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RT > RT-theory > Flashcards

RT-theory Flashcards

1
Q

What kind of radiation beams can be produced by a Linear accelerator?

A

High energy x-ray and electron beams

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2
Q

What kind of radiation is brachytherapy?

A

It uses I131 and produce beta-particles (electrons and positrons)

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3
Q

High-energy beams (photons) deposit energy ______

A

at a greater depht than eletrons

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4
Q

Low-energy beams (electrons) deposit most of the energy_______.

A

Skin surface

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5
Q

Name the 3 processes by which x-ray photons give up their energy when interacting with tissue?

A

Photoelectric effect, compton effect, paired reduction

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6
Q

What is the definition of Gy?

A

1 Gy = 1 J/kg (1 joule absorbed per kg)

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7
Q

Define LET?

A

Linear energy transfer = average energy lost by a particle over a given track length

The biologial effect of a dose of radiation depends on its LET

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8
Q

Give an example of a low LET particle and a high LET particle?

A

low = photons
high = protons

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9
Q

How can DNA be damaged by radiation?

A

Direct - absorption of energy by DNA
Indirect- damage via free radicals

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10
Q

Which radical is most damaging for DNA

A

Hydroxyl radixal - oxidising

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11
Q

How many doble strand breaks are achieved pr. 1Gy?

A

only 25-50

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12
Q

What is mitotic cathastrophy?

A

Occur due to chromosomal aberrations secondary to DNA DSB

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13
Q

What is the bystander effect?

A

When ROS or other factors are released from irradiated cells, causing damage to nearby non-irridated cells

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14
Q

By which two mechanisms can DNA DSBs occur and what are they called?

A

Passage of sinfle paricle = linear effect - seen most with low dosages

Passage of two separat particles causing a SSB = quadratic component - high dose

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15
Q

What is the alpha-beta ratio?

A

Its where the killing via the one hit(linear) and two hit (quadratic) kinetics are equal

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16
Q

Which tumours are considered to have a low a/b ratio?

A

melanoma, prostatic tumour, STS, TCC, OSA

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17
Q

By which modes can a cell die after RT and which one is most common?

A

Apoptosis, necrosis, miotic cathastrophe (most common), terminal growth arres

18
Q

Which 4 cell types are particularly sensitive to apoptotic death?

A

Lymphocytes
Thymocytes
Salivary gland epithelium
Spermatozoa

19
Q

Where in the cell cycle are the cells most senistive to radiaion?

A

M-phase (and late G2)

20
Q

Why are cells in S-phase more resistant to radiation?

A

Due to increasd DNA repair in this phase

21
Q

Which drugs can increase radiosensitisation?

A

Imatinib
PARPi
Carboplatin
Gemcitabine

22
Q

Defnine the following:
CRT
3D-CRT
IMRT
SBRT
Brachy
Plesio

A

CRT- uniform intensity across the RT beam, square
3D-CRT - uniform intensity, irregular beam shape
IMRT - variying intensity of the beam, irregular shape
IGRT - IMRT which changes the size shape and location of tumour or other organs
SBRT - highly conformal, often RT administered in 1-5 fractions
Brachy - directly adjacent tumour or within
Plesio - source of RT placed in contact with exterior surface of the body

23
Q

GTV

A

Gross tumour volume

24
Q

CTV

A

Clinical target volume - allows for tx of microscopic disease beyond edge of visible tumour, or metastatic disease

25
Q

PTV

A

Planned target volume - GTV + CTV + additional margin for set up error, patient movement etc

26
Q

By which cellular mechanisms are DSBs repaired?

A

Homologous recombination - S and G2
Non-homologous end joining - throughout cell cycle

27
Q

What are the risks of SRT?

A

Higher risk of late effects due to high dose in few fractions - it is fractionation and allowing repair which spares the normal tissues

28
Q

What are the reasons combining RT with other treatments may improve outcome?

A
  1. Prevent repopulation
  2. Decrease clonogenic tumour cells
  3. Increase cellular rt sensitivity
  4. Improve reoxygenation
  5. Kill circulating endothelial precursor cells which would replace vasculature destroyd during RT.
29
Q

Name 5 factors impacting tumour senistivity to RT

A
  1. Number of CSC - increased nr = increased RT dose needed
  2. Degree tumour hypoxia
  3. Genetic/epigenetic heterogeneity
  4. Degree of expression/acivation of intracellular pathways (EGFR/MEK/ERK adn PI3K/AKT)
  5. Vascular damage - pro (deprivation of nutrients and cell death con increased hypoxia
30
Q

What is the beneficial effect of hypoxia?

A

The biological effect of RT are enhanced by oxygen
Oxygen can fix the DNA damage caused by hydroxyl radicals and lead to irreparable damage.

31
Q

What are the drugs nimorazole and tirapazamine?

A

Hypoxic cell senzitiser
Drug toxic ony to hypoxic cells

32
Q

When is the cut of between early, early delayed and delayed responses for RT?

A

Acute - less than 3 in rapidly dividing cells
Early delayed - 2 w and 4 months (only neurological tissue)
Late response - over 3 months in slowly dividing cells

33
Q

What are consequential late effects?

A

When severe early reactions result in impaired tissue recovery

34
Q

Which factor influence the risk of acute tox the most?

A

By the time over which a protocol is administered (and total dose)

35
Q

Which factors impact the risk of late tissue tox the most?

A

Fraction size, volume of tissue irradiated and total dose

36
Q

What are functional subunits?

A

How one consider some late responding tissues - arranged in parallel (lung, liver, kidney) or series (spinal cord, intestine)

37
Q

Is it parallel or series tissue which have a reserve capasity when it comes to late RT effect?

A

Parallel

38
Q

What is amifostine?

A

Drug that can be used to reduce sensitivity of normal cells to RT, without influencing tumour cll sensitivity

39
Q

What are the 5 Rs of radiation

A

Repair
Repopulation
Reoxygenation
Redistribution
Radiosensitivity

40
Q

Hyperfractionated

A

o Involves giving a larger total dose
o Fraction size reduced; number of fractions increased
o Treatment time is the same
o Allows higher tumour control (due to higher total dose), whilst reducing late toxicity

41
Q

What is BED used for?

A

A calculation used to predict how changes in dose prescription may affect different cells/tissues based on their alpha/beta ratio

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