- Reasonably well validated experimentally and theoretically
- LQ Model works well if:
- Cell killing is result of DNA damage (DSBs)
- RT is multifractionated and fractions are well-separated
- Irradiation time short and dose rate is constant
- 2 components: α and β:
- α component:
- In low dose region
- Because of lethal damage
- Resulting from single event/hit
- Probability proportional to dose
- Effect ∝ D
- Effect = αD
- Responsible for linear component of cell survival curve
- β component:
- In high dose region
- Because of sublethal damage
- Lethal damage will be a result from 2 events/double hit kill
- Probability to produce 1 SLD is proportional to dose
- Probability to produce 2 SLDs is proportional to square of dose
- Effect ∝ D²
- Effect = βD²
- Responsible for quadratic component of cell survival curve
- α component:
Alpha Beta Ratio:
- Dose at which
- Linear=Quadratic
- α killing = β killing
- αD=βD²
- α/β=D²/D
- α/β=D
- α/β Definition: Dose at which contribution by single hit (Linear) kill becomes equal to double hit (Quadratic) kill
- Unit: Gray
- Application:
-
α/β defines curviness of survival curve, it is the dose at which survival curve starts bending
- Tumours have poor repair capabilities: more lethal damage → high α/β ratio → straighter cell survival curve
- Tissues having good repair cabiliities (e.g. late responding normal tissues) → low α/β ratio → curved cell survival curve
-
Based on α/β ratio, the body tissues have been divided into two category:
- Late Reacting Tissue (e.g. breast and prostate)
- α/β = 1Gy to 7Gy (3Gy)
- Shoulder is more curvy
- Early Reacting Tissue (most tumours)
- α/β = 6Gy to 15Gy (10Gy)
- Shoulder is less curvy
- Late Reacting Tissue (e.g. breast and prostate)
-
Adapting a dose per fraction more than α/β will kill more cells as compared to lesser dose for the same total dose → practically possible only for tumours having low α/β ratio because as normal tissues with a lower α/β will be damaged more if we employ this technique for higher α/β tumours
-
Effect at a given dose
- E = αD+βD²
-
Survival at a given dose
- S = E-αD-βD²
-
BED (Biological Effective Dose)
- n=number of fractions
- d=dose per fraction
- nd=total dose
-
- Limitations of LQ Model
- Does not include effect of redistribution
- Does not include effect of reoxygenation
- Cell survival curve becomes linear at higher doses unlike the bend curve predicted by LQ model
- Data validating LQ model for higher dose per fraction → Missing
