¹⁰³Pd versus ¹²⁵I Plaque Radiation Dose to Normal Ocular Structures in the Treatment of 319 Uveal Melanomas

Abstract

 

Purpose

 

To calculate and compare the pre-operative radiation doses to normal intraocular structures in a large series of patients scheduled for plaque brachytherapy using Pallidum-103 (¹⁰³Pd) versus Iodine-125 (¹²⁵I).

 

Methods

 

We performed a retrospective chart review of 319 patients treated for uveal melanoma in 2006-2012. IRB approval was obtained and the study followed the guidelines of the Declaration of Helsinki. The radiation dose to tumor and normal ocular structures (outside the targeted zone) were calculated prior to surgery for both ¹⁰³Pd and ¹²⁵I Specific intraocular locations included: a 5mm depth, opposite retina, macula, optic disc, lens and the tumor apex (prescription point). Dose measurements were then related to tumor characteristics such as location (anterior versus posterior location), size, and T-stage according to the American Joint Committee on Cancer (AJCC) System.

 

Results

 

Three hundred nineteen patients were diagnosed with T1-T4 uveal melanomas ranging in maximum (apical) height from 1.8 to 14.2 mm (mean 4.0 mm,CI 2.4-2.8). The range of the maximum basal diameter is 1.8 to 21 mm (mean 9.9 mm, CI 9.5-10.3). 21.6% of the tumors are in the anterior location and 78.4% of the tumors are in the posterior location. Table I shows the comparative dosimetry of ¹⁰³Pd versus that of ¹²⁵I to intraocular locations for all tumors. By design, the dose to the prescription point (tumor apex) for ¹⁰³Pd and ¹²⁵I plaques was equivalent. However, ¹⁰³Pd shows a reduced mean doses to all the normal intraocular structures outside the targeted zone when compared to that of ¹²⁵I in all tumors (p < 0.05).

 

Conclusions

 

Radioactive ¹⁰³Pd seeds emit lower energy photons (21 KeV) versus ¹²⁵I (28 KeV). This shift in photon energy leads to shift in radiation exposure within the eye. That is, for an equivalent tumor apex dose the less energetic photons are more quickly absorbed within the melanoma and continue to be more quickly absorbed before they reach most normal intraocular structures. Thus, in a large cohort of patients with uveal melanoma, this study demonstrates that ¹⁰³Pd seeds in standard gold eye plaques offer a method to deliver relatively more radiation within the tumor (mean base to apex) and less dose to most essential intraocular structures.

 

 

Table I: Pre-operative Comparative Dosimetry for All Tumors

 

Keywords: 744 tumors • 671 radiation therapy • 589 melanoma