Fig 1.
(A) Two-dimensional dose distribution displayed using isodose lines on an axial CT-slice. (B) Three-dimensional illustration of two isodose surfaces. Double-headed arrows indicate the distances between two isodose surfaces. The volume and surface area of isodose levels were used to estimate the average distance between two isodose surfaces. V and S represent the volume and surface area of an isodose level, and the subscripts L and H represent the lower and higher doses.
Fig 2.
Geometric objects for verification of the dose gradient index (DGI) calculation.
(A) A sphere and its uniform expansion with a spacing d, (B) a cube and its uniform expansion with a spacing d, (C) an irregular shaped structure, and (D) a multi-layer structure at regular intervals (1 mm), produced by uniform expansion of the structure described in (C).
Fig 3.
Schematic representation of the basic concept of the dose gradient curves (DGC).
(A) The differential DGC and (B) the cumulative DGC.
Table 1.
Dose gradient index (DGI) of the multi-layer structure generated by uniform expansion of an irregularly shaped structure at 1-mm regular intervals.
Table 2.
Dose gradient index (DGI) of SRS plan for a virtual target of 3 cm diameter with a prescription dose of 15 Gy in a single fraction.
(calculation interval = 1%).
Fig 4.
(A) The differential dose gradient curve (dDGC) generated by data from Table 2. The dDGC is a plot in which each point represents the average distance between each isodose interval (the differential dose gradient index; dDGI); the shorter the distance, the steeper the dose gradient. (B) The corresponding cumulative dose gradient curve (cDGC) is a plot of the cumulative dose gradient index (cDGI) generated by summing the DGI values from the prescription dose (100% isodose) to each dose. Each point of the cDGC indicates the average distance from the prescription isodose surface to the corresponding isodose level.
Fig 5.
Plot options for the dose gradient curve (DGC).
Fig 6.
The effect of the calculation interval (step size).
(A) The differential dose gradient curve (dDGC) for varying step sizes ranging from 0.5% to 4%. The dDGC increased with increasing step size. (B) The normalized dDGC showing identical calculation results regardless of the difference in step size. (C) The cumulative dose gradient curve (cDGC) for different step sizes. The cDGC is invariant with respect to changes in step size.
Fig 7.
The dosimetric influence of the number of arcs with respect to the dose gradient.
The combination of the cDGC and the DVH, which were generated from a virtual target with a 1 cm diameter located (A) on the peripheral part of the brain, and (B) near the brainstem. PTV, planning target volume.
Fig 8.
The cumulative dose gradient curve (cDGC) combined with a dose-volume histogram (DVH).
The combined plot illustrates differences in dose gradient (cDGC) and target coverage (DVH) when doses are prescribed (Rx) at 70%, 80%, 85%, and 90% isodose levels in stereotactic radiosurgery (SRS) plans for a virtual target of 3 cm in diameter. PTV, planning target volume.
Fig 9.
The cylindrical sector-like Volume-of-Interest (VOI), with its axis passing through the planning isocenter.
Using the VOI, the DGC would provide the information about the dose gradient in a particular direction.