The photon fluence non-uniformity correction for air kerma near Cs-137 brachytherapy sources

Abstract

The use of brachytherapy sources in radiation oncology requires their proper calibration to guarantee the correctness of the dose delivered to the treatment volume of a patient. One of the elements to take into account in the dose calculation formalism is the non-uniformity of the photon fluence due to the beam divergence that causes a steep dose gradient near the source. The correction factors for this phenomenon have been usually evaluated by the two theories available, both of which were conceived only for point sources. This work presents the Monte Carlo assessment of the non-uniformity correction factors for a Cs-137 linear source and a Farmer-type ionization chamber. The results have clearly demonstrated that for linear sources there are some important differences among the values obtained from different calculation models, especially at short distances from the source. The use of experimental values for each specific source geometry is recommended in order to assess the non-uniformity factors for linear sources in clinical situations that require special dose calculations or when the correctness of treatment planning software is verified during the acceptance tests.

Minimizing the 1/r2perturbation for ideal fluence detectors in small sourceγ-irradiation fields

“…The validity of analytic theories [Kondo andRandolph 1960, Bielajew 1990], has recently brought some criticism because of their restriction to point sources [Vianello and de Almeida 2008]. This hypothesis is supported by detailed Monte Carlo calculations [Rodríguez and de Almeida 2004]. The fluence perturbation for the geometries studied.…”

Establishment of air kerma reference standard for low dose rate Cs‐137 brachytherapy sources

“…Nonuniformity correction factor, both for isotropic electron fluence in the air cavity (Kondo and Randolph ( 37 ) ) as well as for anisotropic electron fluence in the air cavity (Bielajew ( 38 ) ), was also determined and applied while calculating the corrected charge rate. ( 13 , 37 – 39 ) Room scatter correction factor was determined by the shadow shield method and cross‐checked by the Monte Carlo method. The ${\mathrm{normalN}}_{\mathrm{normalK}}$ of the chamber was calculated using decay‐corrected RAKR of the source provided by the IAEA $({\mathrm{normalRAKR}}_{\mathrm{IAEA}})$ and the corrected ionization chamber measured charge rate using the following expression: ${\text{NK}}_{,\text{expt}}(\text{GY}/\mathrm{C})={\text{RAKR}}_{\text{IAEA}}({\text{Gyh}}^{-1})/\text{Corrected}\hspace{0.17em}\text{charge}\hspace{0.17em}\text{rate}({\text{Ch}}^{-1})$ …”

Air kerma standard for calibration of well‐type chambers in Brazil using HDR sources and its traceability

“…This may cause nonuniform electron fluence in the chamber requiring a correction factor to the measurements, 19,20 and is only true, when point or quasipoint sources are used for distance source to the detector larger than 7 cm. 21,22 The magnitude of this correction depends on several parameters, i.e., chamber shape and dimensions, source to chamber distance, source dimension, chamber materials, and the incident energy fluence.…”