SU-E-T-467: Monte Carlo Dosimetric Study of the New Flexisource Co-60 High Dose Rate Source

PURPOSE

Recently, a new HDR 60Co brachytherapy source, Flexisource Co-60, has been developed (Nucletron B.V.). This study aims to obtain quality dosimetric data for this source for its use in clinical practice as required by AAPM and ESTRO.

METHODS

Penelope2008 and GEANT4 Monte Carlo codes were used to dosimetrically characterize this source. Water composition and mass density was that recommended by AAPM. Due to the high energy of the 60Co, dose for small distances cannot be approximated by collisional kerma. Therefore, we have considered absorbed dose to water for r<0.75 cm and collisional kerma from 0.75<r<20 cm. To provide adequate spatial resolution, cells were 0.01 cm in thickness for r<2 cm from the source and a factor of 10 thicker for 2<r<20 cm respectively. Angular sampling was taken every 2°. Additional simulations were performed to obtain SK as recommended by AAPM. Mass-energy absorption coefficients in water and air were consistently derived and used to calculate collisional kerma. Along-away tables and TG-43 formalism parameters and functions were derived. Dosimetric data were also provided following the primary and scatter dose separation for the collapsed cone technique.

RESULTS

TG-43 dosimetry parameters with L = 0.35 cm were obtained. Results performed with both radiation transport codes showed agreement typically within 0.2% for r > 0.8 cm and up to 2% closer to the source. Using Penelope2008 and GEANT4, an average of Î> = 1.085±0.003 cGy/(h U) (with k = 1, Type A uncertainties) was obtained. Dose rate constant, radial dose function and anisotropy functions for the Flexisource Co-60 are compared with published data for other Co-60 sources.

CONCLUSIONS

Dosimetric data are provided for the new Flexisource Co-60 source not studied previously in the literature. Using the data provided by this study in the treatment planning systems, it can be used in clinical practice. This project has been funded by Nucletron BV.

Impact of the Tiloop Bra® mesh in CT images and dose delivery in breast radiotherapy

A new titanized breast mesh, TiLOOP Bra, is currently available for implantation in patients who require radiotherapy. The purpose of this work is to study the dosimetric effect of the presence of a TiLOOP Bra mesh on breast radiation treatment and radiographic imaging. The dosimetric effects have been measured for three X‐ray energies: 1.25 MeV, 6 MV and 18 MV, using radiochromic films placed at three different depths. These depths are representative of mesh location in breast during the radiotherapy treatment and hence, are of interest in this study. In order to assess the disturbance in a radiographic image, different computed tomographic (CT) studies of the mesh have been performed. The absorbed dose differences with and without the mesh are less than 1%. No metallic artifacts have been observed in radiographic images associated with the mesh, nor significant disturbances in Hounsfield Units. The TiLOOP Bra mesh does not disturb the dosimetry of a typical radiotherapy treatment and its influence in the quality of the CT scan required for planning is negligible.

PACS number: 87.55.‐x

Dosimetry comparison between TG-43 and Monte Carlo calculations using the Freiburg flap for skin high-dose-rate brachytherapy

PURPOSE

The purpose of this work was to evaluate whether the delivered dose to the skin surface and at the prescription depth when using a Freiburg flap applicator is in agreement with the one predicted by the treatment planning system (TPS) using the TG-43 dose-calculation formalism.

METHODS AND MATERIALS

Monte Carlo (MC) simulations and radiochromic film measurements have been performed to obtain dose distributions with the source located at the center of one of the spheres and between two spheres. Primary and scatter dose contributions were evaluated to understand the role played by the scatter component. A standard treatment plan was generated using MC- and TG-43-based TPS applying the superposition principle.

RESULTS

The MC model has been validated by performing additional simulations in the same conditions but transforming air and Freiburg flap materials into water to match TG-43 parameters. Both dose distributions differ less than 1%. Scatter defect compared with TG-43 data is up to 15% when the source is located at the center of the sphere and up to 25% when the source is between two spheres. Maximum deviations between TPS- and MC-based distributions are of 5%.

CONCLUSIONS

The deviations in the TG-43-based dose distributions for a standard treatment plan with respect to the MC dose distribution calculated taking into account the composition and shape of the applicator and the surrounding air are lower than 5%. Therefore, this study supports the validity of the TPS used in clinical practice.

Radiation leakage study for the Valencia applicators

INTRODUCTION AND PURPOSE

The Valencia applicators which are accessories of the microSelectron-HDR afterloader (Nucletron, Veenendaal, The Netherlands) are designed to treat skin lesions. These cup-shaped applicators are an alternative to superficial/orthovoltage x-ray treatment units. They limit the irradiation to the required area using tungsten-alloy shielding, and are equipped with a tungsten-alloy flattering filter allowing the treatment of skin tumors, the oral cavity, vaginal cuff, etc. The tungsten-alloy thickness to shield radiation is not the same in all parts of the applicators. This fact led us to question whether the leakage radiation differs depending on where it is measured, and whether this may be relevant in some clinical cases. The purpose of this work is to study from the radiation protection point of view the radiation leakage of the Valencia applicators, and provide a solution for current users and for the manufacturer.

METHODS AND MATERIALS

Simulations based on the Monte Carlo (MC) method using the Geant4 code have been realized studying the dose rate distribution in air around the cup of the Valencia applicators. An experimental study with radiochromic film has also been done to measure the dose distribution in the back side of the applicators and to compare it with MC results.

RESULTS AND CONCLUSIONS

Radiation leakage of up to 170% of the prescribed dose has been found at the back surface of these applicators. Although this side is not usually directed to the patient, in some applications such as the treatment of a lesion on the nose, special care must be taken to avoid unexpected and unnecessary irradiation of the eyes. A possible solution could be to add additional shielding to the applicator in order to reduce this leakage or to put some shielding to protect the eyes. Additionally, a new concept design of the Valencia applicators using more shielding material in the applicator backside is proposed.

QA of dynamic MLC based on EPID portal dosimetry

PURPOSE

Dynamic delivery of intensity modulated beams (dIMRT) requires not only accurate verification of leaf positioning but also a control on the speed of motion. The latter is a parameter that has a major impact on the dose delivered to the patient. Time consumed in quality assurance (QA) procedures is an issue of relevance in any radiotherapy department. Electronic portal imaging dosimetry (EPID) can be very efficient for routine tests. The purpose of this work is to investigate the ability of our EPID for detecting small errors in leaf positioning, and to present our daily QA procedures for dIMRT based on EPID.

METHODS AND MATERIALS

A Varian 2100 CD Clinac equipped with an 80 leaf Millennium MLC and with amorphous silicon based EPID (aS500, Varian) is used. The daily QA program consists in performing: Stability check of the EPID signal, Garden fence test, Sweeping slit test, and Leaf speed test.

RESULTS AND DISCUSSION

The EPID system exhibits good long term reproducibility. The mean portal dose at the centre of a 10 × 10 cm(2) static field was 1.002 ± 0.004 (range 1.013-0.995) for the period evaluated of 47 weeks. Garden fence test shows that leaf position errors of up to 0.2 mm can be detected. With the Sweeping slit test we are able to detect small deviations on the gap width and errors of individual leaves of 0.5 and 0.2 mm. With the Leaf speed test problems due to motor fatigue or friction between leaves can be detected.

CONCLUSIONS

This set of tests takes no longer than 5 min in the linac treatment room. With EPID dosimetry, a consistent daily QA program can be applied, giving complete information about positioning/speed MLC.

TG-43 U1 based dosimetric characterization of model 67-6520 Cs-137 brachytherapy source

PURPOSE

Brachytherapy treatment has been a cornerstone for management of various cancer sites, particularly for the treatment of gynecological malignancies. In low dose rate brachytherapy treatments, 137Cs sources have been used for several decades. A new 137Cs source design has been introduced (model 67-6520, source B3-561) by Isotope Products Laboratories (IPL) for clinical application. The goal of the present work is to implement the TG-43 U1 protocol in the characterization of the aforementioned 137Cs source.

METHODS

The dosimetric characteristics of the IPL 137Cs source are measured using LiF thermoluminescent dosimeters in a Solid Water phantom material and calculated using Monte Carlo simulations with the GEANT4 code in Solid Water and liquid water. The dose rate constant, radial dose function, and two-dimensional anisotropy function of this source model were obtained following the TG-43 U1 recommendations. In addition, the primary and scatter dose separation (PSS) formalism that could be used in convolution/superposition methods to calculate dose distributions around brachytherapy sources in heterogeneous media was studied.

RESULTS

The measured and calculated dose rate constants of the IPL 137Cs source in Solid Water were found to be 0.930 (+/-7.3%) and 0.928 (+/-2.6%) cGy h(-1) U(-1), respectively. The agreement between these two methods was within our experimental uncertainties. The Monte Carlo calculated value in liquid water of the dose rate constant was null set=0.948 (+/-2.6%) cGy h(-1) U(-1). Similarly, the agreement between measured and calculated radial dose functions and the anisotropy functions was found to be within +/-5%. In addition, the tabulated data that are required to characterize the source using the PSS formalism were derived.

CONCLUSIONS

In this article the complete dosimetry of the newly designed 137Cs IPL source following the AAPM TG-43 U1 dosimetric protocol and the PSS formalism is provided.