Optical eye tracking system for real-time noninvasive tumor localization in external beam radiotherapy

PTCOG Ocular Statement: Expert Summary of Current Practices and Future Developments in Ocular Proton Therapy

Although rare cancers, ocular tumors are a threat to vision, quality of life, and potentially life expectancy of a patient. Ocular proton therapy (OPT) is a powerful tool for successfully treating this disease. The Particle Therapy Co-Operative Ocular Group) formulated an Evidence and Expert-Based Executive Summary of Current Practices and Future Developments in OPT: comparative dosimetric and clinical analysis with the different OPT systems is essential to set up planning guidelines, implement best practices, and establish benchmarks for eye preservation, vision, and quality of life measures. Contemporary prospective trials in select subsets of patients (eg, tumors near the optic disc and/or macula) may allow for dosimetric and clinical analysis between different radiation modalities and beamline systems to evaluate differences in radiation delivery and penumbra, and resultant tumor control, normal tissue complication rates, and overall clinical cost-effectiveness. To date, the combination of multimodal imaging (fundus photography, ultrasound, etc), ophthalmologist assessment, and clip surgery with radiation planning have been keys to successful treatment. Increased use of three-dimensional imaging (computed tomography/magnetic resonance imaging) is anticipated although its spatial resolution might be a limiting factor (eg, detection of flat diffuse tumor parts). Commercially produced ocular treatment-planning systems are under development and their future use is expected to expand across OPT centers. Future continuity of OPT will depend on the following: (1) maintaining and upgrading existing older dedicated low-energy facilities, (2) maintaining shared, degraded beamlines at large proton therapy centers, and (3) developing adapted gantry beams of sufficient quality to maintain the clinical benefits of sharp beam conformity. Option (1) potentially offers the sharpest beams, minimizing impact on healthy tissues, whereas (2) and (3) potentially offer the advantage of substantial long-term technical support and development as well as the introduction of new approaches. Significant patient throughputs and close cooperation between medical physics, ophthalmology, and radiation therapy, underpinned by mutual understanding, is crucial for a successful OPT service.

Robot-assisted system for non-invasive wide-range flexible eye positioning and tracking in particle radiotherapy

Particle (proton, carbon ion, or others) radiotherapy for ocular tumors is highly dependent on precise dose distribution, and any misalignment can result in severe complications. The proposed eye positioning and tracking system (EPTS) was designed to non-invasively position eyeballs and is reproducible enough to ensure accurate dose distribution by guiding gaze direction and tracking eye motion. Eye positioning was performed by guiding the gaze direction with separately controlled light sources. Eye tracking was performed by a robotic arm with cameras and a mirror. The cameras attached to its end received images through mirror reflection. To maintain a light weight, certain materials, such as carbon fiber, were utilized where possible. The robotic arm was controlled by a robot operating system. The robotic arm, turntables, and light source were actively and remotely controlled in real time. The videos captured by the cameras could be annotated, saved, and loaded into software. The available range of gaze guidance is 360° (azimuth). Weighing a total of 18.55 kg, the EPTS could be installed or uninstalled in 10 s. The structure, motion, and electromagnetic compatibility were verified via experiments. The EPTS shows some potential due to its non-invasive wide-range flexible eye positioning and tracking, light weight, non-collision with other equipment, and compatibility with CT imaging and dose delivery. The EPTS can also be remotely controlled in real time and offers sufficient reproducibility. This system is expected to have a positive impact on ocular particle radiotherapy.

Multi-camera optical tracking and fringe pattern analysis for eye surface profilometry in ocular proton therapy

Background and purpose

An optical tracking system for high-precision measurement of eye position and orientation during proton irradiation of intraocular tumors was designed. The system performed three-dimensional (3D) topography of the anterior eye segment using fringe pattern analysis based on Fourier Transform Method (FTM).

Materials and methods

The system consisted of four optical cameras and two projectors. The design and modifications to the FTM pipeline were optimized for the realization of a reliable measurement system. Of note, phase-to-physical coordinate mapping was achieved through the combination of stereo triangulation and fringe pattern analysis. A comprehensive pre-clinical validation was carried out. Then, the system was set to acquire the eye surface of patients undergoing proton therapy. Topographies of the eye were compared to manual contouring on MRI.

Results

Pre-clinical results demonstrated that 3D topography could achieve sub-millimetric accuracy (median:0.58 mm) and precision (RMSE:0.61 mm) in the clinical setup. The absolute median discrepancy between MRI and FTM-based anterior eye segment surface reconstruction was 0.43 mm (IQR:0.65 mm).

Conclusions

The system complied with the requirement of precision and accuracy for image guidance in ocular proton therapy radiation and is expected to be clinically tested soon to evaluate its performance against the current standard.

The Impact of Optic Nerve Movement on Intracranial Radiation Treatment

Purpose

In radiotherapy, high radiation exposure to optic nerve (ON) can cause optic neuropathy or vision loss. In this study, we evaluated the pattern and extent of the ON movement using MRI, and investigated the potential dosimetric effect of this movement on radiotherapy.

Methods

MRI was performed in multiple planes in 5 human subjects without optic pathway abnormalities to determine optic nerve motion in different scenarios. The subjects were requested to gaze toward five directions during MRI acquisitions, including neutral (straight forward), left/right (horizontal movement), and up/down (vertical movement). Subsequently, the measured displacement was applied to patients with peri-optic tumors to evaluate the potential dosimetric effect of this motion.

Results

The motion of ON followed a nearly conical shape. By average, the anterior end of ONs moved with 10.8 ± 2.2 mm horizontally and 9.3 ± 0.8 mm vertically, while posterior end has negligible displacement. For patients who underwent stereotactic radiotherapy to a peri-optic tumors, the movement of ON in this measured range introduced non-negligible dosimetric effect.

Conclusion

The range of motion of the anterior portions of the optic nerves is on the order of centimeters, which may need to be considered with extra attention during radiation therapy in treating peri-optic lesions.

Hadron Therapy Achievements and Challenges: The CNAO Experience

Protons and carbon ions (hadrons) have useful properties for the treatments of patients affected by oncological pathologies. They are more precise than conventional X-rays and possess radiobiological characteristics suited for treating radio-resistant or inoperable tumours. This paper gives an overview of the status of hadron therapy around the world. It focusses on the Italian National Centre for Oncological Hadron therapy (CNAO), introducing operation procedures, system performance, expansion projects, methodologies and modelling to build individualized treatments. There is growing evidence that supports safety and effectiveness of hadron therapy for a variety of clinical situations. However, there is still a lack of high-level evidence directly comparing hadron therapy with modern conventional radiotherapy techniques. The results give an overview of pre-clinical and clinical research studies and of the treatments of 3700 patients performed at CNAO. The success and development of hadron therapy is strongly associated with the creation of networks among hadron therapy facilities, clinics, universities and research institutions. These networks guarantee the growth of cultural knowledge on hadron therapy, favour the efficient recruitment of patients and present available competences for R&D (Research and Development) programmes.

Development and validation of a new set-up simulator dedicated to ocular proton therapy at CNAO

An Eye Tracking System (ETS) is used at CNAO for providing a stable and reproducible ocular proton therapy (OPT) set-up, featuring a fixation light (FL) and monitoring stereo-cameras embedded in a rigid case. The aim of this work is to propose an ETS set-up simulation algorithm, that automatically provides the FL positioning in space, according to patient-specific gaze direction and avoiding interferences with patient, beam and collimator. Two configurations are provided: one in the CT room for acquiring images required for treatment planning with the patient lying on a couch, and one related to the treatment room with the patient sitting in front of the beam. Algorithm validation was performed reproducing ETS simulation (CT) and treatment (room) set-up for 30 patients previously treated at CNAO. The positioning accuracy of the device was quantified through a set of 14 control points applied to the ETS case and localizable both in the CT volume and in room X-ray images. Differences between the position of ETS reference points estimated by the algorithm and those measured by imaging systems are reported. The corresponding gaze direction deviation is on average 0.2° polar and 0.3° azimuth for positioning in CT room and 0.1° polar and 0.4° azimuth in the treatment room. The simulation algorithm was embedded in a clinically usable software application, which we assessed as capable of ensuring ETS positioning with an average accuracy of 2 mm in CT room and 1.5 mm in treatment room, corresponding to gaze direction deviations consistently lower than 1°.

Three-dimensional MRI-based treatment planning approach for non-invasive ocular proton therapy

PURPOSE

To develop a high-resolution three-dimensional (3D) magnetic resonance imaging (MRI)-based treatment planning approach for uveal melanomas (UM) in proton therapy.

MATERIALS/METHODS

For eight patients with UM, a segmentation of the gross tumor volume (GTV) and organs-at-risk (OARs) was performed on T1- and T2-weighted 7 Tesla MRI image data to reconstruct the patient MR-eye. An extended contour was defined with a 2.5-mm isotropic margin derived from the GTV. A broad beam algorithm, which we have called πDose, was implemented to calculate relative proton absorbed doses to the ipsilateral OARs. Clinically favorable gazing angles of the treated eye were assessed by calculating a global weighted-sum objective function, which set penalties for OARs and extreme gazing angles. An optimizer, which we have named OPT'im-Eye-Tool, was developed to tune the parameters of the functions for sparing critical-OARs.

RESULTS

In total, 441 gazing angles were simulated for every patient. Target coverage including margins was achieved in all the cases (V95%  > 95%). Over the whole gazing angles solutions space, maximum dose (Dmax ) to the optic nerve and the macula, and mean doses (Dmean ) to the lens, the ciliary body and the sclera were calculated. A forward optimization was applied by OPT'im-Eye-Tool in three different prioritizations: iso-weighted, optic nerve prioritized, and macula prioritized. In each, the function values were depicted in a selection tool to select the optimal gazing angle(s). For example, patient 4 had a T2 equatorial tumor. The optimization applied for the straight gazing angle resulted in objective function values of 0.46 (iso-weighted situation), 0.90 (optic nerve prioritization) and 0.08 (macula prioritization) demonstrating the impact of that angle in different clinical approaches.

CONCLUSIONS

The feasibility and suitability of a 3D MRI-based treatment planning approach have been successfully tested on a cohort of eight patients diagnosed with UM. Moreover, a gaze-angle trade-off dose optimization with respect to OARs sparing has been developed. Further validation of the whole treatment process is the next step in the goal to achieve both a non-invasive and a personalized proton therapy treatment.

Precise delineation and tumor localization based on novel image registration strategy between optical coherence tomography and computed tomography in the radiotherapy of intraocular cancer

Radiation-associated toxicities due to sophisticated ocular anatomy and shape variability of organs at risk (OARs) are major concerns during external beam radiation therapy (EBRT) of patients with intraocular cancer. A novel two-step image registration strategy between optical coherence tomography (OCT) and computed tomography (CT) images was proposed and validated to precisely localize the target in the EBRT of patients with intraocular cancer. Specifically, multiple features from OCT and CT images were extracted automatically, then spatial transformation based on thin-plate spline function was performed iteratively to achieve feature alignment between the CT and OCT images. Finally, an exclusive OR (XOR) algorithm was applied for precise 3D registration using a 3D-mesh model generated from OCT and CT volumes. The accuracy of the proposed novel registration strategy was validated and tested in a schematic-eye phantom with an artificially introduced tumor and in ten patients with confirmed primary and/or secondary intraocular cancer. There was an average registration error and computational time of 0.21  ±  0.05° and 259  ±  5 s, together with an average Dice similarity coefficient and Hausdorff distance of 88.4  ±  0.65 and 0.89  ±  0.09, respectively. The preliminary experimental results demonstrated that the proposed novel strategy to overcome current limitations on eye modeling and to localize precisely the tumor target during EBRT of intraocular cancer is promising.

A platform for patient positioning and motion monitoring in ocular proton therapy with a non-dedicated beamline

PURPOSE

At Centro Nazionale di Adroterapia Oncologica (CNAO, Pavia, Italy) ocular proton therapy (OPT) is delivered using a non-dedicated beamline. This paper describes the novel clinical workflow as well as technologies and methods adopted to achieve accurate target positioning and verification during ocular proton therapy at CNAO.

METHOD

The OPT clinical protocol at CNAO prescribes a treatment simulation and a delivery phase, performed in the CT and treatment rooms, respectively. The patient gaze direction is controlled and monitored during the entire workflow by means of an eye tracking system (ETS) featuring two optical cameras and an embedded fixation diode light. Thus, the accurate alignment of the fixation light provided to the patient to the prescribed gazed direction is required for an effective treatment. As such, a technological platform based on active robotic manipulators and IR optical tracking-based guidance was developed and tested. The effectiveness of patient positioning strategies was evaluated on a clinical dataset comprising twenty patients treated at CNAO.

RESULTS

According to experimental testing, the developed technologies guarantee uncertainties lower than one degree in gaze direction definition by means of ETS-guided positioning. Patient positioning and monitoring strategies during treatment effectively mitigated set-up uncertainties and exhibited sub-millimetric accuracy in radiopaque markers alignment.

CONCLUSION

Ocular proton therapy is currently delivered at CNAO with a non-dedicated beamline. The technologies developed for patient positioning and motion monitoring have proven to be compliant with the high geometrical accuracy required for the treatment of intraocular tumors.

Design and commissioning of the non-dedicated scanning proton beamline for ocular treatment at the synchrotron-based CNAO facility

PURPOSE

Only few centers worldwide treat intraocular tumors with proton therapy, all of them with a dedicated beamline, except in one case in the USA. The Italian National Center for Oncological Hadrontherapy (CNAO) is a synchrotron-based hadrontherapy facility equipped with fixed beamlines and pencil beam scanning modality. Recently, a general-purpose horizontal proton beamline was adapted to treat also ocular diseases. In this work, the conceptual design and main dosimetric properties of this new proton eyeline are presented.

METHODS

A 28 mm thick water-equivalent range shifter (RS) was placed along the proton beamline to shift the minimum beam penetration at shallower depths. FLUKA Monte Carlo (MC) simulations were performed to optimize the position of the RS and patient-specific collimator, in order to achieve sharp lateral dose gradients. Lateral dose profiles were then measured with radiochromic EBT3 films to evaluate the dose uniformity and lateral penumbra width at several depths. Different beam scanning patterns were tested. Discrete energy levels with 1 mm water-equivalent step within the whole ocular energy range (62.7-89.8 MeV) were used, while fine adjustment of beam range was achieved using thin polymethylmethacrylate additional sheets. Depth-dose distributions (DDDs) were measured with the Peakfinder system. Monoenergetic beam weights to achieve flat spread-out Bragg Peaks (SOBPs) were numerically determined. Absorbed dose to water under reference conditions was measured with an Advanced Markus chamber, following International Atomic Energy Agency (IAEA) Technical Report Series (TRS)-398 Code of Practice. Neutron dose at the contralateral eye was evaluated with passive bubble dosimeters.

RESULTS

Monte Carlo simulations and experimental results confirmed that maximizing the air gap between RS and aperture reduces the lateral dose penumbra width of the collimated beam and increases the field transversal dose homogeneity. Therefore, RS and brass collimator were placed at about 98 cm (upstream of the beam monitors) and 7 cm from the isocenter, respectively. The lateral 80%-20% penumbra at middle-SOBP ranged between 1.4 and 1.7 mm depending on field size, while 90%-10% distal fall-off of the DDDs ranged between 1.0 and 1.5 mm, as a function of range. Such values are comparable to those reported for most existing eye-dedicated facilities. Measured SOBP doses were in very good agreement with MC simulations. Mean neutron dose at the contralateral eye was 68 μSv/Gy. Beam delivery time, for 60 Gy relative biological effectiveness (RBE) prescription dose in four fractions, was around 3 min per session.

CONCLUSIONS

Our adapted scanning proton beamline satisfied the requirements for intraocular tumor treatment. The first ocular treatment was delivered in August 2016 and more than 100 patients successfully completed their treatment in these 2 yr.

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

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.

Feasibility study of a non-invasive eye fixation and monitoring device using a right-angle prism mirror for intensity-modulated radiotherapy for choroidal melanoma

We aimed to describe the feasibility and efficacy of a novel non-invasive fixation and monitoring (F-M) device for the eyeballs (which uses a right-angle prism mirror as the optic axis guide) in three consecutive patients with choroidal melanoma who were treated with intensity-modulated radiotherapy (IMRT). The device consists of an immobilization shell, a right-angle prism mirror, a high magnification optical zoom video camera, a guide lamp, a digital voice recorder, a personal computer, and a National Television System Committee standard analog video cable. Using the right-angle prism mirror, the antero-posterior axis was determined coincident with the optic axis connecting the centers of the cornea and pupil. The axis was then connected to the guide light and video camera installed on the couch top on the distal side. Repositioning accuracy improved using this method. Furthermore, the positional error of the lens was markedly reduced from ±1.16, ±1.68 and ±1.11 mm to ±0.23, ±0.58 and ±0.26 mm in the horizontal direction, and from ±1.50, ±1.03 and ±0.48 mm to ±0.29, ±0.30 and ±0.24 mm in the vertical direction (Patient #1, #2 and #3, respectively). Accordingly, the F-M device method decreased the planning target volume size and improved the dose-volume histogram parameters of the organ-at-risk via IMRT inverse planning. Importantly, the treatment method was well tolerated.

Reproducibility of a Noninvasive System for Eye Positioning and Monitoring in Stereotactic Radiotherapy of Ocular Melanoma

Purpose:

Our preferred treatment for juxtapapillary choroidal melanoma is stereotactic radiotherapy. We aim to describe our immobilization system and quantify its reproducibility.

Materials and Methods:

Patients were identified in our radiosurgery database. Patients were imaged at computed tomography simulator with an in-house system which allows visual monitoring of the eye as the patient fixates a small target. All patients were reimaged at least once prior to and/or during radiotherapy. The patients were treated on the CyberKnife system, 60 Gy in 10 daily fractions, using skull tracking in conjunction with our visual monitoring system. In order to quantify the reproducibility of the eye immobilization system, computed tomography scans were coregistered using rigid 6-dimensional skull registration. Using the coregistered scans, x, y, and z displacements of the lens/optic nerve insertion were measured. From these displacements, 3-dimensional vectors were calculated.

Results:

Thirty-four patients were treated from October 2010 to September 2015. Thirty-nine coregistrations were performed using 73 scans (2-3 scans per patient). The mean displacements of lens and optic nerve insertion were 0.1 and 0.0 mm. The median 3-dimensional displacements (absolute value) of lens and nerve insertion were 0.8 and 0.7 mm (standard deviation: 0.5 and 0.6 mm). Ninety-eight percent of 3-dimensional displacements were below 2 mm (maximum 2.4 mm). The calculated planning target volume (PTV) margins were 0.8, 1.4, and 1.5 mm in the anterior–posterior, craniocaudal, and right–left axes, respectively. Following this analysis, no further changes have been applied to our planning margin of 2 to 2.5 mm as it is also meant to account for uncertainties in magnetic resonance imaging to computed tomography registration, skull tracking, and also contouring variability.

Conclusion:

We have found our stereotactic eye immobilization system to be highly reproducible (<1 mm) and free of systematic error.

Phase II multi-institutional clinical trial on a new mixed beam RT scheme of IMRT on pelvis combined with a carbon ion boost for high-risk prostate cancer patients

PURPOSE

Definition of the optimal treatment schedule for high-risk prostate cancer is under debate. A combination of photon intensity modulated radiotherapy (IMRT) on pelvis with a carbon ion boost might be the optimal treatment scheme to escalate the dose on prostate and deliver curative dose with respect to normal tissue and quality of dose distributions. In fact, carbon ion beams offer the advantage to deliver hypofractionated radiotherapy (RT) using a significantly smaller number of fractions compared to conventional RT without increasing risks of late effects.

METHODS

This study is a prospective phase II clinical trial exploring safety and feasibility of a mixed beam scheme of carbon ion prostate boost followed by photon IMRT on pelvis. The study is designed to enroll 65 patients with localized high-risk prostate cancer at 3 different oncologic hospitals: Istituto Europeo di Oncologia, Fondazione IRCCS Istituto Nazionale dei Tumori, and Centro Nazionale di Adroterapia Oncologica. The primary endpoint is the evaluation of safety and feasibility with acute toxicity scored up to 1 month after the end of RT. Secondary endpoints are treatment early (3 months after the end of RT) and long-term tolerability, quality of life, and efficacy.

RESULTS

The study is not yet recruiting; in silico studies are ongoing and we expect to start recruitment by 2017.

CONCLUSIONS

The present clinical trial aims at improving the current treatment for high-risk prostate cancer, evaluating safety and feasibility of a new RT mixed-beam scheme including photons and carbon ions. Encouraging results are coming from carbon ion facilities worldwide on the treatment of different tumors including prostate cancers. Carbon ions combine physical properties allowing for high dose conformity and advantageous radiobiological characteristics. The proposed mixed beam treatment has the advantage to combine a photon high conformity standard of care IMRT phase with a hypofractionated carbon ion RT boost delivered in a short overall treatment time.