Dosimetry of ¹²⁵I and ¹⁰³Pd COMS eye plaques for intraocular tumors: Report of Task Group 129 by the AAPM and ABS

Abstract

Dosimetry of eye plaques for ocular tumors presents unique challenges in brachytherapy. The challenges in accurate dosimetry are in part related to the steep dose gradient in the tumor and critical structures that are within millimeters of radioactive sources. In most clinical applications, calculations of dose distributions around eye plaques assume a homogenous water medium and full scatter conditions. Recent Monte Carlo (MC)-based eye-plaque dosimetry simulations have demonstrated that the perturbation effects of heterogeneous materials in eye plaques, including the gold-alloy backing and Silastic insert, can be calculated with reasonable accuracy. Even additional levels of complexity introduced through the use of gold foil "seed-guides" and custom-designed plaques can be calculated accurately using modern MC techniques. Simulations accounting for the aforementioned complexities indicate dose discrepancies exceeding a factor of ten to selected critical structures compared to conventional dose calculations. Task Group 129 was formed to review the literature; re-examine the current dosimetry calculation formalism; and make recommendations for eye-plaque dosimetry, including evaluation of brachytherapy source dosimetry parameters and heterogeneity correction factors. A literature review identified modern assessments of dose calculations for Collaborative Ocular Melanoma Study (COMS) design plaques, including MC analyses and an intercomparison of treatment planning systems (TPS) detailing differences between homogeneous and heterogeneous plaque calculations using the American Association of Physicists in Medicine (AAPM) TG-43U1 brachytherapy dosimetry formalism and MC techniques. This review identified that a commonly used prescription dose of 85 Gy at 5 mm depth in homogeneous medium delivers about 75 Gy and 69 Gy at the same 5 mm depth for specific (125)I and (103)Pd sources, respectively, when accounting for COMS plaque heterogeneities. Thus, the adoption of heterogeneous dose calculation methods in clinical practice would result in dose differences >10% and warrant a careful evaluation of the corresponding changes in prescription doses. Doses to normal ocular structures vary with choice of radionuclide, plaque location, and prescription depth, such that further dosimetric evaluations of the adoption of MC-based dosimetry methods are needed. The AAPM and American Brachytherapy Society (ABS) recommend that clinical medical physicists should make concurrent estimates of heterogeneity-corrected delivered dose using the information in this report's tables to prepare for brachytherapy TPS that can account for material heterogeneities and for a transition to heterogeneity-corrected prescriptive goals. It is recommended that brachytherapy TPS vendors include material heterogeneity corrections in their systems and take steps to integrate planned plaque localization and image guidance. In the interim, before the availability of commercial MC-based brachytherapy TPS, it is recommended that clinical medical physicists use...

Gold nanoparticle‐based brachytherapy enhancement in choroidal melanoma using a full Monte Carlo model of the human eye

“…In this work, a choroidal melanoma tumor with the apical height of 5 mm (i.e., 6 mm from the exterior surface of the sclera, according to COMS definition for the point of dose prescription (66) ) has been simulated in both phantoms. This tumor has been assumed to be on the lateral portion to the eyeball on the equator.…”

“…The geometric information and composition of the simulated plaque were obtained from some published references 67 , 68 , 69 , 70 , 71 , 72 . Also, the coordinates of the 13 seeds for this plaque are in accordance with the standard position for COMS plaque (66) . Moreover, the detailed geometric and characteristic information of the model 6711 125 I sources have been obtained from the publicly accessible website of the Carleton Laboratory for Radiotherapy physics seeds database (73) .…”

Seed coordinates of a new COMS‐like 24 mm plaque verified using the FARO Edge

“…The coordinates were then corrected to account for the fact that the seed markers were measured at their top surfaces rather than at the height of the seed marker center. These corrections assumed the idealized radius of curvature of the concave side of silastic inserts of 12.3 mm and a 1 mm thickness of silastic to the bottom of the seed groove, as specified in the report of Task Group 129 (8) . Corrections were fairly minor and tended to impact only larger plaque sizes, due to the more extreme difference in angle of the seed markers on outer rings compared to those in more central rings.…”

Episcleral eye plaque dosimetry comparison for the Eye Physics EP917 using Plaque Simulator and Monte Carlo simulation

“…However, the AAPM recommendations in the TG‐43U1 and TG‐43U1S1 reports are limited to infinite homogeneous water medium and do not account for the effects of material heterogeneities, such as the eye plaque and bony orbit, which are of higher effective atomic number ${\mathrm{Z}}_{\text{eff}}$ than water. AAPM Task Group 129 (9) reviewed the dosimetry of eye plaque brachytherapy and evaluated the impact of heterogeneity effects for ${\hspace{0.17em}}^{125}\mathrm{I}$ and ${\hspace{0.17em}}^{103}\text{Pd}$ COMS plaques only; they then made recommendations for treatment planning and quality assurance for eye‐plaque brachytherapy. Since the scope of AAPM TG 129 was limited to COMS plaques, there is a need for further research into the heterogeneity effects of other plaques.…”

“…Eye plaque therapy involves treating the choroidal melanomas with small radioactive sources embedded in a plaque which is placed adjacent to the tumor on the outer scleral surface. PS is based on superposition of dose contributions from individual seeds following the TG‐43 formalism (9) . The software incorporates additional correction factors, such as plaque shell collimation, slot collimation, and carrier attenuation, that allow corrections for source attenuation and scatter (10) .…”