An optimized VMAT planning technique for hippocampal-sparing whole-brain radiotherapy

Whole brain radiation therapy (WBRT) has been shown to provide palliation but with negative neurocognitive effects associated with radiation-induced damage to the hippocampus. Sparing of the hippocampus has been shown to reduce the risk of neurocognitive deficit. The goal of this study is to develop a VMAT planning strategy for WBRT with hippocampal avoidance (HA) that will minimize hippocampal dose while achieving homogeneous coverage of whole brain PTV.

A retrospective study was performed on 10 patients previously treated for intracranial lesions. CT and MRI fused images were used to delineate the whole brain and hippocampus. Strictly following RTOG 0933 atlas guidelines, the hippocampus was manually delineated by a single radiation oncologist. A 4-arc noncoplanar VMAT approach was studied and compared with a 2-arc coplanar method for a prescription of 30 Gy in 10 fractions. We first generated an intermediate plan that optimized only the portion of the whole-brain PTV located farther from the hippocampus. Optimization was then continued by adding the remaining portions of the whole-brain PTV to the optimization structure. In the 4-arc VMAT optimization, a similar intermediate plan was utilized but in addition to this, the hippocampus was blocked for both entry and exit. Dosimetric parameters from both techniques were compared by paired t-test.

When compared to RTOG 0933 dosimetric criteria, both the 2-arc coplanar plans and 4-arc noncoplanar met them with a great margin. However, the 4-arc noncoplanar VMAT approach achieved dramatically better dose sparing for hippocampus as measured by D_0.03cc (10.55 Gy vs 12.66 Gy, p < 0.001), and D_mean (8.43 Gy vs 9.74 Gy, p < 0.001) when compared to the 2-arc coplanar plans. Dose homogeneity of whole brain PTV was also improved substantially in the 4-arc noncoplanar approach as measured by D_2% (32.49 Gy vs 32.90 Gy, p < 0.001), D_98% (28.55 Gy vs 27.97 Gy, p < 0.001), and the homogeneity index (HI) (0.126 vs 0.155, p < 0.001).

Using an intermediate plan in VMAT optimization, both the 2-arc coplanar plans and 4-arc noncoplanar plans meet RTOG 0933 criteria with a great margin, while the 4-arc noncoplanar VMAT planning method for HA-WBRT further improves the hippocampus sparing, as well as whole brain PTV coverage and dose homogeneity.