What Is Cancer? Understanding the Basics

Cancer begins when changes, or mutations, occur in the genes that control how cells grow, divide, and die. Normally the body has checks in place to repair damaged cells or remove them, but in cancer these controls fail, allowing abnormal cells to multiply. Over time these cells can form a mass called a tumour, or in blood cancers, circulate through the bloodstream and bone marrow. Not every tumour is cancerous; benign tumours do not invade nearby tissue or spread.

A cancer diagnosis can feel overwhelming, but understanding that it stems from changes within your own cells, rather than something external, can help make sense of why testing and treatment focus on the tumour's specific characteristics. Your care team will use this biological information to choose the treatments most likely to work for your particular cancer.

Symptoms of cancer vary widely depending on the organ affected and can include unexplained weight loss, persistent fatigue, unusual lumps or swelling, changes in bowel or bladder habits, or abnormal bleeding. Some cancers cause no noticeable symptoms until they are found on screening or imaging done for another reason. Any persistent or unusual symptom should be discussed with a doctor.

Diagnosis usually begins with a physical examination and review of symptoms, followed by imaging tests such as CT, MRI, or PET scans to identify suspicious areas. A biopsy is then performed to remove a small tissue sample, which a pathologist examines under a microscope to confirm whether cancer is present and, if so, what type.

Alongside imaging and biopsy, blood tests, molecular profiling, and genetic testing may be used to better characterise the cancer and check for specific mutations that could influence treatment choice. These additional tests help build a fuller picture of the cancer's behaviour.

Cancers are typically named after the organ or tissue in which they start, such as breast, lung, or colon cancer, and can also be classified by the type of cell involved, such as carcinomas, sarcomas, leukaemias, and lymphomas. Some cancers, like metastatic disease, are described by where they have spread to as well as where they began.

Treatment options can include surgery to remove the tumour, radiation therapy to destroy cancer cells with targeted energy, and systemic therapies such as chemotherapy, targeted therapy, hormone therapy, or immunotherapy that work throughout the body. Many people receive a combination of these approaches, chosen based on the cancer's type, stage, and biology.