Assessment of patterns of infiltration and relapse of patients with glioblastoma of the occipital lobe

Purpose: Target volume delineation protocols for glioblastoma utilise uniform or isotropic expansion around the surgical cavity and residual tumour, without considering specific sites at risk for infiltration. Tumours arising in different neuroanatomical sites may have specific pathways for infiltration. This study aims to review the infiltration and relapse sites for the occipital lobe glioblastoma to determine sites at risk, and potentially improve future target volume delineation protocols. Method: Patients with occipital lobe glioblastoma and managed with EORTC-NCIC Protocol were identified through a prospective database. Based on MRI analysis a qualitative description of sites of tumour infiltration and subsequent relapse was performed. These were categorized into neuroanatomical subsites adjacent to the occipital lobe: Level 1 related to origin gyrus; Level 2 adjacent gyral subsites; Level 3 subsites involved distant regions. These spatial patterns were assessed in relation to three major white matter tracts: inferior longitudinal fasciculus, cingulum, and corpus callosum. Results: 46 patients were analysed. At diagnosis, 20 patients (43.5%) had medial occipital lobe involvement and 26 (56.5%) had lateral involvement. Level 2 and level 3 infiltration were observed in 33 (71.7%) and 27 (58.7%) of patients. Relapse occurred in 43 patients (93.5%), 28% at level 1, 77% at level 2, and 98% at level 3. Level 3 relapse sites included the trigone (70%), splenium (30%), and anterior temporal lobe (35%). Lateral tumours relapsed more commonly in the trigone (75% vs 52.6%) and anterior temporal lobe (50% vs 15.8%, p =0.026), while medial tumours more frequently involved the splenium (47.3% vs 16.7%, p =0.046). Conclusions: Infiltration and relapse of glioblastoma involving the occipital lobe suggests distinct neuroanatomical patterns which may guide radiation therapy target volume delineation.