title: Independent dosimetric assessment of the model EP917 episcleral brachytherapy plaque.
abstract: OBJECTIVE
  To investigate the influence of slot design on dose distributions and dose-volume histograms (DVHs) for the model EP917 plaque for episcleral brachytherapy.
  
  
  METHODS
  Dimensions and orientations of the slots were measured for three model EP917 plaques and compared to data in the Plaque Simulator (PS) treatment planning software (version 5.7.6). These independently determined coordinates were incorporated into the MCNP Monte Carlo simulation environment to obtain dose from the plaques in a water environment and in a clinical environment with ocular structures. A tumor volume was simulated as 5 mm in apical height and 11 mm in basal diameter. Variations in plaque mass density and composition; slot length, width, and depth; seed positioning; and Ag-marker rod positioning were simulated to examine their influence on plaque central axis (CAX) and planar dose distributions, and DVHs.
  
  
  RESULTS
  Seed shifts in a single slot toward the eye and shifts of the(125)I-coated Ag rod within the capsule had the greatest impact on CAX dose distribution. A shift of 0.0994 mm toward the eye increased dose by 14%, 9%, 4.3%, and 2.7% at 1, 2, 5, and 10 mm, respectively, from the inner sclera. When examining the fully-modeled plaque in the ocular geometry, the largest dose variations were caused by shifting the Ag rods toward the sclera and shifting the seeds away from the globe when the slots were made 0.51 mm deeper, causing +34.3% and -69.4% dose changes to the outer sclera, respectively. At points along the CAX, dose from the full plaque geometry using the measured slot design was 2.4%±1.1% higher than the manufacturer-provided slot design and 2.2%±2.3% higher than the homogeneous calculation of PS treatment planning results. The ratio of D10 values for the measured slot design to the D10 values for the manufacturer-provided slot design was higher by 9%, 10%, and 19% for the tumor, inner sclera, and outer sclera, respectively. In comparison to the measured slot design, a theoretical plaque having narrower and deeper slots delivered 30%, 37%, and 62% lower D10 doses to the tumor, inner sclera, and outer sclera, respectively.
  
  
  CONCLUSIONS
  While the measured positions of the slots on the model EP917 plaque were in close agreement (<0.7 mm) with the PS values, small differences in the slot shape caused substantial differences in dose distributions and DVH metrics. Increasing slot depth by 0.1 mm decreased outer scleral dose by 20%, yet shifting the Ag rods in the seeds toward the globe by 0.1 mm increased outer scleral dose by 35%. The clinical medical physicist is advised to measure these types of plaques upon acceptance testing before clinical use to inspect slot shape and position for comparison with data used for treatment planning purposes.
authors.affiliation: Department of Radiation Medicine, University of Kentucky, Lexington, Kentucky 40536.
authors.forename: Prakash
authors.initials: P
authors.lastname: Aryal
authors.affiliation: Department of Radiation Medicine, University of Kentucky, Lexington, Kentucky 40536.
authors.forename: Janelle A
authors.initials: JA
authors.lastname: Molloy
authors.affiliation: Department of Radiation Oncology, Tufts University School of Medicine, Boston, Massachusetts 02111.
authors.forename: Mark J
authors.initials: MJ
authors.lastname: Rivard
mesh_terms: D001918:Brachytherapy / Q000295:instrumentation* / Q000379:methods; D003198:Computer Simulation; D004867:Equipment Design; D005134:Eye Neoplasms / Q000532:radiotherapy*; D009010:Monte Carlo Method; D011874:Radiometry; D011879:Radiotherapy Dosage; D011880:Radiotherapy Planning, Computer-Assisted; D012590:Sclera / Q000528:radiation effects; D012984:Software; D014867:Water
chemical_list: D014867:Water