Radiotherapy Flashcards
(42 cards)
What is external beam radiotherapy?
Treatment is received from an external source
What is brachytherapy?
A sealed radioactive source is placed directly into tumours or body cavities
What is unsealed source radiotherapy?
Where a therapeutic radioisotope is injected/ingested into the body and has an affinity for the target organ
Give an example of unsealed source radiotherapy
Radioiodine for thyroid cancer
Why is radiotherapy an effective way of treating cancer?
Very potent
Relatively low cost
What are the two ways of thinking of X-rays and γ-rays? Give the equations
As waves: c (speed) = λ (wavelength) x ν (frequency)
As photons E (energy of photon) = h (Plank’s constant) X ν
Relationship between wavelength, frequency and Plank’s constant?
As wavelength decreases, frequency increases
Related by Plank’s constant (h)
What is the frequency and wavelength of gamma and X-rays? What does this mean?
High frequency, small wavelength
High photon energy
They are more damaging
What does absorption of energy from radiation lead to (regarding electrons)?
Excitation - moved to a higher shell
Ionisation - ejection of an electron producing positively charged free radicals
What determines if X-rays are absorbed or not?
Depends on the energy of the photos and the chemical composition of the absorbing material
Does the Compton process involve high or low energy protons?
High energy
What happens to the photon in the Compton process when it comes into contact with the absorbing material?
The hooting interacts with loosely bound free elections of low binding energy
Part of the photon energy is given to the electron as KE
Photon is deflected, and proceeds with reduced energy, or a longer wavelength
Are the photons in the photoelectric process high or low energy?
Low energy
What happens when the photon comes into contact with the absorbing material in the photoelectric process?
Photon interacts with a tightly-bound electron of higher binding energy, the photon gives up its energy entirely, the electron is ejected, and the photon is entirely absorbed
What is the photoelectric and Compton process used for? Why?
Photoelectric: diagnostic radiology because photons have lower energy so are absorbed by bone
Compton process: radiotherapy because photons have higher energy and so can pass through bone etc to the target site
What does the Compton process produce after the photons are deflected?
Production of fast electrons which can go on to ionise other atoms of the absorber
Which process is independent of the atomic number of the absorbing species?
The Compton process is independent of atomic number.
What is direct acting radiation?
The atoms of the target molecule (DNA) are ionised
Cannot be modified by sensitisers or protectors
What is indirect radiation?
When the radiation interacts with other molecules to produce free radicals that migrate into the DNA
What is an example of a molecule that indirect radiation can interact with?
Water
To produce a hydroxyl radical, hydrogen ion and an electron
(H2O -> OH + H+ + e-)
What can the effect of the hydroxyl radical which ionises DNA be modified by?
Sensitisers and protectors
What kind of damage can ionising radiation cause to DNA?
Base damage
Sugar damage - eg abasic sites and strand break lesions
Strand breaks - single or double
Why is good to induce double strand breaks?
Of unprepared, thought to be critical cell killing lesions
Their repair is problematic and error-prone
Give some examples of visible chromosomal aberration formation
Ring chromosomes
Di-centric chromosomes
