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Y4 SA - Oncology > Radiotherapy > Flashcards

Radiotherapy Flashcards

(45 cards)

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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?

Study These Flashcards
A

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

20
Q

Describe the main features of electrons for use in radiation

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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?

Study These Flashcards
A

Repair
Repopulate
Redistribution/reassortment
Reoxygenation

22
Q

Describe the repair response to radiotherapy

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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

Study These Flashcards
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

Study These Flashcards
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
Describe the reoxygenation response to radiotherapy
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
How does the timing of radiation affect the dose?
Two doses of radiation given at separate times have less effect than the sum of the two doses given as a single treatment
27
What happens to cells/tissues between radiation treatments?
- 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
Define fractionation
Fractionation is the practice of giving multiple small doses instead of one big one
29
What is the aim of fractionation?
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
What are the affects of larger fraction sizes?
Greater normal tissue damage Especially late responding tissues
31
What are the affects of smaller fraction sizes?
Ideal for most tumours More treatments required for the same anti-tumour effect
32
How is fractionation radiation therapy used in a veterinary practice?
- Once weekly (8Gy to 32Gy) - Monday-Wednesday-Friday (4Gy to 48Gy) - Daily (various eg 3Gy to 48Gy)
33
What are the limitations of fractionation for animals
Requirement for general anaesthesia Cost Owner reluctance - Inconvenience - Frequent visits - Long hospitalisation
34
The response of a patient to fractionated radiotherapy is affected by which factors?
- Tumour growth characteristics - Tumour size - Inherent sensitivity - Tumour type - Tumour site - Patient species
35
How do tumour growth characteristics determine the response to radiotherapy
Slowly dividing tissues may be more radioresistant as fewer cells in sensitive phases
36
How does tumour size determine the response to radiotherapy?
- 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
Name 3 highly radiosensitive tumours
Lymphoma Transmissible venereal tumour Gingival basal cell carcinoma (acanthomatous ameloblastoma)
38
Name some moderately radiosensitive tumours
Oral SCC (dogs) Oral malignant melanoma (dogs) Nasal tumours Perianal adenocarcinoma MCTs Rhinarial SCC (cats) Thyroid carcinomas Brain tumours
39
Name 5 poorly radiosensitive tumours
Fibrosarcomas Haemangiopericytomas Oral SCC (cats) Osteosarcomas Rhinarial SCC - in dogs
40
What are the acute side effects of radiotherapy?
- 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
What are the late side effects of radiotherapy?
- 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
Describe the carcinogenesis of radiotherapy
Radiation therapy is carcinogenic - DNA damage and mutagenesis - Tends to be a long time period before development of malignancy - Usually years in dogs
43
What are some case selection considerations for radiotherpay?
- 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
How can radiotherapy be used in combination with surgery
- 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
For what reasons would radiotherapy be used pre-operatively?
- Reduces tumour burden - Eliminates small numbers of tumour cells at the periphery of the lesion

Y4 SA - Oncology (12 decks)

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