Table of Contents

What Is Oncology?

Oncology is the branch of medicine dedicated to the diagnosis, treatment, and study of cancer — a group of diseases characterized by uncontrolled cell growth that can invade surrounding tissues and spread throughout the body. An oncologist is a physician who specializes in managing cancer from initial diagnosis through treatment and survivorship.

What Cancer Actually Is

Cancer isn’t one disease. It’s more than 200 different diseases that share a common feature: cells that grow without the normal checks and balances.

Your body contains roughly 37 trillion cells, and they’re constantly dividing — producing about 3.8 million new cells every second. Normally, this process is tightly regulated. Cells divide when needed, stop when they should, and self-destruct (a process called apoptosis) when they’re damaged. Cancer happens when these regulatory mechanisms break down.

The breakdown starts with genetic mutations — changes in DNA that alter the instructions a cell follows. Some mutations activate genes that promote cell growth (oncogenes). Others disable genes that normally suppress growth (tumor suppressors). When enough of these mutations accumulate in a single cell — usually 3 to 7 key driver mutations — that cell begins dividing without restraint, ignoring signals to stop, evading the immune system, and eventually invading surrounding tissue.

The resulting mass of abnormal cells is a tumor. But not all tumors are cancer. Benign tumors stay put — they don’t invade neighboring tissue or spread elsewhere. Malignant tumors do both. It’s the ability to invade and metastasize (spread to distant organs) that makes cancer dangerous.

The Three Pillars of Oncology

Oncology divides into three main subspecialties, and most cancer patients encounter all three during their care.

Medical Oncology

Medical oncologists are the quarterbacks of cancer care. They diagnose cancer, determine its stage and grade, develop treatment plans, and administer systemic therapies — treatments that affect the whole body rather than just one area. This includes chemotherapy, immunotherapy, targeted therapy, and hormonal therapy.

Medical oncologists typically manage the overall treatment plan and coordinate with surgical and radiation oncologists. After treatment ends, they provide long-term follow-up surveillance.

Surgical Oncology

Surgical oncologists remove tumors and surrounding tissue, perform biopsies for diagnosis, and sometimes carry out preventive surgeries (like mastectomy for patients with BRCA mutations and very high breast cancer risk). Surgery remains the most common treatment for solid tumors — and for many early-stage cancers, it’s the only treatment needed.

The trend in surgical oncology is toward less radical procedures. Fifty years ago, breast cancer usually meant a radical mastectomy — removal of the entire breast, chest muscles, and all nearby lymph nodes. Today, many patients are candidates for lumpectomy (removing just the tumor and a small margin) followed by radiation, with equivalent survival rates.

Radiation Oncology

Radiation oncologists use high-energy radiation to kill cancer cells or shrink tumors. About half of all cancer patients receive radiation at some point during treatment. Modern radiation technology — including intensity-modulated radiation therapy (IMRT), stereotactic body radiation therapy (SBRT), and proton beam therapy — allows precise targeting of tumors while minimizing damage to surrounding healthy tissue.

A typical radiation treatment course involves daily sessions (fractions) over several weeks — commonly 25 to 30 sessions for many cancers. Each session usually takes 15 to 30 minutes, with the actual radiation delivery lasting only a few minutes.

How Cancer Is Diagnosed and Staged

Diagnosis usually starts with imaging — CT scans, MRIs, PET scans, or ultrasound — and is confirmed by biopsy, in which a sample of the suspicious tissue is examined under a microscope by a pathologist. The pathologist determines whether cells are cancerous and, if so, what type of cancer is present.

Staging describes how far the cancer has spread. The TNM system — used for most solid tumors — evaluates three factors:

T (Tumor): How large is the primary tumor?
N (Nodes): Has the cancer spread to nearby lymph nodes?
M (Metastasis): Has it spread to distant organs?

These factors combine into an overall stage (0 through IV). Stage matters because it guides treatment decisions and predicts outcomes. Stage I colon cancer has a 5-year survival rate above 90%. Stage IV is closer to 14%.

Treatment Approaches

Chemotherapy

Chemotherapy uses drugs that kill rapidly dividing cells. Since cancer cells divide faster than most normal cells, they’re disproportionately affected — but so are other fast-dividing cells like hair follicles, bone marrow, and the lining of the digestive tract. That’s why chemo causes hair loss, suppressed immune function, and nausea.

There are over 100 chemotherapy drugs, often used in combinations. Treatment is typically given in cycles — periods of treatment followed by recovery periods — over weeks or months. The specific regimen depends on the cancer type, stage, and the patient’s overall health.

Immunotherapy

This is the big story in oncology over the past decade. Immunotherapy doesn’t attack cancer directly — it unleashes the body’s own immune system to do the job.

The breakthrough was checkpoint inhibitors. Cancer cells often express proteins like PD-L1 that bind to checkpoints on immune cells, essentially telling them to stand down. Checkpoint inhibitor drugs block those proteins, allowing immune cells to recognize and attack the cancer. The first checkpoint inhibitor, ipilimumab, was approved in 2011 for melanoma. Since then, the class has expanded to treat lung cancer, bladder cancer, kidney cancer, Hodgkin lymphoma, and others.

CAR-T cell therapy goes even further — engineers a patient’s own T cells in a laboratory to recognize specific cancer markers, then infuses them back into the patient. It’s shown remarkable results in certain blood cancers, with some patients achieving complete remission after failing all other treatments.

James Allison and Tasuku Honjo won the 2018 Nobel Prize in Physiology or Medicine for their foundational work on checkpoint inhibitors. Deservedly so — these drugs have genuinely changed the survival curve for several cancer types.

Targeted Therapy

Targeted therapies home in on specific molecular changes that drive a particular cancer. The poster child is imatinib (Gleevec), approved in 2001 for chronic myeloid leukemia (CML). CML is caused by a specific chromosomal abnormality (the Philadelphia chromosome) that produces an abnormal protein. Imatinib blocks that protein. Before Gleevec, the 5-year survival rate for CML was about 30%. After Gleevec, it exceeded 90%.

Other targeted therapies address HER2-positive breast cancer (trastuzumab/Herceptin), EGFR-mutant lung cancer (erlotinib, osimertinib), and BRAF-mutant melanoma (vemurafenib). The catch is that targeted therapies only work if the cancer has the specific target — genomic testing of the tumor is essential to determine eligibility.

Hormonal Therapy

Some cancers — particularly breast and prostate cancers — are fueled by hormones. Hormonal therapy blocks the body’s production of those hormones or prevents cancer cells from using them. Tamoxifen, for example, blocks estrogen receptors in breast cancer cells. Androgen deprivation therapy reduces testosterone levels in prostate cancer. These treatments can be remarkably effective but come with their own side effects — hot flashes, bone loss, fatigue, and sexual dysfunction.

Cancer by the Numbers

The scale of cancer globally is difficult to overstate.

About 20 million new cancer cases were diagnosed worldwide in 2022 (IARC data)
Cancer killed approximately 9.7 million people globally in 2022
In the United States, about 2 million new cases are diagnosed annually
The lifetime probability of developing cancer in the U.S. is roughly 40% for men and 39% for women

But there’s good news too. Overall cancer death rates in the United States have fallen about 33% since their peak in 1991, driven by reductions in smoking, earlier detection, and better treatment. The 5-year survival rate for all cancers combined now exceeds 68%, up from 49% in the mid-1970s.

Some cancers have seen dramatic improvements. Childhood acute lymphoblastic leukemia went from nearly universally fatal in the 1960s to over 90% curable today. Testicular cancer, even when metastatic, has a cure rate above 95%. Melanoma, once a death sentence when advanced, now has immunotherapy options that produce long-term survival in a significant fraction of patients.

Screening and Prevention

The unglamorous truth is that the most effective cancer strategies are prevention and early detection, not treatment.

About 40% of cancers are linked to modifiable risk factors — tobacco use (responsible for about 30% of cancer deaths), obesity, alcohol, physical inactivity, poor diet, UV exposure, and certain infections (HPV, hepatitis B and C, H. pylori). Eliminating tobacco alone would prevent roughly 480,000 deaths per year in the U.S.

Screening — mammograms for breast cancer, colonoscopies for colorectal cancer, low-dose CT scans for lung cancer in heavy smokers, Pap smears and HPV tests for cervical cancer — catches cancer early, when it’s most treatable. Cervical cancer was once a leading cause of cancer death in American women. Regular screening has made it relatively rare in countries with good Pap smear programs.

The Emotional Reality

One thing that doesn’t show up in survival statistics: the psychological weight of cancer. A cancer diagnosis is among the most stressful events a person can experience. Fear, grief, anger, depression, anxiety, disrupted relationships, financial toxicity (cancer treatment costs can be catastrophic, even with insurance), and existential uncertainty are all part of the package.

Good oncology care recognizes this and integrates psychosocial support — social workers, psychologists, support groups, palliative care teams focused on quality of life — alongside medical treatment. The field has gotten better at this, though it still has a long way to go. Treating the disease without treating the person is only half the job.

Frequently Asked Questions

What is the difference between benign and malignant tumors?

Benign tumors are non-cancerous growths that don't invade surrounding tissue or spread to other parts of the body. They can still cause problems by pressing on nearby structures, but they're generally not life-threatening and can usually be removed surgically. Malignant tumors are cancerous — they invade surrounding tissues, can spread (metastasize) to distant organs through the blood or lymphatic system, and can be fatal if untreated. The distinction matters enormously for prognosis and treatment decisions.

What is cancer staging and what do the stages mean?

Staging describes how far a cancer has spread. The most common system uses stages 0 through IV. Stage 0 means abnormal cells are present but haven't invaded nearby tissue (carcinoma in situ). Stage I is a small, localized cancer. Stages II and III involve larger tumors or spread to nearby lymph nodes. Stage IV means the cancer has metastasized to distant organs. Higher stages generally mean more aggressive treatment and lower survival rates, though this varies widely by cancer type.

What are the most common types of cancer?

In the United States, the most commonly diagnosed cancers are breast cancer (about 313,510 new cases projected in 2024), prostate cancer (299,010), lung and bronchus cancer (234,580), colon and rectal cancer (152,810), and melanoma of the skin (100,640). Lung cancer remains the leading cause of cancer death for both men and women, accounting for about 21% of all cancer deaths, despite not being the most commonly diagnosed.

What is immunotherapy and how does it work?

Immunotherapy is a cancer treatment that helps your own immune system recognize and attack cancer cells. Cancer cells often evade the immune system by displaying proteins that act as 'don't attack me' signals. Checkpoint inhibitor drugs — like pembrolizumab (Keytruda) and nivolumab (Opdivo) — block those signals, allowing immune cells to recognize and destroy the cancer. Other approaches include CAR-T cell therapy, which engineers a patient's own immune cells to target specific cancer markers. Immunotherapy has dramatically improved outcomes for several cancer types, including melanoma, lung cancer, and certain lymphomas.