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Table 1 Description of terms and metrics used throughout the manuscript

From: Optimization of the prescription isodose line for Gamma Knife radiosurgery using the shot within shot technique

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.