Radiotherapy and Radiobiology Flashcards
Define radiotherapy.
The medical use of ionising radiation to kill or control cancer cells.
Ionising radiation is radiation with enough energy to removed tightly bound electrons from the orbit of an atom, causing the atom to become charged or ionised.
May be in the form of particulate (electron, neutron, proton) or photon/gamma (from electromagnetic spectrum)
The SI unit is 1 gray (1 joule/kilo)
Contrast the indirect effects to direct effects of radiobiology.
Indirect effects are due to radiation interacting with intra-cellular water to form free radicals (hydroxyl free radicals), which in turn cause damage to DNA.
Direct effects are due to radiation interacting directly with critical cellular structures.
List and describe each of the 5 R’s of radiobiology.
Repair, Repopulation, Redistribution, Re-oxygenation, Radiosensitivity.
Repair - cellular death from radiation can occur in both normal and malignant cells, however repair processes in tumour cells is impaired, the aim of radiation is to achieve maximal death in tumour cells with minimal effects on normal tissue.
Repopulation - the reduction in doubling time of a tumour after commencement of cytotoxic tumour. accelerated repopulation occurs as cells progress from G0, G1 into mitosis.
Redistribution - the tumour cell progression in the cell cycle from radioresistant phase (G0,1) to a radiosensitive phase (M, S)
Re-oxygenation - the well oxygenated cells are more susceptible to SSB and DSB in DNA as oxygen impairs repair. Cells then progress from anoxic to hypoxic to well-oxygenated as previous cells die.
Radiosenstivity - the inheritent susceptibility of certain tissues and tumours to radiation.
Why are radiation doses fractionated?
To allow delivery of a higher dose that in a single fraction would be too toxic to normal tissue, thus the difference in DNA repair capacity of normal tissue versus tumour is exploited.
Why should radiotherapy, once started, be completed?
Because of the risk of accelerated repopulation of cells.
What is brachytherapy?
The use of a radionucleotide, such as Ir-192, which decays and emits elements as it does so into surrounding environment, thus a high dose can be given to a tumour within a short distance of radionucleotide. Used intra-cavity such as tandem and avoids used in gynaecological malignancies.
