Radiation therapy Flashcards
What are the uses of ionizing radiation treatments of cancer?
- Curative (long term tumour control)
- Palliative (short term/ poor prognosis)
What are the goals of palliative ionizing radiation therapy?
- Manage pain
- Help with physical obstructions
- Relief of clinical signs
- Neurological
Does radiation have systemic effects?
No, it is a local therapy with the goal to kill cancer cells with minimum effect on surrounding normal tissues.
How does radiation kill cells?
- Direct effect (DNA damage)
- Indirect effect: free radicals- ionization of water leads to free radicals, oxygen radicals are very reactive and account for most of radiation damage (hypoxia therefore protect cells from radiation damage and so it is less effective on large tumours)
What cell stage does radiation kill cells?
- Cycling death is limited to diving cells and can be achieved at lower doses. Growth fraction and doubling time of cells is important
- interphase death: targets cells NOT in mitosis, higher doses are needed.
Describe dosing of radiation therapy
- Measured in Gray (Gy)
- 1Gy= 1J of energy absorbed by 1kg of tissue
- Dose usually fractioned over multiple treaments (1 treatment is 1 fraction)
e. g. 3Gy x19 fractions= 57Gy total dose)
What are the different methods of delivery for radiation therapy?
- External beam variation (teletherapy): delivered (X-rays or gamma rays)
- Interstitial beam radiation (brachytherapy): implanted (gamma or beta rays)
- Systemic radiation therapy: injected (gamma or beta rays)
Describe Orthovoltage
- type of teletherapy
- patient isn’t radioactive and it is directed into the tumour
- Orthovoltage= low-medium energy range
- Pros= simple, cheap
- Cons= low penetration, max dose goes to the skin
Describe Megavoltage
- Type of teletherapy
- Liner accelerator (c-ray) or cobalt unit (gamma rays)
- 6-10x energy of orthovoltage
- Pros= skin sparing as max dose 0.5cm below skin, less scatter so less radiation sickness, uniform dose to bone and soft tissue
- Cons= more expensive, more expertise needed
In which cases is brachytherapy usually used?
-Treatment of large animals, mainly horses because its expensive
What are the pros of fractionation?
- Normal cells have time to heal (cancer cells heal slower)
- Allows for cancer cells to be reoxigenated
- re-distribution of cancer cells in cell cycle
Which phases of cell cycle are most susceptible to radiation?
G2 and M
What is the difference between hypofractionated and fine fractionation protocols?
- Hypo: few alrge doses given weekly approx 4 weeks. Severe limitations on total dose due to healthy tissue tolerances
- Fine: much more doses given over same time but less energy. Able to achieve high total dose.
Which tumours are resistant to radiation?
- Bone sarcomas (treat palliatively)
- Metastatic LN more than 6cm
- Head/ neck tumours more than 5cm
Is radiation selective to tumour cells?
No
