Accurate localization of intracavitary brachytherapy applicators from 3D CT imaging studies

PURPOSE: To present an accurate method to identify the positions and orientations of intracavitary (ICT) brachytherapy applicators imaged in 3D CT scans, in support of Monte Carlo photon-transport simulations, enabling accurate dose modeling in the presence of applicator shielding and interapplicator attenuation. MATERIALS AND METHODS: The method consists of finding the transformation that maximizes the coincidence between the known 3D shapes of each applicator component (colpostats and tandem) with the volume defined by contours of the corresponding surface on each CT slice. We use this technique to localize Fletcher-Suit CT-compatible applicators for three cervix cancer patients using post-implant CT examinations (3 mm slice thickness and separation). Dose distributions in 1-to-1 registration with the underlying CT anatomy are derived from 3D Monte Carlo photon-transport simulations incorporating each applicator's internal geometry (source encapsulation, high-density shields, and applicator body) oriented in relation to the dose matrix according to the measured localization transformations. The precision and accuracy of our localization method are assessed using CT scans, in which the positions and orientations of dense rods and spheres (in a precision-machined phantom) were measured at various orientations relative to the gantry. RESULTS: Using this method, we register 3D Monte Carlo dose calculations directly onto post insertion patient CT studies. Using CT studies of a precisely machined phantom, the absolute accuracy of the method was found to be +/-0.2 mm in plane, and +/-0.3 mm in the axial direction while its precision was +/-0.2 mm in plane, and +/-0.2 mm axially. CONCLUSION: We have developed a novel, and accurate technique to localize intracavitary brachytherapy applicators in 3D CT imaging studies, which supports 3D dose planning involving detailed 3D Monte Carlo dose calculations, modeling source positions, shielding and interapplicator shielding, accurately.     

Dosimetric characterization of a novel intracavitary mold applicator for 192Ir high dose rate endorectal brachytherapy treatment

The dosimetric properties of a novel intracavitary mold applicator for 192Ir high dose rate (HDR) endorectal cancer treatment have been investigated using Monte Carlo (MC) simulations and experimental methods. The 28 cm long applicator has a flexible structure made of silicone rubber for easy passage into cavities with deep-seated tumors. It consists of eight source catheters arranged around a central cavity for shielding insertion, and is compatible for use with an endocavitary balloon. A phase space model of the HDR source has been validated for dose calculations using the GEANT4 MC code. GAFCHROMIC EBT model film was used to measure dose distributions in water around shielded and unshielded applicators with two loading configurations, and to quantify the shielding effect of a balloon injected with an iodine solution (300 mg I/mL). The film calibration procedure was performed in water using an 192Ir HDR source. Ionization chamber measurements in a Lucite phantom show that placing a tungsten rod in the applicator attenuates the dose in the shielded region by up to 85%. Inserting the shielded applicator into a water-filled balloon pushes the neighboring tissues away from the radiation source, and the resulting geometric displacement reduces the dose by up to 53%; another 8% dose reduction can be achieved when the balloon is injected with an iodine solution. All experimental results agree with the GEANT4 calculations within measurement uncertainties.     

A dual-energy technique for enhanced localization accuracy in intracavitary brachytherapy

The orthogonal imaging method is commonly used for source localization in brachytherapy. In some cases, however, difficulty is encountered in determining the dummy sources because of the presence of either contrast materials or bony structures. We here offer a novel method for source localization utilizing a dual-energy, radiographic technique. In this approach, two sets of orthogonal radiographic images (anterior-posterior and lateral views) are obtained using two different x-ray energies. Image processing (i.e., subtraction between two image sets) is carried out to enhance the source image. In a study performed using a laboratory developed pelvic phantom, it was demonstrated that the dual-energy method could significantly enhance the image quality of the dummy sources, and improve the achievable precision and relative accuracy in localization of source positions. When directly combined with digital imaging modalities, the dual-energy method can be a useful technique to improve the accuracy in brachytherapy source localization from planar radiographs.     

Optimized source selection for intracavitary low dose rate brachytherapy

A procedure has been developed for automating optimal selection of sources from an available inventory for the low dose rate brachytherapy, as a replacement for the conventional trial-and-error approach. The method of optimized constrained ratios was applied for clinical source selection for intracavitary Cs-137 implants using Varian BRACHYVISION software as initial interface. However, this method can be easily extended to another system with isodose scaling and shaping capabilities. Our procedure provides optimal source selection results independent of the user experience and in a short amount of time. This method also generates statistics on frequently requested ideal source strengths aiding in ordering of clinically relevant sources.     

Dose perturbation induced by radiographic contrast inside brachytherapy balloon applicators

Phantom measurements and Monte Carlo calculations have been performed for the purpose of characterizing the dose perturbation caused by radiographic contrast inside the MammoSite breast brachytherapy applicator. Specifically, the dose perturbation is quantified as a heterogeneity correction factor (HCF) for various balloon radii and contrast concentration levels. The dose perturbation is larger for larger balloon radii and higher contrast concentrations. Based on a validated Monte Carlo simulation, the calculated HCF values are 0.99 for a 2 cm radius balloon and 0.98 for a 3 cm radius balloon at 6% contrast concentration levels, and 0.89 and 0.87 for 2 and 3 cm radius balloons, respectively, at 100% contrast concentrations. For a typical implanted balloon radius of 2.4 cm, the HCF values decrease from 0.99 at 6% contrast concentration to 0.90 at 100% contrast concentration. For balloons implanted in patients at our institution, the mean HCF is 0.99, corresponding to a dose reduction of approximately 1%. The contrast effect results in a systematic reduction in the delivered dose, therefore the minimal amount of radiographic contrast necessary should be used.     

Dosimetry of 192Ir sources used for endovascular brachytherapy

An in-phantom calibration technique for 192Ir sources used for endovascular brachytherapy is presented. Three different source lengths were investigated. The calibration was performed in a solid phantom using a Farmer-type ionization chamber at source to detector distances ranging from 1 cm to 5 cm. The dosimetry protocol for medium-energy x-rays extended with a volume-averaging correction factor was used to convert the chamber reading to dose to water. The air kerma strength of the sources was determined as well. EGS4 Monte Carlo calculations were performed to determine the depth dose distribution at distances ranging from 0.6 mm to 10 cm from the source centre. In this way we were able to convert the absolute dose rate at 1 cm distance to the reference point chosen at 2 mm distance. The Monte Carlo results were confirmed by radiochromic film measurements, performed with a double-exposure technique. The dwell times to deliver a dose of 14 Gy at the reference point were determined and compared with results given by the source supplier (CORDIS). They determined the dwell times from a Sievert integration technique based on the source activity. The results from both methods agreed to within 2% for the 12 sources that were evaluated. A Visual Basic routine that superimposes dose distributions, based on the Monte Carlo calculations and the in-phantom calibration, onto intravascular ultrasound images is presented. This routine can be used as an online treatment planning program.     

The development of intracavitary ultrasonic applicators for hyperthermia: a design and experimental study

This study investigated the design concepts and development of a multielement intracavitary ultrasound applicator for use in hyperthermia. A necessary condition imposed on these applicators is that each transducer element be separately powered and produce collimated beams. This way, the power deposition within the target volume can be controlled by varying the power to each element. Theoretical computer simulations (acoustic and thermal) and bench experiments were used to determine the constraints on the transducer element size and the spacing between them. These have shown that the length of the cylindrical segments (or subsections of) must be greater than approximately 10 lambda for proper collimation and that the spacing between them must be less than approximately 1.5 mm for uniform heating. With these design principles in mind, applicators were constructed using sections of cylindrical transducers (wall-thickness resonance). These were surrounded by temperature-controlled circulating water which was enclosed by a latex membrane. This allowed for acoustic coupling and additional control over the depth of the maximum temperature from the cavity wall. This depth could be varied between the cavity surface and up to 1.5 cm for circulating water temperatures between 5 and 42 degrees C, respectively. These applicators were tested in vivo and were able to induce controlled transrectal heating, at depths of 2-3 cm, in the canine rectum and prostate gland.     

Transit dose contributions to intracavitary and interstitial PDR brachytherapy treatments

The objective of this study was to determine the magnitude of transit dose contributions to the planned dose in common intracavitary and interstitial brachytherapy treatments delivered using a pulsed dose rate (PDR) remote afterloader. The total transit dose arises from the travel of the radiation source into (entry) and out of (exit) the applicator, and between the dwell positions (inter-dwell). In this paper, we used a well-type ionization chamber to measure the transit dose component for a PDR afterloader and compared the results against measurements for a high dose rate (HDR) afterloader. Our results show that for typical intracavitary and interstitial treatments, the major contribution to transit dose is from the entry+exit source travel, as the inter-dwell component is effectively compensated for (<0.5%) by the afterloader. The transit dose was generally found to be larger for PDR treatments than for HDR treatments, as it is influenced by the source activity, dwell times and number of radiation pulses. The overall increase in the planned dose contributed by the transit dose in a typical intracavitary PDR treatment was estimated to be <2%, but much higher for interstitial treatments. This study shows that the effect of the transit dose on common clinical intracavitary PDR brachytherapy treatments is practically negligible, but requires attention in highly fractionated large volume interstitial treatments.     

Beta versus gamma dosimetry close to Ir-192 brachytherapy sources

The relative importance of the dose rate component owing to the beta spectrum emitted by 192Ir brachytherapy sources at the short radial distances of interest in intravascular and endobronchial applications is investigated. Separate dosimetric calculations, using Monte Carlo simulations, were performed for the gamma and beta dose rate components of an 192Ir ideal point source as well as real 192Ir source designs used in clinical practice including wire and seed sources and both Nucletron and Varian, old and new, high dose rate (HDR) source designs. A significant dose rate enhancement due to the beta spectrum emitted by 192Ir, greater than 50% for radial distances r<2 mm, was observed for an ideal point source. For real source designs, however, the magnitude of this enhancement was found to depend strongly on the sources' geometric as well as compositional details of the active core and encapsulation. A detectable effect was found for the majority of the investigated sources at radial distances less than 1 mm, but overall findings suggest that the contribution of beta particles is not significant in 192Ir clinical intravascular applications that are currently carried out. However, since treatment of vessels with smaller diameters, in the future, may lead to the development of 192Ir sources and catheters of reduced diameters, the potential effect of the beta spectrum in terms of dose enhancement to tissues in close proximity to 192Ir sources should not be ignored.     

Size and positioning reproducibility of an 192Ir brachytherapy stepping source

In this paper we describe techniques for measuring the dimensions and position reproducibility of an 192Ir brachytherapy stepping source. Measurements were carried out using a 0.25x10x152 mm3 collimator placed in front of a detector of our own design. The brachytherapy source was translated past the collimator in 0.025 mm increments using a stepper motor. The source was found to be 3.58 mm long and 0.69 mm wide, which is in good agreement with the manufacturer's values of 3.5x0.6 mm2. The source position was reproducible to within 0.12 mm.     

Dose distributions produced by shielded applicators using 241Am for intracavitary irradiation of tumors in the vagina.

Dosimetric characteristics of shielded vaginal applicators containing encapsulated 241Am sources are investigated in this work. Encapsulated 241Am sources emit primarily 60-keV photons which are more effectively shielded by thin layers of high atomic number materials than the 662-keV photons from 137Cs sources. With 241Am, it is possible to achieve almost unidirectional irradiation of localized vaginal tumors. The drastic decrease in irradiation volume on the contralateral side (uninvolved with tumor) is observed to decrease dose by up to 20%, even in the forward direction (unshielded side toward the tumor) of the applicator. A possible explanation for the observed effects of shields in both the forward and backward directions is the reduction of scattered photon fluence due to absorption of photons in the lead shield via photoelectric effect. Current theoretical models do not include this perturbation effect caused by shields on brachytherapy applicators.     

Radiochromic film measurement of anisotropy function for high-dose-rate Ir-192 brachytherapy source

The dose distribution produced by the high-dose-rate (HDR) 192Ir source is inherently anisotropic due to self-absorption by the high-density source core, oblique filtration by the source capsule and the asymmetric geometry of the source capsule. To account for the dose distribution anisotropy of brachytherapy sources, AAPM Task Group No 43 has included a two-dimensional anisotropy function, F(r, theta), in the recommended dose calculation formalism. Gafchromic HS radiochromic film (RCF) was used to measure anisotropy function for microSelectron HDR 192Ir source (classic/old design). Measurements were carried out in a water phantom using specially fabricated PMMA cylinders at radial distances 1, 2, 3, 4 and 5 cm. The data so generated are comparable to both experimental and Monte Carlo calculated values for this source reported earlier by other authors. The RCF method described in this paper is comparatively high resolution, simple to use and is a general method, which can be applied for other brachytherapy sources as well.     

Transit dose of an Ir-192 high dose rate brachytherapy stepping source

Clinical dosimetry for high dose rate (HDR) brachytherapy with a single stepping source generally neglects the transit dose. This study investigates the effects of the transit dose in the target volume of an HDR brachytherapy stepping source. A video method was used to analyse the entrance, exit and the interdwell transit speed of the source for different path lengths and step sizes ranging from 2.5 mm to 995 mm. The transit speed was found to vary with the step size and path length. For the travelled distances of 2.5, 5.0, 10.0, 230 and 995 mm, the average transit speeds were 54, 72, 233, 385 and 467 mm s(-1) respectively. The results also show that the manufacturer has attempted to compensate for the effects of interdwell transit dose by reducing the actual dwell time of the source. A well-type chamber was used to determine the dose differences between two sets of measurements, one being the stationary dose only and the other being the sum of stationary and transit doses. Single catheters of active lengths of 20 and 40 mm, different dwell times of 0.5, 1, 2 and 5 s and different step sizes of 2.5, 5 and 10 mm were used in the measurements with the well-type chamber. Most of the measured dose differences between stationary and stationary plus interdwell source movement were within 2%. The additional dose due to the source transit can be as high as 24.9% for the case of 0.5 s dwell time, 10 mm step size and 20 mm active length. The dose difference is mainly due to the entrance and exit source movement but not the interdwell movement.     

Film dosimetry calibration method for pulsed-dose-rate brachytherapy with an 192Ir source

192Ir sources have been widely used in clinical brachytherapy. An important challenge is to perform dosimetric measurements close to the source despite the steep dose gradient. The common, inexpensive silver halide film is a classic two-dimensional integrator dosimeter and would be an attractive solution for these dose measurements. The main disadvantage of film dosimetry is the film response to the low-energy photon. Since the photon energy spectrum is known to vary with depth, the sensitometric curves are expected to be dependent on depth. The purpose of this study is to suggest a correction method for silver halide film dosimetry that overcomes the response changes at different depths. Sensitometric curves have been obtained at different depths with verification film near a 1 Ci 192Ir pulsed-dose-rate source. The depth dependence of the film response was observed and a correction function was established. The suitability of the method was tested through measurement of the radial dose profile and radial dose function. The results were compared to Monte Carlo-simulated values according to the TG43 formalism. Monte Carlo simulations were performed separately for the beta and gamma source emissions, using the EGS4 code system, including the low-energy photon and electron transport optimization procedures. The beta source emission simulation showed that the beta dose contribution could be neglected and therefore the film-depth dependence could not be attributed to this part of the source radioactivity. The gamma source emission simulations included photon-spectra collection at several depths. The results showed a depth-dependent softening of the photon spectrum that can explain the film-energy dependence.     

A standard dosimetry procedure for 192Ir sources used for endovascular brachytherapy

The experimental dosimetry of a high dose rate (HDR) 192Ir source used for the brachytherapy of peripheral vessels is reported. The direct determination of the reference air kerma rate Kr agrees, within the experimental uncertainty, with the results obtained by a well ionization chamber calibrated at the NIST and the manufacturer's certification. A highly sensitive (HS) radiochromic film (RCF), that presents only one active layer, was used for the source dosimetry in a water phantom. The adopted experimental set-up, with the source in its catheter positioned on the RCF plane, seems to have given better accuracy of the RCF optical density measurements. The agreement between the measurement of the dose rate constant DKr (10 mm, pi/2) and the literature data confirmed the coherence of the HS RCF calibration obtained by the kerma in air measurements. The RCF measurements supplied dosimetric information about the dose to water per reference air kerma rate D(r, theta)/Kr along the source transverse bisector axis, the radial dose function g(r) and the anisotropy function F(r, theta). The value D(2 mm, pi/2)/Kr = 22.4 +/- 1.2 cGy h(-1)/(microGy h(-1)) is supplied with a dose uncertainty that is essentially due to the indeterminacy of the source position in the catheter. The data of the radial and anisotropy functions have been compared with Monte Carlo determinations reported in the literature.