late menopause, nulliparity, hormone replacement therapy, and dense breast tissue. Subtypes are defined by hormone receptor (ER/PR) status and HER2 expression: luminal A (ER+, HER2−, low grade), luminal B (ER+, HER2±), HER2-enriched, and triple-negative (ER−, PR−, HER2−). Treatment includes surgery, radiation, endocrine therapy (tamoxifen, aromatase inhibitors), anti-HER2 therapy (trastuzumab), chemotherapy, and immunotherapy.
Colorectal Cancer
The third most common cancer worldwide. Most cases arise from adenomatous polyps through the adenoma-carcinoma sequence driven by APC, KRAS, and TP53 mutations. Screening with colonoscopy reduces incidence and mortality. Treatment depends on stage: surgical resection for localized disease, adjuvant chemotherapy (FOLFOX, FOLFIRI) for node-positive disease, and systemic therapy with or without targeted agents (anti-EGFR, anti-VEGF) for metastatic disease.
Prostate Cancer
The most common cancer in men. Most prostate cancers are adenocarcinomas that grow slowly. PSA screening has reduced mortality but causes substantial overdiagnosis. Active surveillance is appropriate for low-risk disease. Treatment options include radical prostatectomy, radiation therapy, androgen deprivation therapy (ADT), and chemotherapy for castration-resistant disease.
Pancreatic Cancer
Among the most lethal cancers, with a 5-year survival rate of 12%. Most patients present with advanced disease. Risk factors include smoking, obesity, diabetes, chronic pancreatitis, and family history. Surgical resection (Whipple procedure) offers the only chance of cure but is possible in only 15–20% of patients. Chemotherapy (FOLFIRINOX, gemcitabine + nab-paclitaxel) is used in the adjuvant and metastatic settings.
Cancer Treatment
Local Therapies
Surgery. Complete surgical resection with negative margins (R0 resection) is the primary curative treatment for solid organ cancers. Minimally invasive and robotic approaches reduce morbidity.
Radiation therapy. Ionizing radiation damages DNA, preferentially killing rapidly dividing cells. External beam radiation (EBRT) is the most common delivery method. Brachytherapy places radioactive sources within or near the tumor. Stereotactic radiosurgery (Gamma Knife, CyberKnife) delivers precise, high-dose radiation to small targets.
Systemic Therapies
Chemotherapy. Cytotoxic drugs target rapidly dividing cells by interfering with DNA synthesis (alkylating agents, antimetabolites), mitotic spindle function (taxanes, vinca alkaloids), or topoisomerase activity (anthracyclines, irinotecan). Side effects reflect effects on normal rapidly dividing tissues: bone marrow suppression, mucositis, alopecia, and nausea.
Targeted therapy. Small molecule inhibitors and monoclonal antibodies target specific molecular drivers of cancer. Examples include imatinib (BCR-ABL in CML), osimertinib (EGFR mutants in NSCLC), trastuzumab (HER2 in breast cancer), and vemurafenib (BRAF V600E in melanoma). Targeted therapies are generally better tolerated than chemotherapy but invariably lead to resistance through secondary mutations.
Immunotherapy. Immune checkpoint inhibitors (anti-PD-1, anti-PD-L1, anti-CTLA-4) release the brakes on T-cell activity, enabling immune-mediated tumor destruction. They have transformed the treatment of melanoma, lung cancer, renal cell carcinoma, and many other cancers. Chimeric antigen receptor (CAR) T-cell therapy engineers a patient’s own T cells to recognize and kill cancer cells, achieving remarkable results in B-cell malignancies.
Hormonal therapy. Endocrine-sensitive cancers (breast, prostate) depend on hormonal signaling for growth. Strategies include reducing hormone production (aromatase inhibitors, GnRH agonists), blocking hormone receptors (tamoxifen, anti-androgens), or ablating hormone-producing organs (oophorectomy, orchiectomy).
Prevention and Early Detection
Primary prevention. Avoiding tobacco, maintaining a healthy weight, exercising regularly, limiting alcohol, protecting skin from UV exposure, and receiving vaccines against oncogenic viruses (HPV, hepatitis B).
Secondary prevention (screening). Regular screening for breast, cervical, colorectal, and lung cancer in appropriate populations reduces mortality. Emerging technologies include multi-cancer early detection tests (MCEDs) that detect circulating tumor DNA and proteins from multiple cancer types simultaneously.
Chemoprevention. The use of drugs to prevent cancer. Examples include tamoxifen and raloxifene for breast cancer in high-risk women, aspirin for colorectal cancer, and finasteride for prostate cancer.
Survivorship
There are over 18 million cancer survivors in the United States alone. Survivorship care addresses late effects of treatment (cardiotoxicity, secondary malignancies, cognitive impairment, fatigue), surveillance for recurrence, management of chronic pain, psychosocial support, and healthy lifestyle promotion. The transition from active treatment to survivorship requires coordinated care between oncologists, primary care providers, and allied health professionals.
Summary
Cancer is not a single disease but hundreds of distinct entities, each with unique biology and clinical behavior. The hallmarks of cancer provide a conceptual framework for understanding malignant transformation. Accurate diagnosis, staging, and molecular profiling guide treatment decisions. Local therapies (surgery, radiation) cure early-stage disease, while systemic therapies (chemotherapy, targeted therapy, immunotherapy, hormonal therapy) are essential for advanced disease. Prevention, early detection, and survivorship care are integral to reducing the cancer burden. The field continues to evolve rapidly, with immunotherapy, precision oncology, and early detection technologies offering unprecedented opportunities to improve outcomes.