PUB045 The Capability and Characteristic of Helical Tomotherapy at Sparing Hippocampus in Prophylactic Cranial Irradiation

Abstract

Prophylactic cranial irradiation (PCI) is efficient at avoiding lung cancer intra-cranial metastasis. However, PCI may cause neurocognitive impairment because of the hippocampal toxicities. It is reported that hippocampus V7.3Gy > 40% is associated with long-term impairment. Considering helical tomotherapy (HT) has higher modulation capabilities, we investigated the capability and characteristic of HT at sparing hippocampus in PCI. Six anonymous patients previously treated with whole-brain radiotherapy with hippocampal sparing were reviewed. Under T1-weighted MRI, the hippocampus was contoured, and hippocampal avoidance regions (HAR) were created using a 5-mm volumetric expansion around the hippocampus. The prescription dose 3.0Gy * 10 fractions was assigned on planning target volume (PTV) which was defined as whole-brain minus HAR and bilateral hippocampus. During the HT plan optimization process, evaluation factors of PTV and other dosimetry factors were recorded whenever the mean dose of hippocampus decreased by every 2Gy. The average of six HT plans without hippocampus sparing constraints are as follow: hippo_Dmean 27.33±2.22Gy, HAR_Dmean 28.62±1.84Gy, PTV_Dmean 30.77±0.74Gy, PTV_D2% 30.93±0.61Gy, PTV_D98% 29.86±0.34Gy, PTV_V30Gy 0.97±0.02% and conformal index (CI) 0.85±0.03, homogeneity index (HI) 0.04±0.01, plan delivery duration 13.70±1.92min. When V7.3Gy of bilateral hippocampus achieved 40%, the average dosimetry factors are as follow: hippo_Dmean 8.26±0.24Gy, HAR_Dmean 12.88±1.27Gy, PTV_Dmean 31.00±0.47Gy, PTV_D2% 31.97±0.63Gy, PTV_D98% 21.74±5.44Gy, PTV_V30Gy 0.93±0.02%, CI 0.77±0.08, HI 0.35±0.20, plan delivery duration 21.30±4.97min. During the process of hippocampus sparing, HAR_Dmean decreased linearly as well. However, dosimetry variation of PTV presented three steps: plateau region (> 20Gy), gradient region (∼20 to ∼10Gy) and falling region (<10Gy). There’s almost no significant impact on the PTV dosimetry factors when hippo_Dmean was not lower than 15Gy. With the hippocampus dose decreasing, CI decreased linearly and slightly, however, HI was impaired severely in the falling region. Plan delivery duration time was significantly prolonged process (from 13min to 23 min).View Large Image Figure ViewerDownload Hi-res image Download (PPT)