Quantifying the dosimetric influences of radiation coverage and brachytherapy implant placement uncertainty on eye plaque size selection.

Purpose: Quantify the dosimetric adequacy of the 2003 American Brachytherapy Society report tumor margin recommendations for Collaborative Ocular Melanoma Study (COMS) eye plaque size selection for radiation coverage and clinical plaque placement uncertainties.

Methods And Materials: Plaque heterogeneity-corrected dose distributions were generated for the range of available COMS plaque diameters (φplaque) and radionuclides. These dose distributions were used to determine the radiation dose distribution diameter (φ℞) at the eye surface for each plaque as a function of central axis prescription depth (d℞) to assess adequacy of a 2-3-mm margin for various gross tumor volume (GTV) basal diameters (φGTV). Four sets of ellipsoidal tumors (φGTV=5, 8, 11, and 14mm) with a range of apical heights (dGTV=2-8mm) were contoured in a reference CT environment. Plaque placement uncertainties were quantified as circumferential displacements (Δ) at the outer scleral surface. Tumor dose-volume histograms were generated and compared for all Δ with D90 and D95 used to evaluate tumor margin adequacy.

Results: For equivalent φplaque and prescription depths, φ℞ values were typically 0.4-0.8mm less for (103)Pd than for (125)I or (131)Cs. Δ≤3mm resulted in D90 and D95 values as low as 68% and 64% of the prescription dose, respectively. (103)Pd plaque dose distributions were more sensitive than (125)I or (131)Cs to placement uncertainties.

Conclusions: The American Brachytherapy Society-recommended tumor margin may be inadequate for prescription dose coverage given COMS plaque radiation characteristics and placement uncertainties. Better coverage is achieved assuming a GTV-to-planning target volume total basal expansion of 3mm or greater and/or prescribing beyond the tumor apex.