Sensitivity of contributing factors to heterogeneity corrections in dosimetry of ocular brachytherapy using iodine-125 COMS plaques

Purpose:
To investigate sensitivity of contributing factors to heterogeneity corrections in ocular brachytherapy using iodine-125 (¹²⁵I) Collaborative Ocular Melanoma Study (COMS) plaques.

Material and methods:
Using egs_brachy, Monte Carlo (MC) simulations were performed for ¹²⁵I COMS plaques (model: IsoAid IAI-125A). Homogeneous dose (D_HOMO) was estimated under the American Association of Physicists in Medicine Task Group-43 assumptions, with 85 Gy prescribed to a 5 mm depth. Heterogeneous doses (D_HETERO#1-#7) were determined with medium-heterogeneity for the following scenarios: #1. Modulay backing (M) alone, #2. Silastic insert (S) alone, #3. Interseed attenuation effects (I) alone, #4. M + S, #5. M + I, #6. S + I, and #7. M + S + I. For scenarios #1-#7, heterogeneity correction factors (HCFs) were derived from D_HETERO#1-#7/D_HOMO for central axis points up to 22 mm, and for off-axis points (i.e., optic disc, macula, and lens) as a function of distance from tumor to optic disc (DT) or macula (MT), and tumor dimensions in the direction of optic disc (BD) or macula (BM).

Results:
For a 16-mm diameter COMS plaque, in central axis, the dominant factor to heterogeneity corrections (dose reduction by 9.5-13.8% with heterogeneity) was Silastic insert at a depth ≤ 5 mm, whereas both Modulay backing and Silastic insert were contributing (range, 12.2-19.0%) at deeper depths. For off-axis, Silastic insert was the major factor to heterogeneity corrections (≥ 11.8%) at DT (MT) ≤ 6.9 (7.6) mm for optic disc (macula) and (range, 8.9-10.0%) at MT > 18.5 mm for lens, but both Modulay backing and Silastic insert were contributing (range, 18.3-24.4%) at farther DT (MT) for optic disc (macula) and (range, 12.8-18.2%) at MT ≤ 18.5 mm for lens. Interseed effects were small (≤ 1.7% for central axis and ≤ 2.3% for off-axis). Data for the other six plaques (10-14 mm and 18-22 mm diameter) presented similar trends.

Conclusions:
The use of universal HCF(s) for the two dominant factors determined in this study will facilitate heterogeneous dose estimates for each clinical scenario without an MC tool.