Radiobiology > Biologic Interactions > Cel Survival Curve > Curve Indices -- Quasi-Threshold Dose

Cell Survival Curve: Indices, continued

Another aspect of the curve is the so-called quasi-threshold dose (Dq).  This is also known as the sub-lethal dose.

The quasi-threshold dose must be extrapolated from the curve: 

  • Begin by drawing a horizontal line (i.e. parallel to x-axis) from S=1. 
  • Now extend the straight portion of the survival curve back to the horizontal line from unity surviving fraction. 
  • The point of intersection is Dq.


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