Cancer is not a singular disease, but a complex group of over a hundred distinct diseases, all characterized by uncontrolled cell growth. Normally, cells in the body grow and divide in an orderly fashion, replacing old or damaged cells as needed. However, in cancer, this regulated process breaks down, leading to the abnormal proliferation of cells that can form tumors and spread throughout the body.
Why Classifying Cancer Matters
Classifying cancer provides medical professionals with a shared language for accurate diagnosis. This categorization helps determine the likely course of the disease, known as the prognosis. For instance, a higher-grade tumor often indicates a more aggressive cancer that may spread quickly, while a lower-grade tumor suggests a slower-growing cancer.
Classification also guides treatment decisions, enabling doctors to choose effective therapies. Different types and stages often require distinct treatment approaches, such as specific chemotherapies or radiation therapies. Classification allows healthcare providers to compare treatment outcomes and ensure standard guidelines are followed, leading to more tailored patient care.
Classification by Tissue Type
Cancer is primarily classified based on the type of cell or tissue where it first originates. This histological classification helps identify the tumor’s fundamental nature. The International Classification of Diseases for Oncology (ICD-O-3) is a widely used standard for this categorization.
Common classifications by tissue type include:
- Carcinomas: The most common type, accounting for 80% to 90% of all cases. They develop from epithelial cells lining external body surfaces (e.g., skin) and internal organs (e.g., lungs, breast, colon, prostate).
- Sarcomas: Originate in connective or supportive tissues, such as bone, cartilage, fat, muscle, and blood vessels. Examples include osteosarcoma and leiomyosarcoma.
- Leukemias: Cancers that start in blood-forming tissue, primarily the bone marrow, leading to abnormal white blood cells in the bloodstream.
- Lymphomas: Begin in lymphocytes, a type of white blood cell involved in the immune system, affecting the lymphatic system.
- Myelomas: Cancers that specifically arise from plasma cells, another type of immune cell found within the bone marrow.
- Central Nervous System Tumors: Form another category based on their origin within the brain and spinal cord.
Classification by Stage and Grade
Two distinct systems, staging and grading, provide information about the extent and aggressiveness of a cancer. Staging describes how much cancer is in the body and how far it has spread from its original location. The most widely used system is the TNM (Tumor, Node, Metastasis) system, developed by the AJCC and UICC.
The “T” in TNM refers to the size and extent of the primary tumor. A higher number (T1 to T4) indicates a larger tumor or one that has grown more deeply into nearby tissues. The “N” indicates whether the cancer has spread to nearby lymph nodes: N0 means no lymph node involvement, while N1 to N3 show increasing numbers and locations of affected nodes. Finally, “M” signifies whether the cancer has metastasized, or spread to distant parts of the body: M0 means no distant spread, and M1 indicates its presence.
These TNM values are combined to assign an overall stage, from Stage 0 to Stage IV. Stage 0 (carcinoma in situ) means abnormal cells are present but have not spread. Stages I, II, and III indicate increasingly larger tumors or spread to nearby tissues and lymph nodes, while Stage IV signifies that the cancer has spread to distant organs. This system helps predict outlook and informs treatment selection.
Grading, in contrast to staging, describes how abnormal the cancer cells look under a microscope and how quickly they are likely to grow and spread. Pathologists compare tumor cells to healthy cells. Tumors are assigned a grade from 1 to 4.
A Grade 1 tumor has cells that appear relatively normal and are considered well-differentiated, growing slowly. As the grade increases, the cells appear more abnormal or undifferentiated, indicating a higher likelihood of rapid growth and spread. For example, Grade 4 tumors have the most abnormal-looking cells and are the most aggressive.
Some cancers, like prostate cancer, use specific grading systems such as the Gleason score, which assigns scores based on histological patterns unique to those cancers. Both staging and grading provide complementary information for understanding the disease and planning treatment.
Molecular and Genetic Classification
Modern cancer classification increasingly incorporates molecular and genetic characteristics of tumor cells. This approach identifies specific genetic mutations, gene expressions, and biomarkers within the cancer. For example, breast cancer is classified into subtypes like luminal A, luminal B, HER2-positive, and triple-negative based on hormone receptor expression and HER2 status.
Molecular profiling helps identify specific targets for therapy, leading to personalized and effective treatments. Targeted therapies block the effects of particular signaling proteins abnormally active in cancer cells, often with fewer side effects than chemotherapy. Immunotherapies, another form of targeted treatment, activate the body’s own immune system to recognize and destroy cancer cells.
Genetic testing detects mutations in genes like BRCA1 and BRCA2, which are associated with increased cancer risk and inform treatment choices. Precision medicine uses this genomic information to guide medical decisions, tailoring treatments to a patient’s tumor profile. Clinical trials, such as the NCI-MATCH trial, explore how molecular profiling can match patients with various cancer types to targeted treatments based on shared genetic abnormalities. This evolving field promises to enhance treatment efficacy by addressing the molecular drivers of each patient’s cancer.