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Although whole brain radiotherapy (WBRT) provides palliation, reduces local recurrence probability and improves overall survival to patients with brain metastases (BMs), it is well documented that the radiation dose to hippocampus is associated with neurocognitive impairment. Recent published clinical study demonstrated that dose to 100% volume (D_100%) of the hippocampus exceed 9 Gy and maximum dose of the hippocampus exceed 16 Gy in standard WBRT treatment course of 30 Gy/10 fractions were associated with impair memory function. RTOG 0933 phase II trial suggests that minimizing the hippocampus radiation dose to the least possible level may preserve neurocognitive function while not significantly increasing the risk of disease recurrence. In WBRT using conventional double arcs volumetric modulated arc therapy (cVMAT), the large field size creates wide distribution of low dose volume to the hippocampus and other adjacent organs-at-risk (OARs). The present study investigated the potential of a novel technique - hybrid partial-field volumetric modulated arc therapy (HpVMAT) to spare the hippocampus and other OARs during WBRT, and compared with cVMAT.

Computed tomography and magnetic resonance images of 10 patients with brain metastases were retrieved in this study. The hippocampus was manually delineated by single radiation oncologist strictly following the RTOG 0933 atlas definition. Plans delivering 30 Gy in 10 fractions were generated for each patient using cVMAT and our novel HpVMAT technique. cVMAT plans comprise 2 coplanar arcs of 359.8° each and collimator rotation of 30° and 330°. Field size was opened up so that the whole brain PTV was completely covered. HpVMAT consists of a pair of static beams and four 180 ° arcs with field size reduction. Collimator angle of the static fields were selected so that both eye balls can be shielded by the X1 collimator jaw. MLC were used to minimize the irradiated OAR (hippocampus) volume. Plan metrics from both techniques were compared using paired sample t-test. The magnitude and statistical significance of the observed dosimetric differences were quantified. The optimization objectives were not modified or individualized between patients to avoid introducing bias.

Averaged results of planned target volume (PTV) and organs-at-risk (OARs) sparing were summarized in table 1. The results demonstrated that HpVMAT plans are capable to reduce radiation dose to hippocampus, eyes and lenses significantly relative to cVMAT plans, while PTV coverage was not compromised. The HpVMAT technique is also compared favorably to hippocampal sparing reported for RTOG 0933. Beam-on time of HpVMAT had no significant increment compared to cVMAT.

HpVMAT improved hippocampal sparing, while simultaneously spare other OARs such as the eyes and lenses. PTV coverage was not compromised with HVMAT. HpVMAT can be considered for a trial for patients with BMs undergoing WBRT with hippocampal sparing.