Cancer

What is cancer?

In plain words: normally a cell divides only when the body needs more cells, and stops when it is told to. A cancer cell has lost these controls, so it goes on dividing.

Beyond the NCERT text: the National Cancer Institute (US, public domain) defines it in the same spirit: “Cancer is a disease in which some of the body’s cells grow uncontrollably and spread to other parts of the body.”

Contact inhibition

• Normal cells show a property called contact inhibition: contact with other cells inhibits their uncontrolled growth (page 141).
• Cancer cells appear to have lost this property. As a result they just continue to divide, giving rise to masses of cells called tumors (page 141).
• Cells divide through the cell cycle of NCERT Class 11 Biology, Chapter 10; cancer cells keep going round it without the controls that stop normal cells (our reading of NCERT page 141).
• Beyond the NCERT text: the National Cancer Institute notes that “Normal cells stop growing when they encounter other cells”, the same idea in other words.

Benign and malignant tumors

Tumors are of two types (page 141):

• Benign tumors normally remain confined to their original location, do not spread to other parts of the body and cause little damage.
• Malignant tumors are a mass of proliferating cells called neoplastic or tumor cells. These cells grow very rapidly, invading and damaging the surrounding normal tissues. As they actively divide and grow, they also starve the normal cells by competing for vital nutrients.

Reading note (page 141): NCERT describes both kinds of tumor right after saying that cancer cells divide into tumors. Beyond the NCERT text, the National Cancer Institute (US, public domain) says “Tumors can be cancerous or not cancerous (benign)” and “Cancerous tumors may also be called malignant tumors”: only a malignant tumor is a cancer. The same page adds that benign tumors can sometimes be quite large, and some can cause serious symptoms or be life threatening, such as benign tumors in the brain. In an exam, call a benign tumor non-cancerous, and give NCERT’s points: confined, no spread, little damage.

Metastasis

• Cells sloughed off from malignant tumors reach distant sites through the blood, and wherever they get lodged in the body they start a new tumor there (page 141). Beyond the NCERT text: cancer cells can also travel through the lymph system (National Cancer Institute, What Is Cancer?; public domain).

This property called metastasis is the most feared property of malignant tumors.

NCERT Biology (Class 12), Section 7.4, page 141

Causes: carcinogens and oncogenes

The transformation of normal cells into cancerous neoplastic cells may be induced by physical, chemical or biological agents, called carcinogens (page 141):

• Physical: ionising radiations such as X-rays and gamma rays, and non-ionising radiations such as UV, cause DNA damage that leads to neoplastic transformation. UV is also one of the mutagens of NCERT Chapter 4 (page 72).
• Chemical: the chemical carcinogens in tobacco smoke have been identified as a major cause of lung cancer.
• Biological: cancer-causing viruses, the oncogenic viruses, have genes called viral oncogenes.

Several genes called cellular oncogenes (c-onc) or proto oncogenes have also been identified in normal cells. When activated under certain conditions, they could lead to oncogenic transformation of the cells (page 141). Beyond the NCERT text: proto-oncogenes are involved in normal cell growth and division, and become cancer-causing genes (oncogenes) when they are altered or are more active than normal; changes in tumor suppressor genes and DNA repair genes also contribute to cancer (National Cancer Institute, What Is Cancer?; public domain).

Detection and diagnosis

Early detection is essential, because it allows the disease to be treated successfully in many cases (page 141). The methods NCERT names (pages 141–142):

• Biopsy and histopathological studies: a piece of the suspected tissue is cut into thin sections, stained and examined under a microscope by a pathologist.
• Blood and bone marrow tests for increased cell counts, in the case of leukemias.
• Radiography (X-rays), CT (computed tomography) and MRI (magnetic resonance imaging), very useful for cancers of the internal organs. CT uses X-rays to generate a three-dimensional image of the inside of an object; MRI uses strong magnetic fields and non-ionising radiations to detect pathological and physiological changes in living tissue.
• Antibodies against cancer-specific antigens, used to detect certain cancers.
• Molecular biology techniques that detect genes in individuals with an inherited susceptibility to certain cancers. Such people may be advised to avoid the particular carcinogens they are susceptible to, for example tobacco smoke in the case of lung cancer.

Beyond the NCERT text: the National Cancer Institute notes that “cancers of the blood, such as leukemias, generally do not” form solid tumors; this is why leukemias are detected by blood and bone marrow tests (our reasoning).

Treatment of cancer

• Surgery is the first of the common approaches NCERT names (page 142).
• Radiotherapy: tumor cells are irradiated lethally, taking proper care of the normal tissues around the tumor mass (page 142).
• Chemotherapy: several chemotherapeutic drugs kill cancerous cells; some are specific for particular tumors. Most have side effects such as hair loss and anemia (page 142).
• Immunotherapy: tumor cells have been shown to avoid detection and destruction by the immune system, so patients are given biological response modifiers such as α-interferon, which activates their immune system and helps destroy the tumor (page 142).
• Most cancers are treated by a combination of surgery, radiotherapy and chemotherapy (page 142).

Reading note (page 142): NCERT’s opening line names surgery, radiation therapy and immunotherapy as the common approaches, and then describes chemotherapy as well. In an answer on treatment, give all four.

Solved examples

Common mistakes to avoid

Quick revision summary

• Cancer: the breakdown of the mechanisms that control cell growth and differentiation; cancer cells lose contact inhibition and divide into tumors.
• Benign tumors stay confined and cause little damage; malignant tumors of neoplastic cells grow fast, invade tissues and starve normal cells; metastasis, spread through the blood to start new tumors, is their most feared property.
• Carcinogens: physical (ionising X-rays and gamma rays; non-ionising UV), chemical (tobacco smoke and lung cancer), biological (oncogenic viruses with viral oncogenes); cellular oncogenes (c-onc) in normal cells, when activated.
• Detection: biopsy and histopathology; blood and bone marrow tests for leukemias; radiography, CT, MRI; antibodies against cancer-specific antigens; genes for inherited susceptibility.
• Treatment: surgery, radiotherapy, chemotherapy (side effects such as hair loss, anemia) and immunotherapy (α-interferon); usually a combination.

Questions students ask

Can a benign tumor become malignant?

Beyond the NCERT text, some benign growths can turn malignant: the National Cancer Institute (US, public domain, 2026), which calls tumors that are not cancerous benign, says that some tissue changes that are not cancer “may develop into cancer if they are not treated”. Its example is the abnormal mole called a dysplastic nevus, which “can turn into melanoma, although most do not”; NCI adds that some such changes need watching or treatment and others do not.

What are the main types of cancer?

NCERT sorts tumors into two types, benign and malignant (page 141), but beyond the NCERT text cancers themselves are sorted by the kind of cell they start from: the National Cancer Institute (US, public domain, 2026) groups them into carcinomas, the commonest, from the epithelial cells that cover the body’s inner and outer surfaces; sarcomas, from bone and soft tissues such as muscle and fat; leukemias, which NCERT names (page 141), from blood-forming tissue; lymphomas, from lymphocytes; multiple myeloma, from plasma cells; and melanoma, from pigment-making melanocytes. NCI counts more than 100 types, most named after the organ where they begin, such as lung cancer.

Which viruses can cause cancer?

Beyond the NCERT text, the viruses that can cause cancer in people include, in the list of the US National Cancer Institute (2019), high-risk human papillomaviruses (nearly all cervical cancers), hepatitis B and C viruses (liver cancer after chronic infection), Epstein–Barr virus (some lymphomas), HTLV-1 (adult T-cell leukemia/lymphoma) and Kaposi sarcoma-associated herpesvirus. NCERT names oncogenic viruses but no human example (page 141), and in Chapter 9 says that retroviruses in animals can transform normal cells into cancerous cells (page 170).

How many carcinogens are there in tobacco smoke?

Beyond the NCERT text, the National Cancer Institute (US, fact sheet reviewed 2017) counts more than 7000 chemicals in tobacco smoke, at least 250 of them known to be harmful and at least 69 able to cause cancer. Its examples of these carcinogens include benzene, formaldehyde, arsenic, cadmium, polycyclic aromatic hydrocarbons, tobacco-specific nitrosamines and the radioactive element polonium-210. NCERT’s Chapter 7 gives no number and names none of them (pages 141 and 144).

Why is metastasis so dangerous?

Metastasis is dangerous because it takes cancer beyond the place where it began: once cells from a malignant tumor have started new tumors at distant sites (NCERT page 141), the disease is no longer one mass that treatment aimed at a single site can clear (our reasoning). Beyond the NCERT text, the National Cancer Institute (US, public domain, 2026) notes: “Metastatic tumors can cause severe damage to how the body functions, and most people who die of cancer die of metastatic disease.” Treatment can still help, lengthening life for some people and, for others, mainly slowing the cancer or easing its symptoms (NCI).

Practice questions

Try each question first, then open the answer to check your working.

1. Q1. What is contact inhibition, and what happens to it in cancer cells?

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   The property by which contact with other cells inhibits the uncontrolled growth of normal cells; cancer cells appear to have lost it (NCERT page 141).

2. Q2. Name two ionising radiations that act as carcinogens.

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   X-rays and gamma rays (NCERT page 141).

3. Q3. Which tests are used to detect leukemias?

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   Blood and bone marrow tests for increased cell counts (NCERT page 141).