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 (phi(plaque)) and radionuclides. These dose distributions were used to determine the radiation dose distribution diameter ((pp) at the eye surface for each plaque as a function of central axis prescription depth (4) to assess adequacy of a 2-3mm margin for various gross tumor volume (GTV) basal diameters (coGiv). Four sets of ellipsoidal tumors (com,v = 5, 8, 11, and 14 mm) with a range of apical heights (d(GTV) = 2-8 mm) were contoured in a reference CT environment. Plaque placement uncertainties were quantified as circumferential displacements (Delta) at the outer scleral surface. Tumor dose volume histograms were generated and compared for all A with D-90 and D-95 used to evaluate tumor margin adequacy. RESULTS: For equivalent CDplare and prescription depths, phi(BX), values were typically 0.4-0.8 mm less for Pd-103 than for I-125 or (13f) CS. Delta <= 3 mm resulted in D-90 and D-95 values as low as 68% and 64% of the prescription dose, respectively. Pd-103 plaque dose distributions were more sensitive than I-125 or Cs-131 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 3 mm or greater and/or prescribing beyond the tumor apex. Published by Elsevier Inc. on behalf of American Brachytherapy Society.