Dual-source strength seed loading for eye plaque brachytherapy using eye physics eye plaques: A feasibility study

Abstract

Purpose: This study quantifies the dosimetric impact of incorporating two iodine-125 (¹²⁵I) seed source strengths in Eye Physics eye plaques for treatment of uveal melanoma.

Material and methods: Plaque Simulator was used to retrospectively plan 15 clinical cases of three types: (1) Shallow tumors (< 5.5 mm) with large base dimensions (range, 16-19 mm); (2) Tumors near the optic nerve planned with notched plaques; and (3) Very shallow (< 3.0 mm) tumors with moderate base dimensions (range, 13.5-15.5 mm) planned with larger plaques than requested by the ocular oncologist. Circular plaques were planned with outer ring sources twice the source strength of inner sources, and notched plaques with the six seeds closest to the notch at twice the source strength.

Results: In cases of type (1), the dual-source strength plan decreased prescription depth, and doses to critical structures were lower: inner sclera -25% ±2%, optic disc -7% ±3%, and fovea -6% ±3%. In four out of five cases of type (2), the dual-source strength plan decreased prescription depth, and dose to inner sclera was lower (-22% ±5%), while dose to optic disc (17% ±7%) and fovea (20% ±12%) increased. In cases of type (3), a smaller dual-source strength plaque was used, and scleral dose was lower (-45% ±3%), whereas dose to optic disc (1% ±14%) and fovea (5% ±5%) increased.

Conclusions: Dual-source strength loading as described in this study can be used to cover tumor margins and decrease dose to sclera, and therefore the adjacent retina, but can either decrease or increase radiation dose to optic disc and fovea depending on location and size of the tumor. This technique may allow the use of a smaller plaque, if requested by the ocular oncologist. Clinical determination to use this technique should be performed on an individual basis, and additional QA steps are required. Integrating the use of volumetric imaging may be warranted.

Keywords: Eye Physics eye plaques; Plaque Simulator treatment planning system; eye plaque treatment planning; plaque brachytherapy; uveal melanoma.

Fig. 1

To obtain the base plus margin shown in (A), a planning apex of 3.6 mm was required as shown in (B). This planning apex is 1.6 mm deeper than the tumor apex of 2.0 mm

Fig. 2

Representative fundus diagram from the ocular oncologist (A) and its corresponding digitization within the PS TPS (B)

Fig. 3

The three investigated plaques were EP2342 (scenario 1), EP2029N (scenario 2), and EP2031 (scenario 3). Shown in blue are the seed locations loaded with the higher of the two source strengths when planned with dual-source strengths

Fig. 4

Scenario 1: EP2342 plaque isodose lines at the mid-plane with single-source strength plan (A) and dual-source strength plan (B)

Fig. 5

Scenario 2: EP2029N plaque isodose lines on the inner surface of the eye with single-source strength plan (A) and dualsource strength plan (B)

Fig. 6

Scenario 3: isodose lines both at mid-plane and on the inner surface of the eye for EP2342 single-source strength plan (A) and EP2031 dual-source strength plan (B)

Fig. 7

Photographs are taken by the vendor twice during the seed loading process, once after loading the higher source strength seeds (A), and once after loading the remaining source locations with the lower source strength seeds (B). The order numbers in the photos are those associated with the order made for the seeds being loaded at that point in the loading process

Fig. 8

A case of scenario 1 planned with a single-source strength and with seed source strength ratios of 1.5, 2.0, and 3.0

Fig. 9

A case of scenario 3 planned with a single-source strength and with a seed source strength ratio of 2.0 with three seeds closest to the notch and six seeds closest to the notch replaced with higher source strength seeds