✓A dose gradient index (GI) is proposed that can be used to compare treatment plans of equal conformity. The steep dose gradient outside the radiosurgical target is one of the factors that makes radiosurgery possible. It therefore makes sense to measure this variable and to use it to compare rival plans, explore optimal prescription isodoses, or compare treatment modalities.
The GI is defined as the ratio of the volume of half the prescription isodose to the volume of the prescription isodose. For a plan normalized to the 50% isodose line, it is the ratio of the 25% isodose volume to that of the 50% isodose volume.
The GI will differentiate between plans of similar conformity, but with different dose gradients, for example, where isocenters have been inappropriately centered on the edge of the target volume.
In a retrospective series of 50 dose plans for the treatment of vestibular schwannoma, the optimal prescription isodose was assessed. A mean value of 40% (median 38%, range 30–61%) was calculated, not 50% as might be anticipated. The GI can show which of these prescription isodoses will give the steepest dose falloff outside the target.
When planning a multiisocenter treatment, there may be a temptation to place some isocenters on the edge of the target. This has the apparent advantage of producing a plan of good conformity and a predictable prescription isodose; however, it risks creating a plan that has a low dose gradient outside the target. The quality of this dose gradient is quantified by the GI.