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SSC- Biology of Cancer > Radiotherapy in Cancer Management > Flashcards

Radiotherapy in Cancer Management Flashcards

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1
Q

What is radiation therapy?

A

An often used and successful modality in the ‘local’ treatment of cancers

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2
Q

What is the problem with radiation therapy?

A

There is a risk of inducing a variety of human cancers

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3
Q

What cancer treatments are used to achieve local control of the disease?

A
  • Surgery
  • Radiotherapy
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4
Q

What cancer treatments are uesd to treat disseminated disease?

A
  • Chemotherapy
  • Immunotherapy
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5
Q

What cancer treatments are used for pallitation?

A
  • Radiotherapy
  • Chemotherapy
  • Immunotherapy
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6
Q

Is radiotherapy delivered as a monotherapy?

A

Rarely

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7
Q

How is radiotherapy used in conjuction with surgery?

A

It can be used before or after surgery

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8
Q

Why might radiotherapy be used before surgery?

A

To shrink tumour so that it is more operable

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9
Q

What are the types of radiotherapy?

A
  • External beam radiotherapy
  • Brachytherapy
  • Unsealed sources
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10
Q

What happens in external beam radiotherapy?

A

X-rays are generated externally, and precisely targetted into the body

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11
Q

What happens in bracytherapy?

A

A sealed radiation source is inserted into the body

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12
Q

What kind of emitters are used in brachytherapy?

A

Short range, only a few cm

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13
Q

What kind of radiation is used in unsealed source radiotherapy?

A

High energy, short range

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14
Q

Give two examples of medications used in unsealed source radiotherapy

A
  • Radioiodine in thyroid cancer
  • Meta-iodobenzylG in neuroblastoma
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15
Q

What allows unsealed source radiotherapy to work?

A

Some drugs/chemicals have an affinity for certain organs

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16
Q

What are the rules for all forms of radiotherapy?

A
  • Maximise dose to tumour
  • Minimise dose to normal tissue
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17
Q

What % of cancer patients require RT at some stage of their illness?

A

50%

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18
Q

What % of those treated with radiotherapy are treated with curative intent?

A

60%

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19
Q

What % of those treated with curative intent have an >5 year survival?

A

70%

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20
Q

How can radiotherapy improve in the future?

A
  • Improvements in tumour control
  • Reductions in toxicity
  • Early detection
  • Increases in tumour sensitivity
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21
Q

What can x-rays and gamma-rays be thought of as?

A

Waves (λv = cc) or as photons (E = hcv)

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22
Q

What does the equation λv = cc tell you about x-rays?

A

As the frequency goes up, the wavelength must go down to keep Cc constant

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23
Q

What are photons?

A

Packets of energy

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24
Q

What do electromagnetic radiations (x-rays or gamma-rays) interact with?

A

The electronic component of matter

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25
Q

What can absorption of energy from radiation lead to?

A
  • Excitation - loss of an electron to a higher level
  • Ionisation - actual ejection of the electron
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26
Q

What happens when an electron is ejected in ionisation?

A

The molecule has an unpaired electron, and so is a free radical

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27
Q

What is a free radical?

A

A chemically reactive entity that causes chemical changes to exposed atoms and molecules

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28
Q

What damage can be caused by free radicals?

A

Lethal damage or mutation

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29
Q

What kind of damage is good in terms of radiotherapy?

A

Lethal damage, as want to kill the tumour cells

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30
Q

Why it mutation bad in radiotherapy?

A

Because there is a change in function, which can be detrimental in a normal cell

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31
Q

What does the process, at an atomic level, by which x-rays are absorbed depend on?

A

The energy of photons, and the chemical composition of the absorbing material

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32
Q

What process of absorbtion dominates for high energy photons?

A

The Compton Process

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33
Q

What happens in the Compton process?

A

A photon interacts with loosely bound ‘free’ electrons, of low binding energy. Part of the photon energy is given to electron, knocking the electron out. The photon then proceeds with a reduced energy (longer wavelength)

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34
Q

What is the end result of the Compton process?

A

The production of fast electrons, many of which will go on and ionise other atoms of the absorbed. There is also a deflected/scattered photon of reduced energy

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35
Q

What process of absorption dominates for photons of low energy?

A

The photoelectric process

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36
Q

What happens in the photoelectric process?

A

The photon interacts with a tightly bound electron of higher binding energy, and gives up its energy entirely. The electron is ejected, and the photon is entirely absorbed

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37
Q

What is the end result of the photoelectric process?

A

The production of fast electrons but no photon

38
Q

/Where are photons of lower energy used?

A

In diagnostic radiology

39
Q

How do the Compton process and photoelectric process differ, in terms of the relevance of atomic number?

A

The Compton process is independent of the atomic number of the absorbing species, however the photoelectric process varies rapidly with atomic number

40
Q

Where are the differences between the Compton process and the photoelectric process important?

A

In the application of x-rays to diagnosis and therapy

41
Q

Which process is used for radiation therapy/

A

Compton process

42
Q

Why is the Compton process important for radiation therapy?

A

To avoid the problem of differential absorption in tissues, so bone doesn’t shield the tumour

43
Q

What process is used for diagnostic radiology?

A

The photoelectric process

44
Q

Why is the photoelectric process good for diagnostic radiology?

A

Because bone differentially absorbs x-rays, resulting in the visualisation of bone structure

45
Q

In what ways can ionising radiation at a molecular level?

A

Can be directly acting or indirectly acting

46
Q

What is meant by directing acting ionising radiation?

A

If the atoms of the target molecule are ionised

47
Q

Can directly acting ionising radiation be protected against?

A

Generally, can only be shielded, can’t be modified by sensitisers and protectors

48
Q

What is meant by indirectly acting ionising radiation?

A

If the radiation interacts with other molecules to produce free radicals that migrate into the DNA, this is an indirect effect

49
Q

What % of the body is water?

A

70%

50
Q

What happens when ionising radiation hits a water molecule?

A

Produces an electron, which becomes solvated by water, and a H2O<span>*+</span> radical

51
Q

What happens to the H2O*+ radical?

A

It breaks down to produce *OH- and H+

52
Q

What is the problem with the hydroxyl radical (*OH-)?

A

It is very reactive, and generates more free radicals from water molecules

53
Q

What proportion of cellular damage is caused by indirectly acting radication?

A

2/3

54
Q

How can indirectly acting radiation be modified?

A

Sensitisers and protectors

55
Q

What is an important characteristic of ionising radiation, regarding its release?

A

The energy is not uniformly released, but deposited unevenly in discrete nm-localised events of concentrated energy

56
Q

How much energy is released by ionising radiation?

A

About 33eV - enough energy to do considerable amount of damage to biological molecules

57
Q

What is the biological effect of ionising radiation determined by?

A

The photon energy size, not the amount of energy absorbed

58
Q

What is the principle target for the bio-effects of ionising radiation?

A

DNA

59
Q

What lesions can be caused by the interaction of ionising radiation with DNA?

A
  • Base damage
  • Sugar damage
  • Strand breaks
60
Q

Give two examples of base damage that can occur as a result of the interaction of ionising radiation with DNA?

A
  • Thymine glycols
  • 8-hydroxyguanine
61
Q

GIve two examples of sugar damage that can occur due to the interaction of ionising radiation with DNA

A
  • Abasic sites
  • Strand break lesions
62
Q

Why are double strand breaks so important?

A
  • Unrepaired DSBs are thought to be critical cell killing lesions
  • DBS repair is problematic and error-prone in mammalian systems
63
Q

Why is DBS repair problematic?

A

Because you may loose genetic material, causing a sequence change and therefore protein change

64
Q

What is the significance of misrepaired double strand breaks?

A

They are the principle lesions of radiation mutation and visible chromosomal aberration formation

65
Q

How much radiation is given in radiotherapy?

A

Typically, 30 fractions of 2Gy each, given Monday-Friday for 6 weeks

66
Q

What is fractionation of the radiation dose?

A

The splitting of the total dose into many single fractions

67
Q

What is the advantage of fractionation of the radiation dose?

A
  • Produces better tumour control for a given level of normal tissue toxicity than the administration of a large dose
  • Spares normal tissue by allowing for damage repair between doses
68
Q

How does fractionations spare normal tissue?

A

Normal tissue has its full complement of DNA repair, whereas tumour tissue has compromised repair. This means that only the normal tissue can repair its DNA in the interval

69
Q

What happens as solid tumours rapidly grow?

A

They start to exceed their supply of oxygen and nutrients, and tend to become hypoxic

70
Q

What % of solid tumour cells are hypoxic?

A

10-15%

71
Q

What is the problem with hypoxic tumour cells?

A
  • They are radioresistant, but still viable
  • They are more aggressive
72
Q

What is the difference in dose needed to kill hypoxic cells known as?

A

The OER - oxygen enhancement ratio

73
Q

What is the OER at low doses?

A

About 2.5x

74
Q

What can be done to overcome the problem of tumour hypoxia?

A

Dose fractionation

75
Q

How dose dose fractionation overcome the problem of tumour hypoxia?

A

It allows for reoxygenation

76
Q

What does multiple beam radiotherapy allow?

A

The radiologist to superimpose the X-ray dose over the tumour bearing region, to allow high doses to the tumour volume with sparing of adjacent tissue

77
Q

What needs to be done regarding the positioning in mulitiple beam radiotherapy?

A
  • Have to position the patient accurately and reproducibly
  • Have to adjust to take into account tumour shrinkage using image guided ultrasound
78
Q

What is used to shape the beam to the tumour volume?

A

Multilead Collimators

79
Q

What does the use of multiple beam techniques combined with multi-lead collimators allow the radiologist to do?

A

Shape the x-ray beam to the tumour shape to allow a high dose region to fit the tumour volume, with greater sparing of adjacent tissue

80
Q

How is multiple beam radiotherapy developing?

A

Now using an increasing number of beams

81
Q

What is the problem with multiple beam radiotherapy?

A

No region is left unexposed

82
Q

What is it hoped will be achieved with fractionation and multi-beam conformal radiotherapy?

A

The situation of positive therapeutic gain occurs, whereby the dose required to control the tumour is achieved, with an accetable level of normal tissue damage

83
Q

What is a proton?

A

A subatomic particle with a positive electric charge of 1 elementary charge, and a mass of slightly less than a neutron

84
Q

What is the Bragg peak?

A

A pronounced peak on the Bragg curve which plots the energy loss of ionising radiation during its travel through matter. For protons this occurs immediately before the particles come to rest - the Bragg peak

85
Q

What does the depth to which the protons penerate into the patient depend on?

A

The energy of the proton beam

86
Q

What is the result of the precise control of the energy of the proton beam?

A

It means the proton beam stops at the target, and so the target gets more of a radiation dose than the overlying tissue, and the underlying tissue receives no radiation dose.

87
Q

How do x-rays compare to protons in term of penetration?

A

X-rays are more penetrating than protons, and so cannot be stopped at the target so higher doses of radiation are delivered to surrounding tissues

88
Q

What have large scale animal studies and epidemiological studies of human populations shown about radiation?

A

It is a universal carcinogen

89
Q

What is meant by a universal carcinogen?

A

Induces cancer in most tissues, of most species, of all ages

90
Q

What does the universal nature of radiation as a carcinogen relate to?

A

The all-penetrating character of ionising radiation

91
Q

How does the carcinogenic potential and mutagenic potential of radiation compare to other chemical agents/

A

It is a weak carcinogen and mutagen

92
Q

Why is radiation only a weak carciongen and mutagen?

A

Because it is such a good cell killing agent

SSC- Biology of Cancer (11 decks)

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