Purpose: This study aimed to develop a flexible eye shield phantom to acquire artifact-free computed tomography (CT) images for electron beam radiotherapy.
Methods: A flexible eye shield phantom for a newly designed eye shield was fabricated. Because of metal artifacts caused by an eye shield composed of high-density materials such as tungsten or lead, CT image acquisition is not appropriate for treatment planning because of inaccurate dose calculation and organ-at-risk delineation. To acquire artifact-free CT images, a mold of the same size as the outer dimension of the metallic eye shield was manufactured using 3D printing. The flexible eye shield phantom was imaged using a Philips Brilliance CT Big Bore under the same condition as the measurement. The phantom image with an average of 200 Hounsfield unit (HU) was imported into the treatment planning systems (TPS) and assigned a value of 26,750 HU to consider the material density of tungsten. The dosimetric comparison using a 6-MeV electron beam was performed. Measurement was performed using a metal oxide semiconductor field effect transistor detector for point doses at 3 and 10 mm.
Results: The artifact-free CT images using a flexible eye shield phantom without air bubbles were transferred into the TPS. The dose at 10 mm calculated using the TPS agreed with the ionchamber measurements within 2 cGy. Conversely, a larger dose discrepancy between the measured and calculated doses was found at 3 mm depth.
Conclusions: The flexible eye shield phantom was successfully fabricated to apply electron treatment planning by acquiring artifact-free CT images. The dose calculated using the artifact-free image was comparable to the measured dose at lens depth when applying an eye shield.

Purpose: This study aims to evaluate the impact of smaller gantry arc increment (GAI) values on the plan quality and deliverability of volumetric modulated arc therapy (VMAT) for head and neck (HN) and prostate cancer cases using the Monaco treatment planning system. The study investigates whether a smaller GAI can enhance organ at risk (OAR) sparing without compromising target coverage or significantly increasing plan complexity.
Methods: VMAT plans were created for 20 patients (10 HN and 10 prostate cancer) using GAI values of 15° and 30°. Dose-volumetric parameters, such as conformity number, homogeneity and gradient indices, were assessed alongside plan complexity metrics like the modulation complexity score for VMAT (MCSv) and monitor unit (MU). Statistical significance was determined using the Wilcoxon signed-rank test.
Results: For HN cases, a 15° increment significantly reduced the D_0.03cc for the spinal cord and the D_mean for both parotid glands compared to a 30° increment, improving OAR sparing. However, no significant differences were observed in the OAR doses for prostate cases. The 15° increment resulted in higher plan complexity, reflected by a lower MCS_v, but the MU difference was not significant.
Conclusions: Smaller GAI values, such as 15°, can significantly reduce OAR doses in HN VMAT plans, offering potential clinical benefits despite increased plan complexity. However, no substantial advantages were observed in prostate cases. These findings suggest that smaller GAI values may be particularly beneficial for cases requiring high modulation.