What is Cancer?
Under scientific review
Educational content. Use it to learn concepts, not for individual clinical decisions.
Cancer is a group of diseases in which cells grow in a dysregulated way and can invade other tissues. Understanding the process requires looking at changes within cells and at the tissue where they live. “Genetic” does not necessarily mean “inherited”: many changes arise during life. NCI: definition · Cancer genetics.
Distinguish a tumor from cancer; explain clonal selection; recognize what a sample can represent.
Before you begin: Cells, DNA and tissues and basic vocabulary.
- Organized tissue
Cell division and loss respond to tissue signals.
- Clonal expansion
An advantage in a particular environment can favor descendants.
- Diversity and invasion
Subpopulations differ; invasion crosses tissue boundaries.
Growth alone is not cancer
A wound needs new cells to heal. Local signals coordinate division and its reduction as the tissue recovers. Cancer can compromise this coordination through changes in growth, survival and environmental interaction circuits.
Tumor describes a mass; it is not automatically a synonym for malignancy. Some cancers, such as leukemias, do not need to form a solid mass. The comparison below is introductory; actual classification requires pathology and context. NCI.
| Situation | Organization and boundaries | What not to conclude |
|---|---|---|
| Normal growth | Responds to tissue needs and signals | Many dividing cells alone cannot diagnose cancer |
| Benign tumor | Does not invade like a malignant tumor or produce metastases | Benign does not always mean harmless: location and local effects matter |
| Malignant neoplasm | Can invade; some cells can colonize other sites | Not every cancer has metastasized at diagnosis |
Carcinomas originate in epithelial tissues; sarcomas in supporting tissues; leukemias and lymphomas involve hematological lineages. Tissue of origin remains relevant after spread.
How a tumor population evolves
- Variation: cells can acquire genetic changes and different regulatory states. Most variants do not provide a tumor-related advantage.
- Selection: in a particular environment, some differences favor survival or reproduction. The advantage is contextual; there is no universally “best” clone.
- Expansion: descendants share some of these features. Further changes and different environments can produce branches rather than one linear sequence.
- Interaction: nutrients, immunity and treatments change selection pressures. Evolution has no intention or obligatory destination.
This reasoning helps explain heterogeneity: differences between tumors and between regions or cells within a tumor. The clonal evolution hypothesis organized this perspective; it does not mean that every mutation causes progression. Nowell, 1976.
Deeper dive: instability, drivers and passengers
Genomic instability means a greater tendency toward genomic change; it can involve processes ranging from sequence errors to chromosomal changes. It is incorrect to say that every tumor cell switches off all repair or accumulates errors “exponentially.” A driver alteration favors expansion in a particular context; a passenger does not provide that detectable advantage. Recurrence helps investigation but does not replace functional evidence and contextual analysis. Martincorena et al., 2017.
From biology to data
A biopsy combines tumor and non-tumor cells and represents one location at one time. An expression difference may reflect a change within a cell or a change in immune-cell proportions. Data therefore need provenance: tissue, region, method, processing and prior treatment.
For programmers, the implication is concrete: splitting training and test data by patient prevents related samples from creating an inflated impression of performance. Accuracy in one cohort does not prove clinical utility in another.
Tumor mutational burden (TMB) summarizes mutations per length of DNA assessed. As a teaching calculation, 30 qualifying mutations in 1.5 evaluable megabases yield 20 mutations/Mb. Variant selection, coverage and platform affect the number; this invented result does not determine treatment. See biomarkers.
Guided example
Fictional scenario: two regions of one tumor share alteration A; only the second has B.
- Shared A is compatible with common ancestry but does not reconstruct the entire history alone.
- B may characterize a subclone; its absence in the first sample may also depend on sensitivity and sampling.
- The next question is whether B changes a function and whether the result recurs in other regions. “I detected B” and “B causes resistance” are different claims.
Check your understanding
Is every tumor cancer?
No. Tumors can be benign; some cancers also do not form solid masses.
Does genetic mean inherited?
No. A change may be somatic, acquired during life, or germline. Its origin cannot be inferred from the gene name alone.
Does one biopsy represent every cell?
Not necessarily. Region, timing and cellular composition limit the conclusions a sample supports.