Influence of eye movement on lens dose and optic nerve target coverage during craniospinal irradiation

Bianca A.W Hoeben,Enrica Seravalli,Amber M.L Wood,Mirjam Bosman,Witold P Matysiak,John H Maduro,Astrid L.H.M.W Van Lier,Matteo Maspero,Gijsbert H Bol,Geert O Janssens

doi:10.1016/j.ctro.2021.08.009

Abstract

PurposeOptic nerves are part of the craniospinal irradiation (CSI) target volume. Modern radiotherapy techniques achieve highly conformal target doses while avoiding organs-at-risk such as the lens. The magnitude of eye movement and its influence on CSI target- and avoidance volumes are unclear. We aimed to evaluate the movement-range of lenses and optic nerves and its influence on dose distribution of several planning techniques. MethodsTen volunteers underwent MRI scans in various gaze directions (neutral, left, right, cranial, caudal). Lenses, orbital optic nerves, optic discs and CSI target volumes were delineated. 36-Gy cranial irradiation plans were constructed on synthetic CT images in neutral gaze, with Volumetric Modulated Arc Therapy, pencil-beam scanning proton therapy, and 3D-conventional photons. Movement-amplitudes of lenses and optic discs were analyzed, and influence of gaze direction on lens and orbital optic nerve dose distribution. ResultsMean eye structures’ shift from neutral position was greatest in caudal gaze; −5.8±1.2 mm (±SD) for lenses and 7.0±2.0 mm for optic discs. In 3D-conventional plans, caudal gaze decreased Mean Lens Dose (MLD). In VMAT and proton plans, eye movements mainly increased MLD and diminished D98 orbital optic nerve (D98OON) coverage; mean MLD increased up to 5.5 Gy [total ΔMLD range −8.1 to 10.0 Gy], and mean D98OON decreased up to 3.3 Gy [total ΔD98OON range −13.6 to 1.2 Gy]. VMAT plans optimized for optic disc Internal Target Volume and lens Planning organ-at-Risk Volume resulted in higher MLD over gaze directions. D98OON became ≥95% of prescribed dose over 95/100 evaluated gaze directions, while all-gaze bilateral D98OON significantly changed in 1 of 10 volunteers. ConclusionWith modern CSI techniques, eye movements result in higher lens doses and a mean detriment for orbital optic nerve dose coverage of <10% of prescribed dose.

Highlights

Patients with medulloblastomas or other pediatric tumors with lep tomeningeal spread, such as germ-cell tumors, atypical teratoid rhab doid tumors, and ependymomas, have an indication for craniospinal irradiation (CSI) [1]
We evaluated if adding a planning organ-at-risk volume around lenses (PRVlens) and ITVoptic disc during VMAT planning would significantly change Mean Lens Dose (MLD) and D98 orbital optic nerve (D98OON) compared with the original VMAT plans
In this study we evaluated MRI-documented eye structure move ments, and influence of eye movements on lens dose and optic nerve target coverage in CSI plans

Summary

Introduction

Patients with medulloblastomas or other pediatric tumors with lep tomeningeal spread, such as germ-cell tumors, atypical teratoid rhab doid tumors, and ependymomas, have an indication for craniospinal irradiation (CSI) [1]. Leptomeningeal failures of primary brain tu mors in the optic nerves are rarely observed [3,4,5,6]. This can be explained by the fact that optic nerves, even when not targeted during CSI, usually received a therapeutic dose with the lateral opposed beams of the classic 3D-conventional photon technique. Rene et al demonstrated that with modern intensity modulated radiation therapy (IMRT) planning techniques, there is a risk of underdosage of especially the anterior half of the optic nerves if not intentionally targeted, and that there is no margin for setup error regarding optic nerve dose coverage [7]

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