What is the indirect action of cell damage by radiation?
- primary photon interaction producing high energy electrons
- high energy electrons in moving through the tissue produce free radicals in water
- free radicals may produce change in DNA from breakage of chemical bonds
- Changed in chemical bonds result in biological effect
Physical vs Biological planning
- currently used physical quantities as surragotes for biological response
- better approach to use bioloigcal indices ot more directly reflect the clinical goals
What is biologically guided RT?
- use individual patient biological repsonse of tumour and NTT to design dose distribution
- tumour and NTT radiosentivity, oxygenation status and profliferation rate
What is biological based TP?
- use feedback from biological response models
What is the dose reponse model?
- biological cell survival models are required for tumours and normal tissue
- model predict observations seen in clinical dose repsonse data
What is the generalised equivalent uniform dose?
- the uniform dose that would yield the same radiobiological effect as the non-uniform dose
What is the linear quadratic model?
- used to describe the cell surivial curve assuming there are two compenents to cell kill
- the ratio a/b gives the dose at which the linear and quadratic components of cell kill are equal
What does the a describe?
- initial slope of surival curve
- respresents the intrinsic radiosensitivity of the cell
- non-reparable type of cell damage
- linearly dependent on dose
What does the b describe?
- the curvature of the cell survival curve
- repariable type of cell damage with time
- responsible for the dose/fraction variable
- proportional to the sqaure of the dose
What is biologically equivalent dose?
- fractionation schemes for which BED is equal will be equally effective biologically
- clinical a/b only accounts for repair
- K accounts for accelerated repopulation
- LQ model allows comparison of different fractionation scheduales
What is early dose responding?
- occurs immediately or during RT
- cell depletion within rapdily dividing cells
- e.g. skin, muscosal layer of gut causing pain and discomfort
What is late dose resonding?
- start 6-12 months after RT
- cell depletion within slowly dividing cells
e. g. spinal cord and kidneys casuinng irreversible symtoms
What are the general early and late effect tissues?
- early: most tumours, large a/b (10Gy) and a dominates at low dose
- late: normal tissue, small a/b (2Gy) and b has influence at low dose
What are the 5 R’s of RT?
- radiosensitivity
- repair
- repopulation
- redistribution
- reoxygenation
What is radiosensitivity?
- cells have different radiosensitivies
What is repair?
- cells can repair sub-lethal radiation damage
What is repopulation?
- cells repopulated during fractionated RT
What is redistribution?
- in proliferating cell population through the cell cylce increases the cell kill in the fraction therapy relatve to single fraction
What is reoxygenation?
- of hypoxic cells during fractionated RT makes them more sensitive to subsqent dose
What is the radiobiological rationale for fractionation?
- dividing dose spares normal tissue thorugh a repair of sublethal damage between fractions and repopulation of cells
- fraction increases tumour damage through reoxygenation and redistribution
What is the sensitivity to fraction size?
- rapidly prlifering cells (high a/b) and not sensitive to change in fraction size ro dose rate
- slowly proliferating cells (low a/b) are sensitive to dose/fraction and have plenty of repair capability
What size of fractionation are late responding tissue sensitive too?
- large
What is an a/b Gy example of early reacting tissue?
- skin: 9-12
- colon: 9-11
- testis: 12-13
- mucosa: 9-10
What is an a/b Gy example of late reacting tissue?
- kidney: 2-2.4
- rectum: 2.5-5
- lung: 2.7-4
- bladder: 3-7
- SC: 1.8
- Brainstem: 2.2
