A pilot study on geometrical uncertainties for intra ocular cancers in radiotherapy

Abstract

A system for stabilising and monitoring eye movements for linac-based stereotactic radiotherapy associated with the mobile eye, the Eye Tracker, was developed. Whilst the Eye Tracker design is based on a previously reported system, the purpose of this study was to confirm that the modified version can be used with clinically acceptable treatment margins. We report the estimates of the margin required to account for inter- and intra-fraction eye motion based on data from 12 consecutive patients treated with the Eye Tracker system in place. Patients were immobilised in a head and neck mask and were required to fixate on a light source. A camera system monitored eye movements relative to CT simulation baseline measurements. The Exactrac system (Brainlab, Feldkirchen, Germany) combined with the Varian TrueBeamSTx (Varian Medical Systems, Palo Alto, CA) confirmed pre- and intra-treatment setup of the head position. Displacement/rotation of the image of the pupil/iris was determined in the lateral and superior-inferior directions using a video display. A standard margin equation was applied to estimate the margin required to account for inter- and intra-fraction eye movement. The average displacement in both directions was 0.1-0.2 mm (0.36 mm SD). All patients maintained a position within 1 mm of the intended position during treatment. Based on a Bayesian estimation of the systematic and treatment errors, accounting for displacements in two-planes and a standard deviation of the penumbral width of 1.3 mm, the estimated margins to achieve coverage of the GTV with the 95% isodose in 90% of patients was found to be less than 1 mm. Small random and systematic uncertainties due to inter- and intra-fraction movement of the eye were achieved with the Eye Tracker. Whilst the estimated margins are small (<1 mm) they need to be considered in addition to contouring and treatment delivery uncertainties.

An empirical approach to the definition of the target margins in eye radiosurgery

“…Similar systems were described in the following years by some groups of the University Hospital of Toronto, 13 Montréal, 14,15 Melbourne, 16 Osaka, 17 and San Francisco, 18 respectively: the proposed devices were slightly different among them and were applied to SRT both with LINAC and Cyberknife; the Authors claimed reproducibilities ranging from 0.3 to 2 mm.…”

Is It Possible to Preserve Vision without Compromising Metastases-Free Survival by Use of Fully Fractionated Stereotactic Radiotherapy for Posterior Choroidal Melanoma?

“…All patients received SRT to 60 Gy in 30 fractions over 6 weeks. A thermoplastic mask and light/camera for gaze fixation and tracking were utilised for treatment simulation and delivery [8]. A high-resolution T2 sequence planning MR orbit was obtained in a thermoplastic mask and the eye tracker-defined gaze position.…”