Medical Physics : Radiation

Question 1

What equation describes radioactive decay?

Answer 1

Question 2

Define activity

Answer 2

Activity is the rate of change of N

Question 3

What equation describes activity?

Answer 3

Question 4

Write an equation to describe beta minus decay.

Answer 4

Question 5

Write an equation to describe beta plus decay.

Answer 5

Question 6

Write an equation to describe alpha decay.

Answer 6

Question 7

What is absorbed dose and what are its units?

Answer 7

• Absorbed dose is the energy of ionising radiation absorbed per kilogram.
• The unit is gray [Gy] = Joules per kg

Question 8

What is relative biological effectiveness?

Answer 8

The biological effect compared to an equal dose of X-rays.

Question 9

What is the dose equivalent?

Answer 9

Dose equivalent = absorbed dose x RBE

Question 10

What are the two main types of radiotherapy?

Answer 10

Brachytherapy and external beam radiotherapy.

Question 11

How does Brachytherapy work?

Answer 11

In Brachytherapy a sealed radioactive source is placed inside the body at the site of the tumour. A beta emitter can then be used to produce a highly localised dose.

Question 12

How does external beam radiotherapy work?

Answer 12

• Multiple beams of radiation applied to the body from different directions, so as to target dose on the tumour.
• Accurate planning and positioning is crucial

Question 13

Describe positron emission tomography.

Answer 13

• PET generates two dimensional images of the distribution of a positron-emitting radioactive isotope in the body.
• A ring of sensors detect the two gamma photons produced by each positron annihilation.
• There is no need for a collimater as a line can be drawn between the sites of detection of the two photons to establish the positon of the positron source.

Question 14

Describe how a gamma camera works.

Answer 14

• A lead collimater is used to pass only gamma rays travelling ‘vertically’ from subject into the crystal.
• Sodium iodide is used to convert gamma photons into light
• Photomultiplier tubes then convert this light into electrical signals.
• The position on the crystal where the light originates from indicates position of the gamma source.

Question 15

Why does radiotherapy work?

Answer 15

• Cancer cells are more sensitive than normal cells
• Radiation dose is maximised to the tumour whilst limiting the dose in normal tissue.
• Delivered in fraction, allowing time for normal tissue to recover between sucessive radiation exposures.

Question 16

What is the constant in the radioactive decay formula?

Answer 16