Cancer

NCERT grounding

NCERT Class 12 Biology, Chapter 7 — section 7.4 Cancer — anchors this subtopic. The textbook opens with the line that more than a million Indians suffer from cancer and a large number die from it annually, then defines cancer as the breakdown of normal regulation of cell growth and differentiation. NIOS Senior Secondary Biology, Chapter 29 (section on non-communicable diseases), gives a parallel one-line definition — "the uncontrolled and unwanted growth of cells" — and lists the proto-oncogene activation pathway, smoking, tobacco chewing and repeated tissue irritation among the causes.

"Cells sloughed from such tumours reach distant sites through blood, and wherever they get lodged in the body, they start a new tumour there. This property called metastasis is the most feared property of malignant tumours."

— NCERT Class 12 Biology, §7.4

Cancer as deregulated proliferation

In a healthy tissue, cell growth and differentiation are highly controlled and regulated. A cell divides only when signalled to, only as many times as needed, and stops dividing the moment it touches its neighbours. Cancer is the failure of all three controls at once. There is no single "cancer pathogen"; rather, a normal cell accumulates changes that make it deaf to stop signals and blind to its neighbours. The result is an abnormal, unwanted, and apparently unlimited proliferation that produces a mass — a tumour — and, in the malignant form, eventually colonises the rest of the body.

Contact inhibition — the property that is lost

Normal cells in culture, when placed on a dish, divide until they form a single layer that just covers the surface and then they stop. The trigger for stopping is physical contact with neighbouring cells; this is the property called contact inhibition. Cancer cells appear to have lost contact inhibition. They keep dividing on top of each other, piling up into multilayered heaps. In the body, this same loss of contact inhibition allows cancer cells to keep dividing inside an already-crowded tissue and to give rise to the masses of cells called tumours.

Why does growth become "unregulated"?

The molecular machinery that decides whether a cell should divide is encoded by genes called proto-oncogenes (also called cellular oncogenes, or c-onc). These are entirely normal genes — they make growth-factor receptors, signal-transducing proteins, and transcription factors that push the cell through its cycle. When a proto-oncogene is mutated, amplified, or otherwise activated, it becomes a permanently switched-on oncogene that drives the cell to divide regardless of external signals. Combined with loss of contact inhibition and damage to cell-cycle checkpoints, this is the molecular root of oncogenic transformation.

Benign vs malignant tumours

NCERT separates tumours into two clean categories. Benign tumours remain confined to their original location, do not spread to other parts of the body, and cause little damage. Malignant tumours are a mass of proliferating cells called neoplastic or tumour cells — they grow very rapidly, invade and damage surrounding tissues, and starve adjacent normal cells by competing for vital nutrients. The most feared property of malignant tumours, however, is metastasis: cells slough off, travel through the blood, lodge at distant sites, and start new tumours there. Only malignant tumours qualify as "cancer" in the strict NEET-question sense.

Causes — carcinogens

Transformation of normal cells into cancerous neoplastic cells may be induced by physical, chemical, or biological agents. These agents are collectively called carcinogens. They share one common downstream effect — they damage DNA or activate growth-promoting genes — but their identities are sharply distinct, and NEET frequently asks students to match each carcinogen with its category.

Proto-oncogene → oncogene

Inside every normal human cell, NCERT explains, there are genes called cellular oncogenes (c-onc) or proto-oncogenes. These do not "cause cancer" in their normal state — they are part of the regular machinery that turns cell division on and off. They become dangerous only when activated under certain conditions: a mutation, a chromosomal rearrangement, gene amplification, or insertion of a viral promoter. Activation flips a proto-oncogene to an oncogene, and oncogenic transformation of the cell follows. The chain runs as carcinogen → DNA damage → proto-oncogene activation → oncogene → unregulated proliferation → tumour.

Classification of cancers

NCERT does not give an exhaustive classification but NEET routinely asks students to attach a cancer to its tissue of origin. There are four standard categories that you must hold in memory. Carcinomas arise from epithelial tissue — they include the great majority of human cancers, such as those of the lung, breast, colon, skin and most internal organs. Sarcomas arise from connective and supportive tissues of mesodermal origin — bone (osteosarcoma), cartilage, muscle and fibrous tissue. Leukaemias are cancers of the blood-forming cells; the "blood cancers" produce abnormally high numbers of white blood cells in circulation, which is why diagnosis is based on blood and bone-marrow cell counts. Lymphomas are cancers of lymphoid tissue — Hodgkin's and non-Hodgkin's lymphomas — affecting lymph nodes and the spleen.

Detection and diagnosis

Early detection of cancers is essential — NCERT emphasises that it allows the disease to be treated successfully in many cases. The pathologist's starting tool is the biopsy: a piece of the suspected tissue is cut into thin sections, stained and examined under the microscope. This is the histopathological study. Loss of normal tissue architecture, abnormally large nuclei, increased mitotic figures and disordered growth confirm the diagnosis. For leukaemias, where the cancer is in the blood-forming cells of the marrow, the diagnostic tools shift — increased counts of leukocytes in the blood, and bone-marrow tests, point to the disease.

For solid cancers of internal organs that cannot be reached easily by biopsy needle, imaging takes the lead. Radiography (X-rays) gives a quick view of dense tumours. Computed tomography (CT) uses X-rays to generate a three-dimensional image of the internals of an object — useful for tumours of the chest, abdomen and brain. Magnetic resonance imaging (MRI) uses strong magnetic fields and non-ionising radiations to accurately detect pathological and physiological changes in living tissue; it is particularly good for soft tissues. Beyond imaging, monoclonal antibodies raised against cancer-specific antigens are used to flag tumour cells, and molecular-biology techniques can detect inherited-susceptibility genes so that high-risk individuals can be advised to avoid the relevant carcinogens.

Treatment of cancer

The common approaches for the treatment of cancer named in NCERT are surgery, radiation therapy and immunotherapy; in practice most patients receive a combination of surgery, radiotherapy and chemotherapy. Surgery removes accessible solid tumours. In radiotherapy, tumour cells are irradiated lethally, with care taken to spare the normal tissues that surround the tumour mass — the principle being that rapidly dividing tumour cells are more sensitive to radiation-induced DNA damage than the slower-dividing normal cells. In chemotherapy, several chemotherapeutic drugs are used to kill cancerous cells; some are specific for particular tumours, while many are general-purpose cell-cycle inhibitors.

Chemotherapeutic drugs are not selective enough to spare every healthy cell. They preferentially harm rapidly dividing cells — and because hair follicle cells and bone-marrow stem cells also divide rapidly, the majority of chemotherapy drugs cause characteristic side effects such as hair loss and anaemia. NCERT calls these out explicitly. Finally, tumour cells have been shown to avoid detection and destruction by the immune system; to counter this, patients are given biological response modifiers such as α-interferon, which activates the immune system and helps in destroying the tumour.

Worked examples

Common confusion & NEET traps