Feasibility of hippocampal sparing radiation therapy for glioblastoma using helical Tomotherapy Dr Kamalram THIPPU JAYAPRAKASH1,2,3, Dr Raj JENA1,4 and Mrs Karen WILDSCHUT5 1Department of Oncology, Cambridge University Hospitals, Cambridge, United Kingdom 2Department of Oncology, St Luke’s Cancer Centre, Guildford, United Kingdom, 3Department of Clinical and Experimental Medicine, University of Surrey, Guildford, United Kingdom 4Department of Oncology, University of Cambridge, Cambridge, United Kingdom, 5Department of Radiation Physics, Cambridge University Hospitals, Cambridge, United Kingdom Purpose/Objective With improvements in survival for good performance status patients with glioblastoma, some patients will survive to develop significant neurocognitive dysfunction. This retrospective planning study quantifies hippocampal radiation doses in twenty-five patients with glioblastoma receiving radical chemo-radiation therapy, and evaluates the potential for dose reduction using helical Tomotherapy.   Material and Methods We identified twenty-five glioblastoma patients treated with Helical IMRT (Tomotherapy HI-ART, Accuray, Sunnyvale CA, USA) with concurrent and adjuvant temozolomide between October 2011 and December 2013 from our radiotherapy electronic database and conducted a retrospective analysis. Hippocampi (HC) were contoured in CT and MRI co-registered image data sets used for clinical radiotherapy planning and hippocampus planning risk volumes (HC-PRV) were created by adding five-millimetre isotropic margin which were checked by a neuro radiologist* (fig 1). Clinical treatment dosimetry plans were overlaid to obtain dose statistics (fig 2). Four selected patients were planned for hippocampus avoidance radiotherapy (HA) without compromising tumour planning target volume (PTV) coverage using currently established hippocampus dose volume histogram (DVH) based constraints$ (figs 3 & 4). Fig 1 – HC contouring Fig 2 – DVH dose statistics Fig 3 – Treatment plan Fig 4 – HA plan Results Mean HC-PRV maximum, minimum and mean radiation doses were 54.7, 24.15 and 38.62 Gy respectively (table 1). HC-PRV V7.3, V14.9 and V20 (volumes receiving 7.3, 19.9 and 20 Gy respectively) were 99.95%, 98.41% and 95.72% and hippocampus V3 was 100% (table 2). In seventeen patients ipsilateral hippocampi were within PTVs and in seven patients both hippocampi were outside PTVs with only minimal overlapping volumes but DVH based dose constraints were not achieved (tables 3 & 4). Max dose Min dose Mean dose HC-PRV Max dose 66.81 37.80 54.70 HC-PRV Min dose 43.07 6.21 24.15 HC-PRV Ave dose 50.85 20.58 38.62 HC Max dose 60.32 34.16 51.71 HC Min dose 46.13 8.12 28.18 HC Ave dose 49.98 20.98 38.83 Max volume Min volume Mean volume HC-PRV, V 7.3Gy 100 99.15 99.95 HC-PRV, V 14.9Gy 73.59 98.41 HC-PRV, V 20Gy 47.50 95.72 HC, V 3Gy Max volume Min volume Mean volume HC-PRV in PTV (cc) 10.16 1.55 HC-PRV, V 7.3Gy (%) 100 HC-PRV, V 14.9Gy (%) HC-PRV, V 20Gy (%) 95.55 99.74 HC, V 3Gy (%) Max volume Min volume Mean volume HC-PRV in PTV (cc) 20.19 9.34 Rt HC-PRV in PTV (cc) 19.14 5.36 Lt HC-PRV in PTV (cc) 10.11 3.93 HC-PRV, V 7.3Gy (%) 100 99.15 99.81 HC-PRV, V 14.9Gy (%) 73.59 94.55 HC-PRV, V 20Gy (%) 47.50 85.98 HC, V 3Gy (%) HC & HC-PRV doses DVH constraints analysis Volumetric analysis Volumetric analysis With hippocampus avoidance radiotherapy planning, in four patients HC-PRV minimum doses and in three patients mean HC-PRV doses were reduced and significant reductions in DVH based dose constraints were achieved in three patients when compared to clinical treatment plans (table 5). Location Right frontal Right temporal Left parietal Posterior fossa Group Treatment HA HC-PRV max dose 53.17 51.88 51.28 45.33 56.88 55.91 52.63 61.02 HC-PRV min dose 14.96 2.27 12.4 2.85 31.63 4.11 25.34 6.28 HC-PRV mean dose 33.39 9.09 28 6.71 43.09 15.55 42.79 37.79 V 7.3Gy 100 58.7 29.17 67.43 99.29 V 14.9Gy 25.51 98.24 16 38.91 91.4 V 20Gy 16.22 82 12.12 28.68 80 Conclusion Our analysis showed hippocampus PRVs received significant radiation doses and currently established hippocampus DVH parameters were not achieved during cranial radiotherapy for glioblastoma using Helical IMRT without hippocampus avoidance planning. Our planning study demonstrated significant dose reductions were possible with hippocampus avoidance radiotherapy planning in selected patients. More clinically correlated DVH objectives for hippocampus are required for better optimisation for hippocampus avoidance cranial radiotherapy in glioblastoma for this to be considered for all patients. *Hippocampal-Sparing Whole Brain Radiotherapy: A “How-To” Technique, Utilizing Helical Tomotherapy and LINAC-based Intensity Modulated Radiotherapy; Int J Radiation Oncol Biol Phys, Vol. 78, Issue 4, 1244–1252, 2010 $Hippocampal Dosimetry Predicts Neurocognitive Function Impairment After Fractionated Stereotactic Radiotherapy for Benign or Low-Grade Adult Brain Tumors; Int J Radiation Oncol Biol Phys, Vol. 85, No. 2, 348-354, 2013 $Preservation of Memory With Conformal Avoidance of the Hippocampal Neural Stem-Cell Compartment During Whole-Brain Radiotherapy for Brain Metastases (RTOG 0933); A Phase II Multi-Institutional Trial; JCO, Vol. 32, No.34 3810-16, 2014 Contact: k.thippujayaprakash@nhs.net Key words; glioblastoma, hippocampus avoidance radiotherapy, neurocognitive function