HyperArc radiotherapy for unresectable benign orbital tumours-cohort study and dosimetric comparison study.

Objective: To report the clinical application, dosimetric features, efficacy, and toxicity profile of HyperArc (HA) for benign orbital tumours not amenable to surgical resection.

Methods: A retrospective interventional cohort study. Gross target volume included the radiologically evident tumour and the optic nerve (excluded in case of haemangioma). Dosimetry was compared between HA and volumetric modulated arc therapy (VMAT) radiotherapy. Patients were treated with HA and followed-up clinically and radiologically for response and toxicity assessment.

Results: Eight patients were included in our study, six patients with an optic nerve sheath meningioma, one cavernous haemangioma and one orbital schwannoma. All patients demonstrated tumour regression, mean tumour volume prior to treatment of was 4916 mm³ and reduced to 3239 mm³ (P = .03). Three of eight patients showed improvement of visual acuity, three retained excellent pre-treatment vision and two patients had a reduction of vision. HA and VMAT planning target volume coverage dosimetry was similar (D95%: 98.7% and 98.6%, P > .05). The dosimetry of the contralateral lens (32.2 vs 69.8 Gy), lacrimal gland (1.7 vs 7.8 Gy), optic nerve (9.0 vs 26.6 Gy), nasal cavity (10.2 vs 20.6 Gy) and ipsilateral temporal lobe (4.9 vs 11.6 Gy) was significantly improved (P < .001) with HA.

Conclusion: This is the first reported clinical application of HA for benign orbital tumours. HA was an effective and well tolerated treatment modality. HA offered better dosimetry for some of the OARs compared to VMAT.

Advances in knowledge: This is the first article reporting the use of the HA system for planning and delivery of radiotherapy for orbital tumours.