Oncology: Radiotherapy

Question 1

What are the forms of radiotherapy?

Answer 1

Brachytherapy- close
* Direct application
* Implantation- iridium wires
* Systemic administration- iodine

Teletherapy- far
* External beam

Question 2

How is radioation produced for teletherapy?

Answer 2

Linear accelerators or natural radioactive decay

Question 3

What are the different types of electromagentic radiation?

Answer 3

X-rays
Gamma rays
Electrons

Question 4

How does high energy electromagnetic radiatino transfer energy?

Answer 4

Low linear energy transfer
* Lose energy slowly as passes through tissues
* Deep penetration
* Must consider the effects on deep structures

Dose initially increases to a max then decreases with depth- change to have highest dose at place indicated

Indirectly ionising

Question 5

What is the compton effect?

Answer 5

• When a X-ray photon interacts with another particle (electron)
• X-ray photon loses energy

Question 6

Why is the highest dose not at the surface of the skin?

Answer 6

There is a build up effect

Question 7

If the highest dose is not at the skin, how is the highest dose controlled to be at the skin?

Answer 7

Bolus- acts like the skin therefore skin below gets highest dose

Question 8

1. What is the target for therapeutic photon radiation?
2. How is the target affected?

Answer 8

1. DNA- but very small
2. Damage is caused by ionisation of water molecules- free radicles damage DNA

Question 9

How can photon radiation damaged rapidly reversed?

Answer 9

Damage is reversible unless it is fixed by oxygen
Oxygen inhibits the repair of free radical induced damage

Question 10

After cell damage from photon radiation, how does cell death occur?

Answer 10

• Induction of apoptosis
• Permanent cell cycle arrest
• Mitotic catastrophe

Damage often not expressed until cell tried to divide

Question 11

Lineator

What does this image show?

Answer 11

Multileaf collimator- tumour shaped

Question 12

What is the benefit of multiple beams?

Answer 12

Multiple beams can increase tumour dose while sparing surrounding tissue

Question 13

How are electons different to photons?

Answer 13

• Directly ionising
• High linear energy transfer- rapidly lose energy
• Useful for superficial tumours

Question 14

What are the 4 Rs of radiotherapy?

Answer 14

• Repair
• Repopulation
• Redistribution
• Reoxygenation

Question 15

How is repair prevented?

Answer 15

Tumour cells and normal cells have similar repair capacities- higher in hypoxic tumour cells

Fractionation
* Total dose of radiation required is less if a few large doses are given then lots of smaller ones

Question 16

How are areas repopulated after radiation?

Answer 16

• Seen in rapidly dividing tissues
• Cells recruited from G0
• Protects rapidly dividing normal tissues
• Not useful is tumours also rapidly dividing

Question 17

What is redistribution?

Answer 17

Cells in late G2 and M are more sensitive to radiation
* Cells can all become synchronised- as they are all recruited at the same time

Give more then one fraction

Question 18

What is the importance of reoxygenation?

Answer 18

• Many solid tumours have poor blood supply
• Areas of hypoxia/necrosis

Need to be well oxygenated to be targeted
Reoxygenation may occur after therapy

Question 19

What is fractionation?

Answer 19

• Giving multiple small doses instead of one big one
• Between big ones- repair and repopulation can occur

Question 20

What is the advantage of fractionation?

Answer 20

• Reduce normal tissue toxicity
• Achieve better tumour cell kill

Question 21

What are the limits for fractionation for animals?

Answer 21

• Requirment for GA
• Cost
• Owner reluctance- inconvenience, visits, hospitalisation

Question 22

How does tumour volume affect radiation?

Answer 22

Smaller tumours are more sensitive
* More rapidly dividing, higher growth fraction, more cells in sensitive phases
* Less likely to have hypoxic cells
* Easily to dose acurately and evenly

Question 23

What tumours are highly radiosensitive?

Answer 23

• Lymphoma
• Transmissible venereal tumour
• Gingival basal cell carcinoma

Question 24

What tumours are moderately radiosensitive?

Answer 24

• Oral SCC (dogs)
• Oral malignant melanoma
• Nasal tumours
• Perianal adenocarcinoma
• MCTs
• Rhinarial SC (cats)
• Thyroid carcinomas
• Brain tumours

Question 25

What tumours are poorly radiosensitive?

Answer 25

• Fibrosarcomas
• Haemangiopericytomas
• Oral SCC (cats)
• Osteosarcomas
• Rhinarial SCC (dogs)

Question 26

Why can side effects be delayed?

Answer 26

Delayed
* Damage not apparent until cells try to divide

Question 27

What are acute side effects?

Answer 27

Affects rapidly dividing cells
* Skin
* MMS
* Erythema/desquamation

Question 28

What are late side effects?

Answer 28

• Damage to tissues and their microvasculature
• Reduced healing capacity

Question 29

Why should irradiation be avoided in young patients?

Answer 29

Radiation therapy is carcinogenic

Question 30

When is radiotherapy indicated over surgery?

Answer 30

Surgery- complete excision is the treatment of choice

Radiotherapy may be appropriate where complete excision cannot be achieved

Question 31

How can radiotherapy be used along side surgery?

Answer 31

• Post op- common (after wound healing)
• Intra-op- not commmon
• Post-op- eliminate tumour burden- occasional

Question 32

Which species are very good for showing very little clinical signs of radiotherapy?

Answer 32

Cats

Question 33

When should radiotherapy referral be considered?

Answer 33

Prior to any surgery
Worst case scenario
* Scar
* No record of pre-treament tumour
* Large recurrent mass after incision