Seminar 9 - Radiotherapy in cancer management

Question 1

Outline the types of radiation therapy.

Answer 1

1. External beam radiotherapy
2. Brachytherapy - sealed sources
3. Unsealed sources: radioiodine - thyroid cancer, meta-iodobenzylG - neuroblastoma

Question 2

Describe the Compton process.

Answer 2

• High energy photons
• Photons interacts with loosely bound free electrons of low energy
• Part of photon energy is given to electron as KE
• Photon is deflected and proceeds with a reduced energy: longer wavelength
• The end result is the production of fast electrons, which may go on to ionise other atoms of the absorber, plus a deflected/scattered photon with lower energy
• Independent of the atomic number of the absorbing species
• In radiation therapy, important to avoid the problem of differential absorption in tissues - bone vs. soft tissue

Question 3

Describe the photoelectric process.

Answer 3

Photons of lower energy - diagnostic radiology

• Photons interact with tightly bound electrons of higher binding energy
• Photon gives up its energy entirely
• Electron is ejected, and the photon is entirely absorbed
• End result is production of fast electrons but the photon is entirely absorbed
• Varies rapidly with the atomic number (Z^4)
• Diagnostic radiology - photoelectric process is important because bone differentially absorbs X-rays, resulting in the familiar appearance of radiographs

Question 4

Describe the spindle assembly checkpoint.

Answer 4

• Quality control checkpoint
• Allows mitosis to progress only if the chromosomes are properly attached to the spindle apparatus
• If there is the “GO” signal, SAC promotes degradation of cyclin B and allows mitosis to progress

Question 5

What is APC/C?

Answer 5

The anaphase promoting complex - E3 ubiquitin ligase which promotes degradation of cyclin B1 and securin

Question 6

State the components of SAC.

Answer 6

1. BubR1
2. MAD2
3. BUB3

Question 7

Distinguish between the direct and indirect effects of ionising radiation.

Answer 7

1. Direct: ionising radiation interacts with the atoms of the target molecule (DNA) –> ionised: generally cannot be modified by sensitisers and protectors
2. Indirect: ionising radiation interacts with other molecules to produce free radicals that migrate to the DNA
   - Accounts for 2/3rds of cellular damage, modified by sensitisers and protectors

Question 8

Outline the features of spurs and blobs.

Answer 8

Spur: 4nm diameter, 3 ion pairs
Blob: 7 nm diameter, 12 ion pairs

Question 9

What is the biological effect of ionising radiation determined by?

Answer 9

Not by amount of energy absorbed, but photon energy size

Question 10

What type of damage does interaction of ionising radiation with DNA cause?

Answer 10

1. Base damage: thymine glycols, 8-hydroxyguanine
2. Sugar damage: abasic sites, strand break lesions
3. Strand breaks: single strand breaks, double strand breaks