Evaluation Of Gamma Knife Plans Using Dose-volume Histograms

Keywords: gamma knife, dose planning, Dose distribution, radiosurgery, software

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     The objectives of this study were to study the effects of shots and plugs, and plan evaluation in Gamma Knife using the DVH analysis. The comparisons of a single shot (SS) vs. multiple shot (MS), no plug vs. plugs, and plan evaluation were performed for six different diseases. The diseases studied were arteriovenous malformation (AVM; N=10), brain metastasis (BM; N=20), meningioma (MN; N=19), pituitary tumor (PT; N=10), vestibular schwannoma (VS; N=13), and trigeminal neuralgia (TN; N=16).
     The relationships between dose and volume coverage were measured at 90%, 80%, 70%, 60%, 50%, 40%, and 30% of the maximum doses using dose-volume histograms. We set the control volume covered by the 50% of the maximum dose as unity. Then, volumes covered by different percentages of the maximum dose were divided by the control volume. This method was used for the analysis of six diseases. The impact of plug patterns was analyzed for VS and TN. And the conformality index (CI) was calculated to evaluate the Gamma Knife plans for BM, MN, and VS.
     The mean prescribed doses for six diseases were 18.7, 18.6, 13.6, 15.0, 12.4 Gy to 50%, and 72 Gy to 80%, respectively.  Average volume ratios (AVRs) covered by SS for TN were 0.15, 0.34, 0.52, 0.73, 1.40 and 2.11 times the control volume while AVRs covered by MS in the first five diseases were 0.04, 0.15, 0.35, 0.64, 1.46, and 2.21, at 90%, 80%, 70%, 60%, 40%, and 30% of the maximum dose, respectively.  Plug patterns did not affect much on the relationship of dose and volume coverage in TN whereas a maximum of 14% difference was observed in VS. The CI values were 1.81, 1.66 and 1.43 for BM, MN, and VS, respectively.
     This is a retrospective study.
     Greater volumes were covered by the higher percentage isodose lines (= 60%) in SS than in MS.  
     Plug patterns did not affect much on the volume-dose ratio relationships; however, they produced the greater volume ratio covered by the low percentage isodose line (e.g. 30%). The CI orders from the most conformal to the least were VS>MN>BM in this study.


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