Feasibility study of the Fricke chemical dosimeter as an independent dosimetric system for the small animal radiation research platform (SARRP)

Determination of the correction factors used in Fricke dosimetry for HDR 192Ir sources employing the Monte Carlo method

PURPOSE

Fricke dosimetry has shown great potential in the direct measurement of the absolute absorbed dose for ¹⁹²Ir sources used in HDR brachytherapy. This work describes the determination of the correction factors necessary to convert the absorbed dose in the Fricke solution to the absorbed dose to water.

METHODS

The experimental setup for Fricke irradiation using a ¹⁹²Ir source was simulated. The holder geometry used for the Fricke solution irradiation was modelled for MC simulation, using the PENELOPE.

RESULTS

The values of the factors determined for validation purposes demonstrated differences of less than 0.2% when compared to the published values. Four factors were calculated to correct: the differences in the density of the solution (1.0004 ± 0.0004); the perturbations caused by the holder (0.9989 ± 0.0004); the source anisotropy and the water attenuation effects (1.0327 ± 0.0012); and the distance from the center of the detection volume to the source (7.1932 ± 0.0065).

CONCLUSION

Calculated corrections in this work show that the largest correction comes from the inverse squared reduction of the dose due to the point of measurement shift from the reference position of 1 cm. This situation also causes the correction due to volume averaging and attenuation in water to be significant. Future versions of the holder will aim to reduce these effects by having a position of measurement closer to the reference point thus requiring smaller corrections.

A pilot study of a postal dosimetry system using the Fricke dosimeter for research irradiators

Cobalt-60 irradiators and soft X-ray machines are frequently used for research purposes, but the dosimetry is not always performed using the recommended protocols. This may lead to confusing and untrustworthy results within the conducted research. Postal dosimetry systems have already been approved by the IAEA, with thermoluminescence dosimeters (TLD) and optically stimulated luminescence (OSL) as the most commonly used dosimeter systems in these cases. The present study tests the Fricke dosimeter properties as a potential system to be used in postal dosimetry for a project using research irradiators. The Fricke solution was prepared according to the literature, and the linearity and fading tests were performed accordingly. All calculated doses were measured using a NE2571 Farmer ionization chamber as a reference. Doses ranging from 25 to 300 Gy were delivered by a research irradiator, with 150 kV and 22 mA to the Fricke solutions inside polyethylene (PE) bags (4 × 4 × 0.2 cm³). The results compared with the ionization chamber showed a linear response to the range of doses used. Fading tests showed no significant difference for the absorbed doses over 9 days, with a maximum difference of 1.5% found between days 0 and 3. The Fricke dosimeter presented good linearity, for low and high doses, and low uncertainties for the fading even for 9 days after irradiation. These preliminary results are motivating, and as the next step, we intend to design a postal dosimetry system using the PE bags of Fricke solution.

Technical Note: Fricke dosimetry for blood irradiators

PURPOSE

The Fricke dosimeter has been shown to be a viable option as an absorbed dose standard. This work aims to provide the dose distribution in an irradiator container during blood irradiation using Fricke dosimetry.

METHODS

Measurements were performed using a Gammacell Elan 3000 blood irradiator at Hemocenter in Rio de Janeiro, Brazil. A specific phantom was constructed and patented by the authors to perform these measurements. Fricke solution was prepared according to international protocols, and polyethylene bags filled with Fricke solution (n = 19) were spatially distributed within the phantom. Control bags were also submitted to the same process, except the irradiation. The irradiation time was calculated to give 25.7 Gy to the central portion of the phantom, the same dose used for blood bags.

RESULTS

Encouraging results were obtained with an overall uncertainty of 2.1% (k = 1). The obtained results were compared with the doses calculated by the physicist from Hemocenter based on parameters provided by the manufacturer. The mean dose delivered to the Fricke bag in the center of the phantom (cavity 2) was 28.7 ± 0.5 Gy, which is 12% higher than the planned dose of 25.7 Gy.

CONCLUSIONS

The obtained results showed that the setup (Fricke and phantom) is able to perform dosimetry for blood irradiators. The delivered dose was higher than expected. This highlights the importance in controlling all the parameters during irradiation to ensure the correct dose for all irradiated bags.

Determination of the absorbed dose to water for medium-energy x-ray beams using Fricke dosimetry

PURPOSE

For x-ray beams in the low and medium energy range, reference dosimetry is established in terms of air kerma. Fricke dosimetry has shown great potential in the absolute measurements of the absorbed dose to water for high-energy ranges. Therefore, the main purpose of this work was to compare the absorbed dose to water for medium-energy x-ray beams obtained through Fricke dosimetry with that obtained from the air kerma rate.

METHODS

To determine the absorbed dose to water using Fricke dosimetry, the polyethylene bags methodology was chosen. Fricke solution was irradiated at four different beam qualities. The absorbed dose to water values obtained using Fricke dosimetry were compared to those obtained using the standard protocol, using the Z-score.

RESULTS

Values of the Z-score were <2 for all measurements of absorbed dose to water, which means that the values obtained using Fricke dosimetry are equivalent to those obtained using the reference protocol. The combined standard uncertainty for the absorbed dose to water obtained by Fricke dosimetry was lower than that obtained with the ionization chamber.

CONCLUSIONS

Chemical dosimetry using a standard FeSO₄ solution has been demonstrated to be a potential option as a standard for the quantity absorbed dose to water for medium kV x-ray qualities.