Oncology: Radiotherapy Flashcards
What are the forms of radiotherapy?
Brachytherapy- close
* Direct application
* Implantation- iridium wires
* Systemic administration- iodine
Teletherapy- far
* External beam
How is radioation produced for teletherapy?
Linear accelerators or natural radioactive decay
What are the different types of electromagentic radiation?
X-rays
Gamma rays
Electrons
How does high energy electromagnetic radiatino transfer energy?
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
What is the compton effect?
- When a X-ray photon interacts with another particle (electron)
- X-ray photon loses energy
Why is the highest dose not at the surface of the skin?
There is a build up effect
If the highest dose is not at the skin, how is the highest dose controlled to be at the skin?
Bolus- acts like the skin therefore skin below gets highest dose
- What is the target for therapeutic photon radiation?
- How is the target affected?
- DNA- but very small
- Damage is caused by ionisation of water molecules- free radicles damage DNA
How can photon radiation damaged rapidly reversed?
Damage is reversible unless it is fixed by oxygen
Oxygen inhibits the repair of free radical induced damage
After cell damage from photon radiation, how does cell death occur?
- Induction of apoptosis
- Permanent cell cycle arrest
- Mitotic catastrophe
Damage often not expressed until cell tried to divide
Lineator
What does this image show?
Multileaf collimator- tumour shaped
What is the benefit of multiple beams?
Multiple beams can increase tumour dose while sparing surrounding tissue
How are electons different to photons?
- Directly ionising
- High linear energy transfer- rapidly lose energy
- Useful for superficial tumours
What are the 4 Rs of radiotherapy?
- Repair
- Repopulation
- Redistribution
- Reoxygenation
How is repair prevented?
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
How are areas repopulated after radiation?
- Seen in rapidly dividing tissues
- Cells recruited from G0
- Protects rapidly dividing normal tissues
- Not useful is tumours also rapidly dividing
What is redistribution?
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
What is the importance of reoxygenation?
- Many solid tumours have poor blood supply
- Areas of hypoxia/necrosis
Need to be well oxygenated to be targeted
Reoxygenation may occur after therapy
What is fractionation?
- Giving multiple small doses instead of one big one
- Between big ones- repair and repopulation can occur
What is the advantage of fractionation?
- Reduce normal tissue toxicity
- Achieve better tumour cell kill
What are the limits for fractionation for animals?
- Requirment for GA
- Cost
- Owner reluctance- inconvenience, visits, hospitalisation
How does tumour volume affect radiation?
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
What tumours are highly radiosensitive?
- Lymphoma
- Transmissible venereal tumour
- Gingival basal cell carcinoma
What tumours are moderately radiosensitive?
- Oral SCC (dogs)
- Oral malignant melanoma
- Nasal tumours
- Perianal adenocarcinoma
- MCTs
- Rhinarial SC (cats)
- Thyroid carcinomas
- Brain tumours
What tumours are poorly radiosensitive?
- Fibrosarcomas
- Haemangiopericytomas
- Oral SCC (cats)
- Osteosarcomas
- Rhinarial SCC (dogs)
Why can side effects be delayed?
Delayed
* Damage not apparent until cells try to divide
What are acute side effects?
Affects rapidly dividing cells
* Skin
* MMS
* Erythema/desquamation
What are late side effects?
- Damage to tissues and their microvasculature
- Reduced healing capacity
Why should irradiation be avoided in young patients?
Radiation therapy is carcinogenic
When is radiotherapy indicated over surgery?
Surgery- complete excision is the treatment of choice
Radiotherapy may be appropriate where complete excision cannot be achieved
How can radiotherapy be used along side surgery?
- Post op- common (after wound healing)
- Intra-op- not commmon
- Post-op- eliminate tumour burden- occasional
Which species are very good for showing very little clinical signs of radiotherapy?
Cats
When should radiotherapy referral be considered?
Prior to any surgery
Worst case scenario
* Scar
* No record of pre-treament tumour
* Large recurrent mass after incision
