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Y4 SA - Oncology > Radiotherapy > Flashcards

Radiotherapy Flashcards

1
Q

Name the two main forms of radiotherapy

A

Brachytherapy
Teletherapy

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2
Q

Describe the different forms of brachytherapy

A
  • Direct application
  • Implantation
  • Systemic administration
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3
Q

Describe teletherapy

A

External beam
Most common
Linear accelerator

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4
Q

Radiation is produced by … or …

A

Linear accelerators or radioactive decay

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5
Q

Describe the three forms of electromagnetic radiation

A
  • X-rays
  • Gamma rays (cobalt sources)
  • Electrons
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6
Q

Describe low linear energy transfer as a form of high energy electromagnetic radiation

A

X-rays, gamma rays
- Lose energy slowly as passes through tissues
- Deep penetration
- Must consider the effects on deep structures

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7
Q

Describe indirectly ionising transfer as a form of high energy electromagnetic radiation

A

Absorption by the Compton effect
Interaction of photons with water molecules important

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8
Q

What is the Compton effect?

A
  • An x-ray photon transfers energy to an electron in the target tissue, causing ejection of this electron and scattered secondary photon (as the energy transfer is not absolute)
  • The secondary photons interact with tissue again and again.
  • Interaction is with outer electrons
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9
Q

What happens if the maximum dose isn’t absorbed at the surface of a tissue?

A

Build up affect

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10
Q

The xray dose at the depths of tissues is affected by what factors?

A

Beam energy
Field size and shape
Scatter
Tissue irradiation
Inverse square law

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11
Q

What is the critical target for therapeutic radiation?

A

DNA

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12
Q

Why is DNA a difficult target?

A

DNA is very small, so chances of an incident photon directly damaging (hitting) it are low

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13
Q

Describe how indirect damage works

A

Ionisation of water molecules
Water molecules around the DNA are ionised
Free radicals are generated
DNA is damaged by the free radicals

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14
Q

Why does DNA damage need to be ‘fixed’

A

The damage by free radicals to DNA is rapidly reversible unless it is fixed (made permanent) by oxygen
In hypoxic cells DNA is rapidly repaired as there is no oxygen

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15
Q

How does oxygen fix DNA damage?

A

Oxygen inhibits the repair of free radical induced damage
- Forming irreversible peroxides with the injured biomolecules
- “fixing” (making permanent) the radiation damage
- In absence of oxygen, damage is rapidly repaired

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16
Q

When using high energy electromagnetic radiation cell death occurs due to?

A
  • Induction of apoptosis: Lymphoid cells are sensitive to this
  • Permanent cell cycle arrest
  • Mitotic catastrophe
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17
Q

How does a Linac (used for external beam radiation treatments) work?

A
  1. Electron gun
  2. Accelerating wave-guide (travelling or standing wave)
  3. Magnetron
  4. RF wave-guide
  5. Photon target
  6. Photon jaws
  7. Flattening filter
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18
Q

How is the beam of radiation treatments shaped?

A

Simply with jaws
- like an x-ray
- rectangular field
Tumour shaped
- Multileaf collimator
MARGINS!

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19
Q

What is the advantage of multiple radiation beams?

A

Multiple beams (fields) can increase tumour dose while sparing surrounding tissue

20
Q

Describe the main features of electrons for use in radiation

A
  • Directly ionising
  • High linear energy transfer: loses energy rapidly as passes through tissue, ionisation only occurs superficially
  • Can treat superficial tumours
  • Useful for our patients
  • Electrons are very light compared to the nuclei of target tissue, and they lose a lot of their energy in a single interaction
21
Q

What are the 4 responses of the to radiotherapy?

A

Repair
Repopulate
Redistribution/reassortment
Reoxygenation

22
Q

Describe the repair response to radiotherapy

A
  • Most repair timescale about 6 hours
  • Repair of sublethal or potentially lethal damage to cells after exposure occurs rapidly
  • Tumour cells and normal cells generally have similar repair capacities: some tumours are really good at it e.g. malignant melanoma
  • Total dose of radiation required to kill cells is less if a few large doses rather than lots of smaller doses are given
  • Fractionation
23
Q

Describe the repopulation response to radiotherapy

A
  • Seen in rapidly dividing tissues
  • Cells are recruited from G0
  • Protects rapidly dividing normal tissues
  • Rapidly dividing tumours also repopulate effectively
24
Q

Describe the redistribution/reassortment response to radiotherapy

A

Cells are more sensitive to radiation in some phases of the cell cycle than others (late G2 and M)
- Cells may become synchronised in the post treatment period
- Synchrony is soon lost
- Timescale over which redistribution occurs is variable and poorly defined

25
Q

Describe the reoxygenation response to radiotherapy

A

Important factor
- Many solid tumours have poor blood supply: sinusoidal vessels, poorly formed microvasculature
- Areas of hypoxia/necrosis
- Reoxygenation may occur after therapy
- Euoxic cell death
- Changes in tumour vascularity

26
Q

How does the timing of radiation affect the dose?

A

Two doses of radiation given at separate times have less effect than the sum of the two doses given as a single treatment

27
Q

What happens to cells/tissues between radiation treatments?

A
  • Between treatments cells can REPAIR sublethal damage
  • Normal tissue and tumour tissue can REPOPULATE from cells that are in resting phases/cycle arrest
  • Want to allow some reoxygenation of cells in hypoxic areas so tumour cells are more susceptible to subsequent treatment
28
Q

Define fractionation

A

Fractionation is the practice of giving multiple small doses instead of one big one

29
Q

What is the aim of fractionation?

A

Aim is to achieve better tumour cell kill for any given level of normal tissue toxicity
Allows higher total doses of radiation to be given

30
Q

What are the affects of larger fraction sizes?

A

Greater normal tissue damage
Especially late responding tissues

31
Q

What are the affects of smaller fraction sizes?

A

Ideal for most tumours
More treatments required for the same anti-tumour effect

32
Q

How is fractionation radiation therapy used in a veterinary practice?

A
  • Once weekly (8Gy to 32Gy)
  • Monday-Wednesday-Friday (4Gy to 48Gy)
  • Daily (various eg 3Gy to 48Gy)
33
Q

What are the limitations of fractionation for animals

A

Requirement for general anaesthesia
Cost
Owner reluctance
- Inconvenience
- Frequent visits
- Long hospitalisation

34
Q

The response of a patient to fractionated radiotherapy is affected by which factors?

A
  • Tumour growth characteristics
  • Tumour size
  • Inherent sensitivity
  • Tumour type
  • Tumour site
  • Patient species
35
Q

How do tumour growth characteristics determine the response to radiotherapy

A

Slowly dividing tissues may be more radioresistant as fewer cells in sensitive phases

36
Q

How does tumour size determine the response to radiotherapy?

A
  • Smaller tumours are more sensitive to radiation
  • More rapidly dividing, higher growth fraction, more cells in sensitive phases
  • Less likely to contain large numbers of hypoxic cells
  • Easy to dose accurately and evenly
37
Q

Name 3 highly radiosensitive tumours

A

Lymphoma
Transmissible venereal tumour
Gingival basal cell carcinoma (acanthomatous ameloblastoma)

38
Q

Name some moderately radiosensitive tumours

A

Oral SCC (dogs)
Oral malignant melanoma (dogs)
Nasal tumours
Perianal adenocarcinoma
MCTs
Rhinarial SCC (cats)
Thyroid carcinomas
Brain tumours

39
Q

Name 5 poorly radiosensitive tumours

A

Fibrosarcomas
Haemangiopericytomas
Oral SCC (cats)
Osteosarcomas
Rhinarial SCC - in dogs

40
Q

What are the acute side effects of radiotherapy?

A
  • Affect rapidly dividing tissues: Skin, Mucous membranes, Erythema/desquamation
  • Develop during or soon after treatment
  • Resolve within a few weeks of cessation of therapy
  • Worse if treatment course is compressed
41
Q

What are the late side effects of radiotherapy?

A
  • Affect slowly dividing tissues
  • Develop many weeks, months or years after treatment
  • Damage to tissues and their microvasculature
  • Potentially very serious e.g. Ischaemic necrosis of brain or bone tissue
  • Many inconsequential/ less serious late effects: Alopecia, Skin fibrosis etc
  • Reduced healing capacity
  • Vascular damage and organ depletion
42
Q

Describe the carcinogenesis of radiotherapy

A

Radiation therapy is carcinogenic
- DNA damage and mutagenesis
- Tends to be a long time period before development of malignancy
- Usually years in dogs

43
Q

What are some case selection considerations for radiotherpay?

A
  • Is treatment given with palliative or curative intent?
  • Would combined therapy be better than radiation alone?
  • What will be effect on adjacent structures?
  • Tumours of head, neck, extremities
  • Concurrent disease
  • Costs
  • Which protocol? Daily or M-W-Fr or Weekly
44
Q

How can radiotherapy be used in combination with surgery

A
  • Post-operative - Commonly used
  • After RADICAL surgical debulking
  • First treatment after wound healing
  • Intra-operative: application of radiotherapy to unresectable, otherwise inaccessible tumours at the time of surgery
  • Pre-operative or neo-adjunctive
45
Q

For what reasons would radiotherapy be used pre-operatively?

A
  • Reduces tumour burden
  • Eliminates small numbers of tumour cells at the periphery of the lesion

Y4 SA - Oncology (12 decks)

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