Course 25.3Lesson 1 of 3

Tumours and cancer: causes, detection and treatment

A normal cell stops dividing when it touches other cells. A cancer cell does not, and it forms a tumour. A benign tumour stays in one place, and a malignant tumour invades and spreads by metastasis. Carcinogens damage the genes, and proto-oncogenes turn into oncogenes. Cancer is found by biopsy and imaging and is treated by surgery, radiation, chemotherapy and immunotherapy.

about 30 min1 simulation1 practice question

Lesson 1 of 3Show all lessons

Try the code and the simulations beside the lesson.

Cells that forget to stop

A cut on the skin heals, and then the new cells stop growing. How do they know when to stop?

Normal cells show contact inhibition. When they touch each other, they stop dividing. This keeps a tissue to its right size.

Cancer cells have lost this control. They go on dividing again and again, and they form a mass of cells called a tumour, or a neoplasm. Cancer is a disease of cells that divide without control.

Benign and malignant tumours

Three panels side by side. Normal tissue: a single layer of regular round cells, each with a nucleus, in a neat grid. A note says that cells that touch stop dividing, which is contact inhibition. Benign tumour: an oval lump of crowded cells inside a clear boundary. A note says that it stays in one place inside its boundary and does little damage. Malignant tumour: an irregular mass with uneven edges that invades the tissue around it. A red arrow leads from the mass into a blood vessel in which red cells travel, and a second arrow leads from the vessel to a small new tumour at a distant place. A note says that the tumour grows fast and invades, and that the cells travel in the blood, which is metastasis.

A benign tumour stays in the place where it began. It is usually enclosed, and it normally does little damage. It may press on a nearby part, and then it needs to be removed.
A malignant tumour is a mass of cells that divide fast. They invade and damage the tissues around them. Cells of the tumour may be carried by the blood to far places. They lodge there and start new tumours. This spread is metastasis, and it is the most dangerous feature of cancer.

So a tumour is not always cancer. Only a malignant tumour is cancer.

What causes cancer?

A normal cell changes into a cancer cell by a change in its genes. Agents that can cause this change are carcinogens. There are three kinds.

Physical agents. The ionising radiations, such as X-rays and gamma rays, and the non-ionising ultraviolet (UV) rays damage the DNA and can lead to cancer.
Chemical agents. Many chemicals are carcinogens. Tobacco smoke is the main cause of lung cancer.
Biological agents. Some viruses, the oncogenic viruses, carry genes called viral oncogenes.

A flow diagram. Three boxes on the left are headed carcinogens. Physical agents: X-rays, gamma rays and UV. Chemical agents: for example tobacco smoke. Biological agents: oncogenic viruses. An arrow from each box leads to a box in the middle: the cellular proto-oncogene, a normal gene for cell growth. An arrow marked activated leads to a box on the right: oncogene, cells divide without control. An arrow from it leads to a note that the cells form a tumour, or neoplasm. A note under the middle box says that oncogenic viruses may also carry viral oncogenes into the cell.

Normal cells hold genes called cellular proto-oncogenes. A proto-oncogene helps to control the growth of the cell, and it is harmless. If a carcinogen activates it, it becomes an oncogene, and the cell divides without control.

This part is not in the NEET syllabus, but A-Level and IB ask it. Cells also have tumour suppressor genes, such as p53, which stop cell division or start the death of a damaged cell. Cancer often needs both the activation of oncogenes and the loss of these genes.

Finding cancer

Biopsy and histopathology. A small piece of tissue is taken, cut into thin slices, stained and studied under the microscope.
Blood and bone marrow tests. They show an increased number of cells in leukaemia, which is cancer of the white blood cells.
Radiography, CT and MRI. X-ray pictures, computed tomography and magnetic resonance imaging show the position of a tumour inside the body.
Antibodies against antigens that are special to a cancer are used to detect some cancers.
Molecular tests can find the genes that make some people more likely to get a cancer.

Treating cancer

Doctors treat most cancers by a combination of these.

Surgery removes the tumour.
Radiation therapy aims radiation at the tumour to kill its cells. The doctors take care not to harm the normal tissue around it.
Chemotherapy uses drugs that kill cancer cells. The drugs also harm some normal cells, so the side effects include hair loss and anaemia.
Immunotherapy uses substances called biological response modifiers, such as α-interferon. They activate the immune system and help it to destroy the tumour.

How long does a tumour take to grow?

A tumour starts from one cell, which divides into two, then four, then eight. After $d$ doublings there are $2^{d}$ cells. About a billion cells weigh about 1 gram, which is the size of a small lump. Thirty doublings give $2^{30}$ , which is a little over a billion. This is a rule of thumb from medical textbooks, and NCERT does not give it.

A graph with the number of doublings of a tumour from 0 to 40 on the horizontal axis and the number of cells, as a power of 10, on the vertical axis. A straight rising red line starts at 1 cell. Marked points on it are: about 10 to the power 3 cells after 10 doublings; about 10 to the power 9 cells after 30 doublings, which is about 1 gram, a lump of about 1 cubic centimetre; and about 10 to the power 12 cells after 40 doublings, which is about 1 kilogram.

You should find that a tumour can grow for years before a lump is large enough to feel or to see. This is why a check up that finds a tumour early is so useful.

Try it in Python

The program counts the cells after the doublings.

Practise with new numbers

Common mistakes

Saying that every tumour is cancer. A benign tumour is not cancer.
Confusing metastasis with fast growth. Metastasis is the spread of cancer cells to other places.
Saying that a proto-oncogene is a cancer gene. It is a normal gene. It becomes an oncogene when it is activated.
Putting UV rays among the ionising radiations. X-rays and gamma rays are ionising. UV is non-ionising.
Saying that chemotherapy and radiation therapy are the same. Chemotherapy uses drugs. Radiation therapy uses radiation on the tumour.

Remember these

Normal cells show contact inhibition. Cancer cells do not.
Benign: stays at its place, does little damage. Malignant: invades and spreads by metastasis.
Carcinogens: physical (X-rays, gamma rays, UV), chemical (tobacco smoke), biological (oncogenic viruses).
Proto-oncogene (normal) becomes an oncogene when it is activated.
Detection: biopsy, blood and bone marrow tests, X-rays, CT, MRI. Treatment: surgery, radiation, chemotherapy, immunotherapy.

Check yourself

Try each one on your own first. If you are stuck, open a hint. Then check with Show the answer.

What is contact inhibition?

How is a malignant tumour different from a benign tumour?

What is metastasis?

Name one physical, one chemical and one biological carcinogen.

What is the difference between a proto-oncogene and an oncogene?

References

World Health Organization (2024). Fact sheet: Cancer.
Weinberg R. A. (2014). The Biology of Cancer, 2nd edition, Garland Science, chapters 1 and 2.
Betts J. G. et al. (2022). Anatomy and Physiology 2e, OpenStax, chapter 3, the section on cancer and the cell cycle.
NCERT (2023). Biology, Textbook for Class XII, chapter 7.