Portal dosimetry in wedged beams
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Journal of Applied Clinical Medical Physics
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Portal dosimetry using electronic portal imaging devices (EPIDs) is often applied
to verify high-energy photon beam treatments. Due to the change in photon energy
spectrum, the resulting dose values are, however, not very accurate in the case of
wedged beams if the pixel-to-dose conversion for the situation without wedge is
used. A possible solution would be to consider a wedged beam as another photon
beam quality requiring separate beam modeling of the dose calculation algorithm.
The aim of this study was to investigate a more practical solution: to make aSi
EPID-based dosimetry models also applicable for wedged beams without an
extra commissioning effort of the parameters of the model. For this purpose two
energy-dependent wedge multiplication factors have been introduced to be applied
for portal images taken with and without a patient/phantom in the beam. These
wedge multiplication factors were derived from EPID and ionization chamber
measurements at the EPID level for wedged and nonwedged beams, both with
and without a polystyrene slab phantom in the beam. This method was verified for
an EPID dosimetry model used for wedged beams at three photon beam energies
(6, 10, and 18 MV) by comparing dose values reconstructed in a phantom with
data provided by a treatment planning system (TPS), as a function of field size,
depth, and off-axis distance. Generally good agreement, within 2%, was observed
for depths between dose maximum and 15 cm. Applying the new model to EPID
dose measurements performed during ten breast cancer patient treatments with
wedged 6 MV photon beams showed that the average isocenter underdosage of
5.3% was reduced to 0.4%. Gamma-evaluation (global 3%/3 mm) of these in vivo
data showed an increase in percentage of points with γ ≤ 1 from 60.2% to 87.4%,
while γmean reduced from 1.01 to 0.55. It can be concluded that, for wedged beams,
the multiplication of EPID pixel values with an energy-dependent correction factor
provides good agreement between dose values determined by an EPID and a TPS,
indicating the usefulness of such a practical solution.
Como referenciar
SPREEUW, HANNO; ROZENDAAL, ROEL; CAMARGO, PRISCILLA; MANS, ANTON; WENDLING, MARKUS; OLACIREGUI-RUIZ, IGOR; SONKE, JAN J.; HERK, MARCEL van; MIJNHEER, BEN. Portal dosimetry in wedged beams. Journal of Applied Clinical Medical Physics, v. 16, n. 3, p. 244-257, 2015. DOI: 10.1120/jacmp.v16i3.5375. Disponível em: http://repositorio.ipen.br/handle/123456789/23857. Acesso em: 30 Dec 2025.
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.