Assessing Ir-192 as an alternative to I-125 in ophthalmic treatment
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2020
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Radiotherapy and Oncology
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Purpose or Objective: Brachytherapy sources for ocular melanoma usually
contain Co-60, I-125, Pd-103 or Ru/Rh-106 as
radionuclides. Ir-192 is not a preconized radioactive
material for this purpose, although it is used for other
brachytherapy applications. Higher mean energy from
Ir-192 emission (ca. 380 keV) may be a reason for the
preference of I-125 (35 keV) or Pd-103 (21 keV) over it,
since low penetration is desired on the small structures of
the human eye. This is not, however, an excluding
criterion, considering Co-60 and Ru/Rh-106 have even
higher mean energies.
The demand in Brazil for lower-cost seeds to treat ocular
melanoma lead to the development of an Ir-192 seed to
make treatment more accessible, but since it is not used
as an ophthalmic brachytherapy source, before its
dosimetry is considered, one should care about the
possibility of using it over more stablished materials.
Considering this, the aim of this work is to assess the
possibility of using Ir-192 seeds as ophthalmic
brachytherapy sources by comparing some dosimetric
parameters of a new seed model with the most stablished
I-125 seed in literature, OncoSeed 6711. Material and Methods: As an initial study on the topic, this work relies only on
Monte-Carlo simulations using MCNP4C transport code.
Parameters analyzed are air-kerma strength, dose-rate
constant and depth-dose curve, attention given to points
within the human eye dimensions. The medium considered
was homogeneous water, as it is a good approximation to
the eye tissues in terms of composition and density and
allows for future comparisons with TG-43 based
calculations.
OncoSeed 6711 is not produced anymore, but its long term
as the reference source for dosimetry was considered. A
20 mm COMS ophthalmic applicator was also modeled and
considered to be fully loaded with each seed model to
compare the same parameters at a realistically clinical
approach. Results: As expected, due to the higher energy of the Ir-192
emission spectrum, dose fall-off on the transversal axis of
the seeds is less pronounced for the new seed model. The
steeper dose gradient for I-125 is also visible on the doserate
constant value. The effect of using a COMS applicator
only strengthens this characteristic. Depth-dose curves
were calculated up to the distance of 5 cm, both for a
single seed and for an applicator fully loaded with 24
seeds. All the eye components relevant for dosimetry are located within this range, like the cells of the crystallin
and the optical nerve. Conclusion: If one expects to use Ir-192 as an alternative to I-125 in
ophthalmic cancer treatment, at least the dosimetry
following TG-43 protocol should be carried with utmost
attention, as undesirable dose to healthy nearby tissues is
unavoidable. Crafting a different applicator most suited
for this radionuclide is a possibility that can be taken into
account. Another recommendation is to go beyond TG-43
water-based protocol and actually estimate dose to
relevant eye components.
Como referenciar
ANGELOCCI, L.; NOGUEIRA, B.R.; SOUZA, C.D. de; ZEITUNI, C.A.; ROSTELATO, M.E.C.M. Assessing Ir-192 as an alternative to I-125 in ophthalmic treatment. Radiotherapy and Oncology, v. 152, p. S735-S736, 2020. Supplement 1. DOI: 10.1016/S0167-8140(21)01402-X. Disponível em: http://repositorio.ipen.br/handle/123456789/32695. Acesso em: 14 May 2024.
Esta referência é gerada automaticamente de acordo com as normas do estilo IPEN/SP (ABNT NBR 6023) e recomenda-se uma verificação final e ajustes caso necessário.