Novel low-kVp beamlet system for choroidal melanoma

Abstract

Background: Treatment of choroidal melanoma with radiation often involves placement of customized brachytherapy eye-plaques. However, the dosimetric properties inherent in source-based radiotherapy preclude facile dose optimization to critical ocular structures. Consequently, we have constructed a novel system for utilizing small beam low-energy radiation delivery, the Beamlet Low-kVp X-ray, or "BLOKX" system. This technique relies on an isocentric rotational approach to deliver dose to target volumes within the eye, while potentially sparing normal structures.

Methods: Monte Carlo N-Particle (MCNP) transport code version 5.0(14) was used to simulate photon interaction with normal and tumor tissues within modeled right eye phantoms. Five modeled dome-shaped tumors with a diameter and apical height of 8 mm and 6 mm, respectively, were simulated distinct positions with respect to the macula iteratively. A single fixed 9 x 9 mm2 beamlet, and a comparison COMS protocol plaque containing eight I-125 seeds (apparent activity of 8 mCi) placed on the scleral surface of the eye adjacent to the tumor, were utilized to determine dosimetric parameters at tumor and adjacent tissues. After MCNP simulation, comparison of dose distribution at each of the 5 tumor positions for each modality (BLOKX vs. eye-plaque) was performed.

Results: Tumor-base doses ranged from 87.1-102.8 Gy for the BLOKX procedure, and from 335.3-338.6 Gy for the eye-plaque procedure. A reduction of dose of at least 69% to tumor base was noted when using the BLOKX. The BLOKX technique showed a significant reduction of dose, 89.8%, to the macula compared to the episcleral plaque. A minimum 71.0 % decrease in dose to the optic nerve occurred when the BLOKX was used.

Conclusion: The BLOKX technique allows more favorable dose distribution in comparison to standard COMS brachytherapy, as simulated using a Monte Carlo iterative mathematical modeling. Future series to determine clinical utility of such an approach are warranted.

Figure 1

Diagrammatic representation of BLOKX rotational technique (left); Siemens Orthopantomograph x-ray unit model #0P 10 A reconstructed with the mechanical ability to move in three-dimensional space about an isocentric point (right). The source to axis distance is adjustable, with a 14 cm range.

Figure 2

Spatial location of intra-ocular tumor models, showing 90°(A), 60°(B), 45°(C), 30°(D) and 270°(E) positions.

Figure 3

Experimentally derived energy spectrum of BLOKX utilized for Monte Carlo calculations.

Figure 4

Axial view beam directions for tumor at 45° position; a) Original x-ray beam setup for irradiating the tumor based on TLD measurements, b) Revised x-ray beam direction used to spare direct irradiation of the macula.

Figure 5

MCNP output simulation of BLOKX procedure.

Figure 6

MCNP output simulation of eye-plaque procedure.

Figure 7

Tumor volume dose distribution comparison of BLOKX and eye plaque methods for tumor at 45° position, with legend.

Figure 8

Tumor volume dose distribution comparison of BLOKX and eye plaque methods for tumor at 90° position, with legend.