4.10 Radioactive sources in radiotherapy

Question 1

What is the definition of radioactivity?

Answer 1

A meterial that spontaneously emits radiation

Question 2

What is the definition of ‘Activity’?

Answer 2

number of decays per second

Question 3

What is the SI unit for Acitivity?

Answer 3

1 Becquerel (Bq) per second

Question 4

What is Radioactive Decay?

Answer 4

Rearrangement of a nucleus to change to a more stable configuration.
In a group, the number of radiaoactive atoms is declining exponentially.

Question 5

What is alpha decay?

Answer 5

Nucelus is too heavy (Z82+) and so loses 2 protons and 2 nuetrons (helium atom)

Z -2
A -4

Question 6

What is Beta- decay?

Answer 6

Neutron rich nucleus
Neutron turns into = Proton + electron + anti-neutrino

Electron lost - Nucleus has excess energy released as gamma radiation

Z +1
A same

Question 7

What is Beta+ decay?

Answer 7

Proton rich nuclei.

Extra proton turns into a neutron by releasing a positron.
Positron combines with an electron to release to photons of 511 keV each which travel in opposite directions (detected by PET (neutrino also released)

Question 8

What is electron capture?

Answer 8

Proton rich

electron + Proton -> Neutron

Outer orbiting electron then fills the vacancy left by the captured electron (moves from higher to lower energy state) excess energy released (Auger electron/ characteristic x-ray)

Question 9

What type of decay to proton heavy nuceli undergo?

Answer 9

1. Positron/Beta + decay
2. Electron capture

Question 10

What type of decay do Neutron heavy nuclei undergo?

Answer 10

beta - decay

Question 11

What is Physical Half life? (Tp)

Answer 11

Radioacitve decay - time tken for radioactivty to reduce by half due to decay

Question 12

What is biological half life? (Tb)

Answer 12

Biological clearance of unsealed radiation - Time for the body to clear half the substance (excretion)

Question 13

What is the effective half life? (Te)

Answer 13

Combined physical and biological half lives - net rate of clearance from the body due to radiaoactive decay and biological clearance

T1/2 = ln(2)/ λ

1/Te = 1/Tp + 1/Tb

Question 14

What contributes to the biological half life?

Answer 14

The “radiopharmaceutical”
1. The pharmaceutical
2. The radioactive material

Question 15

How does the pharmaceutical affect the biological half life?

Answer 15

The properties of the pharmaceutical control where the agent will go in the body and how long it will stay there

Need high uptake in target tissue and low uptake in other tissues

Question 16

How does the radiactivity contribute to the biological half life?

Answer 16

the properties of the radioactive material control how much radiation dose the exposed tissue will receive

Question 17

What are the 2 radioactive sources in therapy?

Answer 17

1. Sealed - used in brachytherapy, uncapsulated in metal
2. Unsealed - liquid or gas form. Commonly for diagnostics e.g. bone scans/PET. Sometimes therpapy.

Question 18

What are the ideal features of unsealed radionucelide therapy?

Answer 18

• High uptake in target tissue
• Low uptake in other tissue
• Emitter of non-penetrating radiation to minimuse dose to OAR - Pure Beta emitter
• Moderately long effective half-life (few days - weeks)
• If it also happens to emit hamma ray ~200keV can be used for imaging to see where it has been delivered to

Question 19

What is the source in unsealed radionucleide therapy?

Answer 19

The tissue contiaing the radioactivity

Question 20

What is the target of radionucelide therapy?

Answer 20

Tissue receiving the dose

Thyroid gland source and target

Question 21

What is a common unsealed radioactive source?

Answer 21

Iodine 131

Question 22

What is Iodine 131 used for?

Answer 22

Na-I for hyperthyroid and thyroid cancer

I-mIBG (noradrenaline analogue) for NET
I-mAb for NHL

Question 23

What type of radioactive decay does Iodine 131 undergo?

Answer 23

Beta minus - emits an electron

Question 24

What is the stable isotope of Iodine-131?

Answer 24

Xenon 131

Question 25

What type of radiation and doses does Iodine 131 emit?

Answer 25

• Beta - 191 keV
• Gamma 360-600 keV
• X-rays so can image to look at uptake

Question 26

How long is the half life of Iodine 131?

Answer 26

8 days

Question 27

What are the safety rules after Iodine 131 treatment and permanent sealed source treatment?

Answer 27

• Avoid children and pregnant others for 2 months
• Avoid public places
• Avoid sexual contact
• Avoid sharing of towels
• Avoid food prep for others

Question 28

What is strontium-89 used for?

Answer 28

Behaves like calcium in the body so metabolic uptake in the bones

Question 29

What type of radiation does strontium 89 emit?

Answer 29

Beta emissions 583 keV

up to 8mm in the tissue

Question 30

What is the half life of strontium 89?

Answer 30

50 days

Question 31

What is brachytherapy?

Answer 31

Use sealed sources close to or within a target to deliver therapuetic radiation

Question 32

What are the different types of brachytherapy (aka where the source goes in the body)?

Answer 32

• Interstitial - source placed within the tumour
• Intracavity - source enters the body through a cavity
• Intraluminal - lung, oesophagus, nronchus
• Surface - near the skin by moulds/applicators

Question 33

What are the two types of prostate brachytherapy?

Answer 33

LDR (low dose) 0.4 - 2Gy per hour via permanent implants/seeds

HDR (High dose) >12Gy per hour