Eyelid lesions necessitate the use of metallic eye shields in radiation therapy to minimize complications such as cataract, keratopathy, retinopathy, and optic neuropathy [1-5]. Proposed for electron beam or kilo-voltage photon beam treatments, metallic eye shields made of high Z materials such as lead or tungsten should meet three essential requirements when positioned between the eyeball and the eyelid: (1) harmless to normal orbital tissues, (2) sufficient protection for the cornea and lens, and (3) no attenuation of the local therapeutic effect by partially shielding the planning target volume. Accurate estimation of the dose delivered to the surrounding area is crucial when using metallic eye shields [6,7]. However, the presence of a metallic eye shield in computed tomography (CT) images poses challenges in treatment planning systems (TPS) because of difficulties in acquiring precise CT numbers and the delineation of surrounding tissues caused by severe metal artifacts. To address this, a nonmetallic dummy shield was fabricated as a substitute for metallic eye shields. Studies have shown the successful implementation of machined acrylic dummy shields for accurate dose calculation using a commercial TPS with CT-based planning and Monte Carlo (MC) simulation [8]. In addition, the fabrication of the dummy eye shields using 3D scanning and printing techniques has been demonstrated, showing effective adoption for treating ocular tumors with electron radiotherapy [9,10]. However, dummy eye shields made of 3D printing materials such as polylactic acid or acrylonitrile butadiene styrene exhibit shore hardness levels of ≥70D, potentially causing eye damage. Consequently, additional packaging is required to avoid direct eye contact [10].

In this study, a flexible eye shield phantom was fabricated using a 3D-printed mold and casting method. The dose distribution calculated using the flexible dummy shield with an assigned CT value was compared with the dose measured with a metal shield using a metal oxide semiconductor field effect transistor (MOSFET) detector.

Fig. 4 shows flexible eye shield phantom which has the same appearance as the newly designed eye shield consisting of small, medium, and large size. The silicone used in the flexible eye shield phantom which is harmless, does not generate metal artifacts in CT images, so it is not only advantageous for delineating the surrounding organ-at-risk (OAR) and target volume, but also resolve error in CT-based dose calculations caused by metal artifacts.

Fig. 5 illustrates the differences in dose distribution between when performing dose calculations based on CT images acquired using flexible eye shield phantom and CT images using the eye shield made by high density material. The HU value corresponding to tungsten was overridden in the flexible eye shield phantom, and a 6-MeV beam with 100 MU was irradiated.

Table 2 compares dose calculated by the eMC algorithm with flexible eye shield phantom to dose measured by MOSFET. A 3-mm bolus was used to simulate the eyelid depth, resulting in mean differences between measured and calculated doses at the eyelid of 36.2%, 34.2%, and 34.1% for small, medium, and large sizes, respectively. In contrast, at the position for measuring transmitted dose beyond the eye shield, the mean differences were 1.3, 0.9, and 0.8 cGy for small, medium, and large sizes, respectively.

Compared with the traditional rigid acrylic material dummy, the flexible eye shield phantom may provide greater comfort to patients and to potentially reduce the risk of eye damage. Moreover, the relatively low cost of manufacturing a flexible eye shield phantom using silicone compared to processing acrylic makes it more suitable for single-patient use to prevent infections. The dose distribution similar to previous studies was acquired by using CT imaging without metal artifact. On the contrary, dose evaluation was made difficult by metal artifact when performing dose calculation using CT image with the eye shield. The MOSFET measurements showed relatively good agreement at a depth of 10 mm simulating the lens depth. These results were consistent with previous studies comparing calculated dose using acrylic dummy shield [7-9]. In addition, Kang et al. [7,8] showed a difference of approximately −20% in the dose at the eyelid compared with the MC results in a commercial TPS using a pencil beam algorithm, which is similar to the underestimated dose at the eyelid as shown in this study. These results indicated that dummy shields can be applicable for estimating the dose below the shield in TPS; however, they may be inappropriate for evaluating eyelid doses in clinical practice. These results can be inferred from the finding that the eMC algorithm, as shown in previous studies, cannot properly consider backscatter from higher density materials [11]. In this study, the HU was assigned to extremely high-density materials such as tungsten, which could have emphasized the discrepancies. Therefore, for accurate assessment of doses around high Z materials, using a fully MC calculation is recommended rather than using an algorithm that provides mass densities of 5 preset materials. In the future, flexible eye shield phantom would be applied to fully MC dose calculations and for comparative evaluation with algorithms of existing commercially available TPS.

In this study, flexible dummy shield phantoms for newly designed eye shields, offering a more comfortable fit for patients than existing acrylic dummy shields, were fabricated. In addition, it was possible to acquire artifact-free CT images, which not only facilitates accurate delineation of the OAR and target volume but also indicates its suitability for use in dose calculation.