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. 2020 Mar;21(3):45-51.
doi: 10.1002/acm2.12818. Epub 2020 Feb 11.

Strategies to optimize stereotactic radiosurgery plans for brain tumors with volumetric-modulated arc therapy

David Wang  1 , Albert DeNittis  1   2 , Yibing Hu  1
Affiliations

Strategies to optimize stereotactic radiosurgery plans for brain tumors with volumetric-modulated arc therapy

David Wang et al. J Appl Clin Med Phys. 2020 Mar.

Abstract

Purpose: Prescription practice in SRS plans for brain tumors differs significantly for different modalities. In this retrospective study, the strategies to optimize SRS plans for brain tumors with volumetric arc therapy (VMAT) were presented.

Methods: Fifty clinically treated cases were stratified by the maximum target size into two groups (≥ 2 cm in 25 cases and <2 cm but ≥1 cm in 25 cases), which were optimized using traditional LINAC MLC-based approaches with the average prescription isodose line (P-IDL) of (91.4 ± 0.6)%. Four to five plans have been created for each case with variation of the P-IDL from 65% to 90%. The optimization strategies to select an optimal P-IDL, to use tuning structures within the target and beyond as well as to use NTO (normal tissue objectives), were applied to all new plans.

Results: The optimal P-IDL was found to be around 75%. After applying the new optimization strategies with an average P-IDL of 74.8%, the mean modified gradient index (mGI) and V12 were reduced by 25% and 35%, respectively for both groups. The Paddick conformity index (PCI) was averagely improved by 8%. The average homogeneity index (HI) and focal index (FI) were increased by 22% and 50%, respectively. The mGI was inversely proportional to the PTV volumes. The shape of the dose distribution in target was also changed from concave to convex. The comparison of PCI with historical data from other institutes and modalities shows that the plans in this study have the best conformity near the target.

Conclusions: With the new optimization strategies for VMAT SRS plan of brain tumor more conformal plans in both high and intermediate dose region (~50% of the PD) were created, in which the dose in the core of the target was notably increased while V12 and mGI were significantly decreased, and PCI was improved. The mGI was inversely proportional to the PTV volumes. The optimal P-IDL is around 75%. The average PCI is the best in this study compared with the published historical data. These strategies are applicable to treatment planning for multiple brain and liver tumors where sparing the tissue peripheral to the target is critical.

Keywords: FI; HI; PCI; SRS; V12; VMAT; brain metastasis; mGI; plan optimization; prescription isodose line.

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Conflict of interest statement

There are no conflict of interests.

Figures

Figure 1
Figure 1
Illustration of the tuning ring structures. The partial body in the figure at the left up corner shows the contour of the partial body and aims at speed up optimization process due to reduced volume. The structures, PartialBody‐(PTV + 2 cm) in green, R1 in blue, R2 in pink and CTVCore in red, are shown at the top right of the Figure. The structures, PartialBody‐(PTV + 8 mm) and PartialBody‐(PTV + 5 mm), are shown at the bottom left and right of the figure, respectively. One can see that the three structures, PartialBody‐(PTV + 2 cm), PartialBody‐(PTV + 8 mm), and PartialBody‐(PTV + 5 mm), are partially overlapped.
Figure 2
Figure 2
Variation of mGI and the relative change of V12 with the P‐IDL.
Figure 3
Figure 3
Variation of PCI with the P‐IDL. PCI, Paddick conformity index.
Figure 4
Figure 4
Variation of the relative change of FI and MU with P‐IDL.
Figure 5
Figure 5
Comparison of typical dose distributions in the target for the treated plan (Old, solid line) and the retrospective study plan (New, dotted line).
Figure 6
Figure 6
mGI was categorized into six groups according the PTV volumes for the retrospective study plans.
Figure 7
Figure 7
Comparison of PCI of this study to historical data. PCI, Paddick conformity index.
See this image and copyright information in PMC

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