Term/metric | Equation | Description |
---|---|---|
Dose gradient | General term referencing the rapid fall-off in dose along the target periphery. | |
Penumbra | \( \left|{d}_{IDL_{80\%}}-{d}_{IDL_{20\%}}\right| \) | The distance between two IDLs that lie within the dose gradient. Traditionally, the IDLs are chosen as the 80% and 20% lines. |
Gradient distance | \( \left|{d}_{IDL_{RX}}-{d}_{IDL_{RX}\times f}\right| \) | Distance between two relative IDLs, where the first is the prescription IDL and the second is the prescription IDL multiplied by a factor ranging from 0.2–0.9. |
Gradient index | \( \frac{V_{1/2 RX}}{V_{RX}} \) | Ratio of volume enclosed by half the prescription dose to that enclosed by the prescription dose. |
Prescription isodose diameter | \( \left|{d}_{Left,{IDL}_{RX}}-{d}_{Right,{IDL}_{RX}}\right| \) | Diameter of the prescription isodose volume as visualized in the axial plane, i.e. distance between prescription IDLs located on the left and right side of the shot center. |
Coverage | \( \frac{V_{RX}\cap {V}_{target}}{V_{target}} \) | Fraction of target volume within prescription isodose volume. |
Conformity | General term referencing the degree to which the prescription isodose is contained within the target volume. | |
Selectivity | \( \frac{V_{RX}\cap {V}_{target}}{V_{RX}} \) | Fraction of prescription isodose volume within target volume. Also, the inverse of the conformity index multiplied by coverage. |