TU‐E‐201‐03: Eye Lens Dosimetry in Radiotherapy Using Contact Lens‐Shaped Applicator
Authors: Park J. 
Published: June 2015

Madan M. Rehani, Massachusetts General Hospital and Harvard Medical School, Boston

Methods for Eye Lens Dosimetry and Studies On Lens Opacities with Interventionalists

Radiation induced cataract is a major threat among staff working in interventional suites. Nearly 16 million interventional procedures are performed annually in USA. Recent studies by the principal investigator's group, primarily among interventional cardiologists, on behalf of the International Atomic Energy Agency, show posterior subcapsular (PSC) changes in the eye lens in 38–53% of main operators and 21–45% of support staff. These changes have potential to lead to cataract in future years, as per information from A‐Bomb survivors. The International Commission on Radiological Protection has reduced dose limit for staff by a factor of 7.5 (from 150 mSv/y to 20 mSv/y). With increasing emphasis on radiation induced cataracts and reduction in threshold dose for eye lens, there is a need to implement strategies for estimating eye lens dose. Unfortunately eye lens dosimetry is at infancy when it comes to routine application. Various approaches are being tried namely direct measurement using active or passive dosimeters kept close to eyes, retrospective estimations and lastly correlating patient dose in interventional procedures with staff eye dose. The talk will review all approaches available and ongoing active research in this area, as well as data from surveys done in Europe on status of eye dose monitoring in interventional radiology and nuclear medicine. The talk will provide update on how good is Hp(10) against Hp(3), estimations from CTDI values, Monte Carlo based simulations and current status of eye lens dosimetry in USA and Europe.

The cataract risk among patients is in CT examinations of the head. Since radiation induced cataract predominantly occurs in posterior sub‐capsular (PSC) region and is thus distinguishable from age or drug related cataracts and is also preventable, actions on awareness can lead to avoidance or even prevention.

Learning Objectives:

To understand recent changes in eye lens dose limits and thresholds for tissue reactions

To understand different approaches to dose estimation for eye lens

To learn about challenges in eye lens opacities among staff in interventional fluoroscopy

Di Zhang, Toshiba America Medical Systems, Tustin, CA, USA Eye lens radiation dose from brain perfusion CT exams

CT perfusion imaging requires repeatedly exposing one location of the head to monitor the uptake and washout of iodinated contrast. The accumulated radiation dose to the eye lens can be high, leading to concerns about potential radiation injury from these scans. CTDIvol assumes continuous z coverage and can overestimate eye lens dose in CT perfusion scans where the table do not increment. The radiation dose to the eye lens from clinical CT brain perfusion studies can be estimated using Monte Carlo simulation methods on voxelized patient models. MDCT scanners from four major manufacturers were simulated and the eye lens doses were estimated using the AAPM posted clinical protocols. They were also compared to CTDIvol values to evaluate the overestimation from CTDIvol. The efficacy of eye lens dose reduction techniques such as tilting the gantry and moving the scan location away from the eyelens were also investigated. Eye lens dose ranged from 81 mGy to 279 mGy, depending on the scanner and protocol used. It is between 59% and 63% of the CTDIvol values reported by the scanners. The eye lens dose is significantly reduced when the eye lenses were not directly irradiated. CTDIvol should not be interpreted as patient dose; this study has shown it to overestimate dose to the eye lens. These results may be used to provide more accurate estimates of actual dose to ensure that protocols are operated safely below thresholds. Tilting the gantry or moving the scanning region further away from the eyes are effective for reducing lens dose in clinical practice. These actions should be considered when they are consistent with the clinical task and patient anatomy.

Learning Objectives:

To become familiar with method of eye dose estimation for patient in specific situation of brain perfusion CT

To become familiar with level of eye lens radiation doses in patients undergoing brain perfusion MDCT

To understand methods for reducing eye lens dose to patient

Jong Min Park, Department of Radiation Oncology, Seoul National University Hospital, Seoul, Korea

Eye lens dosimetry in radiotherapy using contact lens‐shaped applicator

Dose calculation accuracy of commercial treatment planning systems is relatively low at shallow depths. Therefore, in‐vivo measurements are often performed in the clinic to verify delivered doses to eye lens which are located at shallow depth. Current in‐vivo dosimetry for eye lens during radiotherapy is generally performed with small in‐vivo dosimeters on the surface of patient eyelid. Since this procedure potentially contains considerable uncertainty, a contact lens‐shaped applicator made of acrylic (lens applicator) was developed for in‐vivo measurements of eye lens dose during radiotherapy to reduce uncertainty. The lens applicator allows the insertion of commercially available metal oxide semiconductor field effect transistor (MOSFET) dosimeters. Computed tomography (CT) images of an anthropomorphic phantom with and without the lens applicator were acquired. A total of 20 VMAT plans were delivered to an anthropomorphic phantom and the doses with the lens applicator and the doses at the surface of the eyelid were measured using both micro and standard MOSFET dosimeters. The differences in measured dose at the surface of the eyelid from the calculated lens dose were acquired. The differences between the measured and the calculated doses at the lens applicator, as well as the differences between the measured and the calculated doses at the surface of the eyelid were acquired. The statistical significance of the differences was analyzed. The average difference between the measured and the calculated dose with the lens applicator was 16.8 % ± 10.4 % with a micro MOSFET dosimeter and 16.6 % ± 10.9% with a standard MOSFET dosimeter. The average difference without the lens applicator was 35.9% ± 41.5% with micro MOSFET dosimeter and 42.9% ± 52.2% with standard MOSFET dosimeter. The maximum difference with micro MOSFET dosimeter was 46% with the applicator and 188.4% without the applicator. For the standard MOSFET dosimeter, the maximum difference was 44.4% with the applicator and 246.4% without the applicator. The lens applicator allowed reduction of the differences between the calculated and the measured dose during in‐vivo measurement for the eye lens as compared to in‐vivo measurement at the surface of the eyelid.

Learning Objectives:

To understand limitations of dose calculation with commercial treatment planning system for eye lens during radiotherapy

To learn about current in‐vivo dosimetry methods for eye lens in the clinic

To understand limitations of in‐vivo dosimetry for eye lens during radiotherapy

Di Zhang is an employee of Toshiba America Medical Systems