Proton Therapy for Uveal Melanoma on a Pencil Beam Scanning Gantry

Abstract

Purpose: We present our experience treating ocular tumors in a standard pencil beam scanning (PBS) gantry room without apertures, which could broaden access to proton therapy for patients with ocular cancer globally. Besides, this study explores the dosimetric benefits of beam-specific apertures.

Methods and materials: We retrospectively evaluated 11 consecutive patients with uveal melanoma treated in a clinic gantry room. The dose deviations between the planned and received by the patient were investigated by assessing the forward calculation of the treatment plan on the synthetic computed tomography of cone beam computed tomography. Each plan was forward calculated with a beam-specific brass aperture (BSA) using a Monte Carlo algorithm to explore dosimetric improvements. We compared the plan quality to the delivered plan (DP) using target coverage (D95%) and mean/maximum doses to the adjacent organs.

Results: A close agreement between the planned and delivered dose was achieved, with D95% deviations within 3.6% for all treatments, maintaining dose constraints for critical organs. Similar target coverage was reached, with D95% at 101% ± 1.0% (DP) and 101% ± 3.2% (BSA). BSA was effective (P < .05) in reducing the mean [D _Mean (DP, BSA)Gy] and maximum [D _Max (DP, BSA)Gy] dose to organs: retina D _Mean (37.7, 29.5), cornea D _Mean (10.7, 2.4), conjunctiva D _Mean (13.6, 4.1), lacrimal gland D _Mean (25.5, 14.1), optic nerve D _Mean (19.6, 13.1), lens D _Max (22.4, 8.5), cornea D _Max (24.4, 10.2), eyebrow D _Max (15.3, 6.8). BSA lowered the mean dose to surrounding organs and significantly decreased the maximum dose to nonabutting organs (lens, cornea, eyebrow), but had little impact on the maximum dose to the abutting organs (retina, optic nerve).

Conclusions: We demonstrate the successful implementation of ocular proton treatment with a standard PBS gantry beamline without apertures. The beam-specific apertures effectively reduced doses to the organs adjacent to the target in the PBS proton treatment while maintaining similar target coverage. This approach offers an opportunity to expand access to ocular proton therapy widely.

Figure 1

The target volume was delineated and illustrated on an axial section of the simulation computed tomography (CT) for 11 patients in this study. The last figure in the third row shows the custom-molded head mask (a) with an eye gaze fixation device (b) attached to the mask and a marker (c) for the patient to gaze at. The images in the last row show the kV and cone beam computed tomography (CBCT) for patient 8 acquired for alignment, and the alignment contours from treatment planning.

Figure 2

The differences in dosimetry metrics between the planned and the delivered doses. The differences were calculated as (delivered – planned dose)/planned dose in percentage. The planned dose was obtained from the simulation computed tomography (CT). The delivered dose was obtained from the synthetic CT.

Figure 3

Comparison of dose distribution for a single lateral field (first row) and all 3 fields (second row) among the delivered plan (left) and aperture plan (right, 2 cm air gap) for the eighth patient. Yellow arrows: 2 beam angles for the coplanar fields; the third beam is a noncoplanar field and is not shown here.

Figure 4

Box plot of the (a) mean and (b) maximum organ dose comparison for delivered plans and plans with beam-specific apertures. Each box consists of 11 patients' doses of different organs-at-risk (OARs). The lowest, highest, 25th percentile, 50th percentile, 75th percentile, and average doses are presented in the plot. +: Average dose. The dosimetry of different plans is compared, and differences with P < .05 are tagged with stars (*).