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Number 268
The Biology of skin cancer
Each year there are 45,000 new cases of skin cancer diagnosed in the UK – 123 a day! 6000 of these are malignant melanomas, which kill
1600 people annually. The incidence of malignant melanoma is increasing rapidly and the main reason is sunbathing.
This Factsheet:
• Describes the causes and characteristics of malignant melanomas
• Describes the most common type of exam questions on this topic
• Outlines the most common errors and misconception that Chief Examiners have reported in students’ exam scripts
A malignant melanoma is a type of cancer – a mass of cells that divide continuously and in an uncontrolled way. Malignant means that the
cancer can spread to other parts of the body (Fig 1).
Fig. 1 Types of skin cancer
Hair follicle
Horny layer
Squamous cells: Cancers take the
form of scaly lumps, ulcers,
nodules or sores that will not heal
Epidermis
Melanocytes: Produce the protective
pigment melanin but can develop
malignant melanomas (MM) which,
if undetected, can rapidly spread
around the body. Moles can also
develop into MM
Melanocytes
Sebaceous glands
Basal cells: The most common
skin cancer – basal cell
carcinoma develops here
Dermis
Sweat glands
Free nerve endings
Lymphatic
channels
Blood vessels
Nerve fibres
Subcutaneous
layer
Pacinian corpuscles
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268 The Biology of skin cancer
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Malignant melanoma is the most dangerous type of skin cancer and
its incidence is increasing rapidly (Fig.2).
Cancer is uncontrolled mitosis
Fig.2 Malignant melanoma
The rate of cell division is controlled by two types of genes:
1. proto-oncogenes which stimulate cell division
2. tumour suppressor genes that slow cell division
A mutation in a proto-oncogene results in an oncogene that
then stimulates cells to divide too quickly.
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cases of malignant
melanomas
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A mutation in a tumour suppressor gene can cause it to become
inactivated and the rate of cell division again becomes
uncontrolled.
10
rate per
100 000
population
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The resulting mass of unspecialized, non-functioning cells is
known as a tumour.
death from malignant
melanomas
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Typical Exam Question
4
(a) How do benign tumours differ from malignant tumours? (2)
(b) Describe two ways in which tumours may damage the body
(2)
(c) Outline the link between excessive exposure to the sun and
skin cancer (3)
(d) (i) Fair - skinned people are at a greater risk of skin cancer
than dark - skinned people when sunbathing.
Explain why (1)
(ii) Early humans with dark skins evolved in tropical Africa
and slowly spread out across the world. Natural selection
then resulted in a range of skin pigmentation in different
populations.
Suggest the advantage of pale skin in humans living in
Northern Europe (1).
2
0
1970
1980
1990
Year
2000
2010
Mortality has not increased as fast as incidence because of better
diagnosis, better public awareness with more careful self-checking
and better treatment.
Ultraviolet radiation
Both UV-A and UV-B (Fig.3) in sunlight can cause DNA mutations
that result in skin cancer.
(a) benign tumours do not cause cancer;
benign tumours do not invade other tissues causing damage;
if parts of benign tumours break off and spread they do not
result in new tumours/metastasis;
(b) may directly damage or exert pressure on organs/blood
vessels;
may cause blockages/obstructions;
(c) solar radiation contains UV radiation;
UV causes mutation of genes which control division;
ref to proto-oncogenes;
ref to tumour suppression genes;
(d) (i) fair skin has less melanin which protects against UV Reject
“ no melanin”
(ii) less blocking of UV needed to synthesise vitamin D;
Fig.3 The electromagnetic spectrum
gamma rays
x-rays
infrared
UV
microwaves radio waves
UVA UVB
visible light
Violet
380
Indigo
440
Blue
480
Green Yellow Orange
540
600
640
red
680
720
Fig.4 summarises one sequence of events that results in malignant
skin cancer.
Markscheme
Fig. 4 The development of malignant skin cancer
Extract from Chief Examiner’s Report
Candidates struggled to explain how tumours actually damage
the body. Some believed that metastasis involved whole tumours
traveling around the body. Many believed that tumours cause
blood clots. Only the most able candidates made any reference
to proto-oncogenes or tumour suppression genes. Common
misconceptions were that UV radiation interacts with melanin to
cause skin cancer and that dark-skinned people had a darker
pigment than fair-skinned people, rather than just more melanin.
UV radiation damages DNA
È
Mutation in the genes responsible for cell division and growth
È
Abnormal, uncontrolled pattern of cell division and growth to
form a tumour (undifferentiated mass of cells)
È
Tumour may damage surrounding cells or damage or block blood
vessels
È
Cancerous cells break off and cancer spreads via blood or lymph
to other body organs (metastasis)
Hint: This is a very common exam problem
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Bio Factsheet
268 The Biology of skin cancer
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Exam Hints
1. Don’t call it ultraviolent radiation
2. Don’t write “ Cancer is rapid cell division”. Instead write “ Cancer is uncontrolled cell division”
3. Learn to spell melanin
4. Don’t confuse melanin with melanoma
5. It isn’t true that fair-skinned people have no melanin
6. Cancer cells can’t be passed on from one generation to another!
Treating skin cancer
Table 1 summarises the main ways of treating skin cancer.
Table.1 Treating skin cancer
Technique
Procedure
Surgery
Cancerous cells are cut out along with some healthy surrounding tissue
Cryotherapy
Small areas of cancerous cells e.g. basal cell carcinomas are frozen with liquid nitrogen. A blister and then
scab forms and after two weeks the scab falls off and the cancer cells should have been killed
Radiotherapy
Often used for basal and squamous cell carcinomas and following surgery to make sure all the cancerous
cells are dealt with. Video of radiotherapy at Royal Marsden Hospital: http://www.macmillan.org.uk/
Cancerinformation/Cancertreatment/Treatmenttypes/Radiotherapy/Radiotherapy.aspx
Photodynamic therapy
Uses light and a light-sensitive drug to kill cancerous cells
Topical chemotherapy
Used for early squamous cell and superficial basal cell carcinomas
Topical immunotherapy Used for some basal cell and squamous cell carcinomas
Typical Exam Question
(a) Some anti-cancer drugs work by attacking tumours. What is a tumour? (1)
(b) Melanoma is a particularly aggressive form of skin cancer and in the past few patients survive for more than five years. However,
the Human Genome Project has proved extremely useful in helping develop new drugs that, in clinical trials, are showing great
promise. Suggest how work on the Human Genome Project has helped in the development of more effective drugs (2)
(a) mass of cells dividing uncontrollably;
(b) Human Genome Project has identified genes/alleles/mutations linked to cancers/melanomas;
New drug effectively targets these genes/alleles;
ref to mechanism e.g. prevents gene expression/translation;
Markscheme
How Science Works: Sun Protection Factors
Suncreams help to block UV radiation from the Sun. The effectiveness of different creams is rated on a scale referred to as the Sun
Protection Factor (SPF) based on how long it takes skin to develop sunburn relative to unprotected skin. Scientists investigating
different suncreams have shown that the SPF scale is unreliable because:
•
•
•
•
manufacturers failed to consistently test levels of sunburn
the ease of sunburn depends on a person’s skin type
manufacturers used different UV frequencies in their tests
it measures protection against sunburn not protection against skin cancer
Question:
Why do you think the manufacturers chose protection against sunburn rather than protection against the risk of developing skin
cancer as the basis of the SPF scale?
Hopefully, you identified factors such as:
• Studying sunburn is a lot faster than studying skin cancer
• Many other factors influence cancer
• A large sample would be needed to study cancer, thus it would be more expensive
• Monitoring sunburn is easier than monitoring skin cancer
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Bio Factsheet
268 The Biology of skin cancer
www.curriculum-press.co.uk
Practice Questions
Markschemes
1. Scientists investigated a new form of chemotherapy for treating
skin cancer.
The graph shows the effect of different doses of the
chemotherapy drug on both tumour cells and on healthy body
cells.
1. (a) UV damages DNA in skin cells/ causes mutation;
Uncontrolled cell division/cell division exceeds cell death;
Disruption of tumour suppressor genes / oncogenes;
Tumour is undifferentiated mass of cells;
Damages tissues/blood vessels/healthy cells;
A
B
Percentage
of cells killed
C
(b) A kills a proportion of cancer cells but a low proportion of
healthy cells;
But many cancer cells are not killed;
So multiple treatments required;
B kills a high proportion of cancer cells but a low proportion
of healthy cells;
so would cause fewer side effects;
C kills all tumour cells;
but too many healthy cells;
Healthy
cells
Tumour
cells
(c) (i) drug affects allele expression;
mutation/mutated protein not produced;
drug kills melanoma cells rapidly;
melanoma cells don’t divide/ are replaced with normal
body cells;
via mitosis/ named process in mitosis;
Dose of drug
(a) Describe the sequence of events leading to the formation of a
skin cancer tumour (3).
(b) Outline the relative advantages and disadvantages of doses A,
B and C (4).
(ii) randomised trial;
double blind/ neither doctors not patients know who is
getting the drug or a placebo;
uses large number of patients;
quantitative/statistical analysis of effectiveness of the
drug;
(c) Scientists conducted clinical trials of a new drug aimed at treating
malignant melanomas. The drug appears to achieve a 45%
shrinkage of melanomas in just 12 weeks.
(i) Suggest how the new drug may have caused the melanoma
to shrink in only 12 weeks (3).
(ii) The drug is now entering a phase III trial, before it can be
approved for use. Explain what happens in a phase III trial
(2)
2. (a) malignant – capable of spreading and causing harm;
melanoma – cancer/cells undergoing uncontrolled division;
(b) melanomas take time to develop/exposure when young but
cancer triggered later;
early effects sub-lethal;
metastasis affects vital organs after many years;
immune system becomes less effective as people age;
cumulative exposure to UV over the years;
2. The graph shows the death rate from malignant melanomas in
fair-skinned men in different age groups.
20
Death rate 15
per 100,000
of the
population 10
5
0 20 30 35 40 45 50 55 60 65 70 75 80
Age at death/years
(a) Explain the term malignant melanoma (2)
(b) Suggest three reasons for the trend shown in the graph (3)
Acknowledgements:
This Factsheet was researched and written by Kevin Byrne..
Curriculum Press, Bank House, 105 King Street, Wellington, Shropshire, TF1 1NU.
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permission of the publisher. ISSN 1351-5136
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