9) Radiotherapy Flashcards
How is radiotherapy currently used in cancer treatment?
Used in the local control of the disease
Can be used before or after surgery
Combined with chemo
For palliation
What are the different types of radiotherapy?
External beam radiotherapy - potent
Brachytherapy (sealed source) - short range
Unsealed sources e.g. radioiodine in thyroid cancer
What are the advantages of using radiotherapy?
Cost effective, curative, potent
What are the effects of absorption of energy from radiation on atoms?
Excitation - raising electron to a higher energy level
Ionisation - ejection of the electron
Explain the Compton process and which photons this happens in:
High energy photons (radiotherapy) - produces fast electrons that can go on to ionise other atoms and produce deflected/scattered photon of reduced energy
Explain the photoelectric process and which photons this happens in:
Lower energy (diagnostic radiology) - produces fast electrons but photon entirely absorbed
Why is the Compton process suited to radiation therapy?
Avoids the problem of differential absorption in tissues so can reach the tumour
Why is the photoelectric process suited to diagnostic therapy?
Allows differential absorption of X rays to provide contrast between tissues on radiograph
Explain the direct and indirect effects of ionising radiation:
Direct - atoms of target molecule are ionised
Indirect - production of free radicals that migrate to DNA - can be modified by protectors
What are spurs and blobs?
Photon’s energy deposited in concentrated packets. Spurs are 3 ion pairs and blobs are 12 ion pairs
Why is radiation dangerous?
Concentrated energy that can cause damage to DNA
What are the effects of ionising radiation on DNA?
Base damage e.g. thymine glycols
Sugar damage - abasic sites
Strand breaks - single or double
Why are double strand breaks particularly bad?
Critical cell killing lesions as the DSB repair is error prone
Explain the process of fractionation:
Splitting of the total radiation dose into many single fractions
What are the advantages of fractionation?
Better tumour control for a given level of normal tissue toxicity
Spares normal tissue by allowing repair between doses
How can DNA damage in cancer be exploited in therapy?
Genomic instability in cancer cell so further genomic damage by therapy can kill cells as they are unable to repair the damage (damage to DNA repair genes)
What are some examples of hypoxia markers?
HIF-1 and CA IX
Why are hypoxic tumour cells a problem?
Radioresistant but still viable, also more aggressive
How is the problem of tumour hypoxia overcome in therapy?
Dose fractionation allows reoxygenation of hypoxic cells so a great proportion of cells are radiosensitive
What is multiple beam radiotherapy?
Allows radiologist to superimpose the X-ray dose over the tumour using multiple beams from different direction. High dose to tumour, sparing adjacent tissue e.g. liver, kidney, spinal cord
What are the function of multileaf collimators?
Allows shaping of beam to tumour volume
What is the Bragg peak and how can this be used in therapy?
Depth to which protons penetrate is determined by energy of beam so energy can be controlled so beam stops at target. Where protons stop is Bragg peak
Why can proton therapy be advantageous to x-ray therapy?
Less damage to adjacent tissue by radiation as proton beam can stop at target tissue
What is the risk of x-ray radiation?
Acts as a weak carcinogen and mutagen but small risk compared to other hazards in society
