Comparison of I-125 sources used for permanent interstitial implants

A Monte Carlo investigation of the dose distribution for new I-125 Low Dose Rate brachytherapy source in water and in different media

Permanent and temporary implantation of I-125 brachytherapy sources has become an official method for the treatment of different cancers. In this technique, it is essential to determine dose distribution around the brachytherapy source to choose the optimal treatment plan. In this study, the dosimetric parameters for a new interstitial brachytherapy source I-125 (IrSeed-125) were calculated with GATE/GEANT4 Monte Carlo code. Dose rate constant, radial dose function and 2D anisotropy function were calculated inside a water phantom (based on the recommendations of TG-43U1 protocol), and inside several tissue phantoms around the IrSeed-125 capsule. Acquired results were compared with MCNP simulation and experimental data. The dose rate constant of IrSeed-125 in the water phantom was about 1.038 cGy·h−1U−1 that shows good consistency with the experimental data. The radial dose function at 0.5, 0.9, 1.8, 3 and 7 cm radial distances were obtained as 1.095, 1.019, 0.826, 0.605, and 0.188, respectively. The results of the IrSeed-125 is not only in good agreement with those calculated by other simulation with MCNP code but also are closer to the experimental results. Discrepancies in the estimation of dose around IrSeed-125 capsule in the muscle and fat tissue phantoms are greater than the breast and lung phantoms in comparison with the water phantom. Results show that GATE/GEANT4 Monte Carlo code produces accurate results for dosimetric parameters of the IrSeed-125 LDR brachytherapy source with choosing the appropriate physics list. There are some differences in the dose calculation in the tissue phantoms in comparison with water phantom, especially in long distances from the source center, which may cause errors in the estimation of dose around brachytherapy sources that are not taken account by the TG43-U1 formalism.

Stranded seed displacement, migration, and loss after permanent prostate brachytherapy as estimated by Day 0 fluoroscopy and 4-month postimplant pelvic x-ray

PURPOSE

The aim of the study was to determine the incidence of local displacement, distant seed migration to the chest, and seed loss after permanent prostate brachytherapy (PPB) with stranded seeds (SSs) using sequential two-dimensional fluoroscopic pelvic and chest x-rays.

METHODS AND MATERIALS

Between October 2010 and April 2014, a total of 137 patients underwent PPB and 4-month followup pelvic and chest x-ray imaging. All patients had exclusively SSs placed and an immediate postimplant fluoroscopic image of the seed cluster. Followup x-ray images were evaluated for the number, location, and displacement of seeds in comparison to Day 0 fluoroscopic images. Significant seed displacement was defined as seed displacement >1 cm from the seed cluster. Followup chest x-rays were evaluated for seed migration to the chest.

RESULTS

Seed migration to the chest occurred in 3 of the 137 patients (2%). Seed loss occurred in 38 of the 137 patients (28%), with median loss of one seed (range, 1-16), and total seeds loss of 104 of 10,088 (1.0%) implanted. Local seed displacement was seen in 12 of the 137 patients (8.8%), and total seeds displaced were 0.15% (15/10,088).

CONCLUSIONS

SS placement in PPB is associated with low rates of substantial seed loss, local displacement, or migration to the chest. Comparing immediate postimplant fluoroscopic images to followup plain x-ray images is a straightforward method to supplement quality assurance in PPB and was found to be useful in identifying cases where seed loss was potentially of clinical significance.

A comparison study on various low energy sources in interstitial prostate brachytherapy

PURPOSE

Low energy sources are routinely used in prostate brachytherapy. (125)I is one of the most commonly used sources. Low energy (131)Cs source was introduced recently as a brachytherapy source. The aim of this study is to compare dose distributions of (125)I, (103)Pd, and (131)Cs sources in interstitial brachytherapy of prostate.

MATERIAL AND METHODS

ProstaSeed (125)I brachytherapy source was simulated using MCNPX Monte Carlo code. Additionally, two hypothetical sources of (103)Pd and (131)Cs were simulated with the same geometry as the ProstaSeed (125)I source, while having their specific emitted gamma spectra. These brachytherapy sources were simulated with distribution of forty-eight seeds in a phantom including prostate. The prostate was considered as a sphere with radius of 1.5 cm. Absolute and relative dose rates were obtained in various distances from the source along the transverse and longitudinal axes inside and outside the tumor. Furthermore, isodose curves were plotted around the sources.

RESULTS

Analyzing the initial dose profiles for various sources indicated that with the same time duration and air kerma strength, (131)Cs delivers higher dose to tumor. However, relative dose rate inside the tumor is higher and outside the tumor is lower for the (103)Pd source.

CONCLUSIONS

The higher initial absolute dose in cGy/(h.U) of (131)Cs brachytherapy source is an advantage of this source over the others. The higher relative dose inside the tumor and lower relative dose outside the tumor for the (103)Pd source are advantages of this later brachytherapy source. Based on the total dose the (125)I source has advantage over the others due to its longer half-life.

Studies on the development of ¹⁶⁹Yb-brachytherapy seeds: New generation brachytherapy sources for the management of cancer

This paper describes development of (169)Yb-seeds by encapsulating 0.6-0.65 mm (ϕ) sized (169)Yb2O3 microspheres in titanium capsules. Microspheres synthesized by a sol-gel route were characterized by XRD, SEM/EDS and ICP-AES. Optimization of neutron irradiation was accomplished and (169)Yb-seeds up to 74 MBq of (169)Yb could be produced from natural Yb2O3 microspheres, which have the potential for use in prostate brachytherapy. A protocol to prepare (169)Yb-brachytherapy sources (2.96-3.7 TBq of (169)Yb) with the use of enriched targets was also formulated.

Dosimetric comparison of four new design 103Pd brachytherapy sources: optimal design using silver and copper rod cores

Four new brachytherapy sources, IRA1-103Pd, IRA2-103Pd, IRA3-103Pd, and IRA4-103Pd, have been developed at Agricultural, Medical, and Industrial Research School and are designed for permanent implant application. With the goal of determining an optimal design for a 103Pd source, this article compares the dosimetric properties of these sources with reference to the authors' earlier IRA-103Pd source. The four new sources differ in end cap configuration and thickness and in the core material, silver or copper, that carries the adsorbed 103Pd. Dosimetric data derived from the authors' Monte Carlo simulation results are reported in accordance with the updated AAPM Task Group No. 43 report (TG-43U1). For each source, the authors obtained detailed results for the dose rate constant lambda, the radial dose function g(r), the anisotropy function F(r, theta), and the anisotropy factor phi(an)(r). In this study, the optimal source IRA3-103Pd provides the most isotropic dose distribution in water with the dose rate constant of 0.678(+/-0.1%) cGy h(-1) U(-1). The IRA3-103Pd design has a silver rod core combined with thin-wall, concave end caps. Finally, the authors compared the results for their optimal source with published results for those of other source manufacturers.

Evaluation of the dose distribution for prostate implants using various 125I and 103Pd sources

Recently, several different models of 125I and 103Pd brachytherapy sources have been introduced in order to meet the increasing demand for prostate seed implants. These sources have different internal structures; hence, their TG-43 dosimetric parameters are not the same. In this study, the effects of the dosimetric differences among the sources on their clinical applications were evaluated. The quantitative and qualitative evaluations were performed by comparisons of dose distributions and dose volume histograms of prostate implants calculated for various designs of 125I and 103Pd sources. These comparisons were made for an identical implant scheme with the same number of seeds for each source. The results were compared with the Amersham model 6711 seed for 125I and the Theragenics model 200 seed for 103Pd using the same implant scheme.

Development of a spherical (125)i-brachytherapy seed for its application in the treatment of eye and prostate cancer

Palladium-coated silver beads of 0.5 mm (phi) were used to adsorb (125)I, encapsulated inside a titanium capsule by an Nd:YAG laser, for use as a brachytherapy source. Experimental conditions, such as feed activity, carrier concentration, and reaction time, were optimized for the maximum adsorption of (125)I. Uniform distribution of radioactivity on the source core was ascertained by the autoradiography technique. Leachability of (125)I was found to be <0.01%. The dose-rate constant of the new source was estimated to be 1.045 cGyh(-1)U(-1). This newly developed source could be an alternative to other (125)I sources.

Comprehensive I-125 multi-seed comparison for prostate brachytherapy: Dosimetry and visibility analysis

PURPOSE

To compare the visibility of different manufacturers I-125, seeds, and to investigate the effect of differences in dosimetry on treatment planning.

MATERIALS AND METHODS

Oncura Oncoseed, Oncura Echoseed, IBT Intersource, Bebig Isoseed and Nucletron Selectseed were investigated. The point dose at increasing distances from each seed type was calculated for three different angles; theta=0 degrees, 45 degrees and 90 degrees (where theta=0 degrees lies parallel to seed length). 10 patient plans were used to assess the effect of a change in dosimetry on treatment planning and quality of prostate and rectum implant indices such as Vp100, Vp200, Dp90, Vr100 and Vr69. All implant indices and dosimetry data were compared to Oncoseed. Visibility under X-ray, fluoroscopy, CT and MRI was investigated using prostate phantoms created in-house. Statistical significance was calculated using paired two-tailed t-tests.

RESULTS

Dosimetric analysis was carried out for seeds of the same source strength. Differences in dose increase closer to the centre of each source, with the largest changes occurring for the angle theta=0 degrees. Selectseed and Isoseed seed types provide a consistently lower dose in all three directions. Changes to Vp100 are small and statistically insignificant for all seeds except Selectseed which shows a statistically significant decrease of 0.04% (p=0.02). Changes to Vp150 and Vp200 are statistically significant (p<0.01), with Intersource showing the greatest increase in both values. Selectseed shows a decrease in both Vp150 and Vp200. Echoseed shows an increase in both Vp150 and Vp200. Changes to D90 are statistically significant (p<0.01), with Intersource showing the greatest increase, followed by Isoseed then Echoseed. Selectseed shows a decrease in D90. For Vr100 there is no statistically significant change for any seed type. However, all seeds except Selectseed show a statistically significant increase in the value of Vr69, with Intersource showing the greatest increase. On fluoroscopy and X-ray images, Intersource seeds appear least visible, Echoseed and Oncoseed are similar, and Isoseed and Selectseed are most visible. Ultrasound greyscale beam profiles show that all seed images have a FWHM larger than the Oncoseed image. The CT greyscale beam profiles are similar for all seed images. The MRI signal voids are similar for all seed images except Intersource which shows a larger signal void.

CONCLUSIONS

The greatest changes to point dose occur at very close distances to the seeds. Changing seed type may require a treatment replan to maintain satisfactory DVH criteria. Visibility on US and CT is similar, though it may vary on MRI, X-ray and fluoroscopy.

On the physical, spectral, and dosimetric characteristics of a new I125 brachytherapy source

A new 125I source under the name Braquibac has been developed in Argentina for interstitial brachytherapy applications. The aim of this work is to study the new seed's design and to calculate its dosimetric parameters. Radiographic and destructive tests were carried out on inactive seeds to determine the physical characteristics of the source. Values of g(r), Lambda, F(r, Theta), and phi(an)(r), were obtained in water and air by simulation using the MCNP5 Monte Carlo code according to the methodology recommended in TG-43 and updated in TG-43U1. The dose rate constant was determined to be 0.937+/-0.004 cGy h(-1) U(-1) (overall statistical uncertainty +/-2.7%). Sk per unity activity was calculated to be 0.671+/-0.003 cGy cm2 h(-1) mCi(-1) by simulation of the seed in dry air using point detectors. Spectroscopic studies for both the new and the Amersham model 6711 seed were performed using an HPGe planar detector. The emission spectra of both seeds proved to be very similar. The anisotropy of the total photon intensity in air was measured in planes containing the seed's short and long axes using the HPGe detector. The minimum photon intensity for the new seed was 31.14+/-3.10% of the transverse intensity.

Multiple‐estimate Monte Carlo calculation of the dose rate constant for a cesium‐131 interstitial brachytherapy seed

The purpose of this study was to calculate a more accurate dose rate constant for the 131Cs (model CS-1, IsoRay Medical, Inc., Richland, WA) interstitial brachytherapy seed. Previous measurements of the dose rate constant for this seed have been reported by others with incongruity. Recent direct measurements by thermoluminescence dosimetry and by gamma-ray spectroscopy were about 15% greater than earlier thermoluminescence dosimetry measurements. Therefore, we set about to calculate independent values by a Monte Carlo approach that combined three estimates as a consistency check, and to quantify the computational uncertainty. The calculated dose rate constant for the 131Cs seed was 1.040 cGy h(-1) U(-1) for an ionization chamber model and 1.032 cGy h(-1) U(-1) for a circular ring model. A formal value of 2.2% uncertainty was calculated for both values. The range of our multiestimate values were from 1.032 to 1.061 cGy h(-1) U(-1). We also modeled three 125I seeds with known dose rate constants to test the accuracy of this study's approach.

Dosimetry of a thyroid uptake detected in seed migration survey following a patient's iodine-125 prostate implant and in vitro measurements of intentional seed leakages

As a quality control procedure, a post-implant seed migration survey has been accomplished on 340 prostate cancer patients since November 2001. Pulmonary seed embolization and intracardiac seed embolization have been detected. A case of thyroid uptake due to leaking iodine-125 (I-125) sources was also seized. In order to determine the dose to the thyroid, a dosimetry method was developed to link in vivo measurements and the cumulated dose to the thyroid. The calculated source leakage half-life in the case was approximately 15 days based on the measurements and the estimated cumulated dose to thyroid was 204 cGy. It is concluded that one seed was leaking. In order to verify the in vivo measurements, intentional in vitro seed leakage tests were performed. A seed was cut open and placed in a sealed glass container filled with a given volume of saline. The I-125 concentration in the saline was subsequently measured over a period of six months. Consistent in vivo and in vitro results were obtained. Recent incidents of seed leaks reported from other centers have drawn practitioners' attention to this problem. In order to make the measurements more useful, the seed leakage tests were expanded to include I-125 seeds from six other vendors. The results show that the leakage half-lives of those seeds varied from nine days to a half-year. Two seed models demonstrated least leakage. Since the measurements lasted for six months, the escape of iodine resulted from oxidation of iodide in the saline was a concern for the measurement accuracy. As a reference, another set of leakage tests were performed by adding sodium thiosulfate salt (Na2S2O3 x 5 H2O) to the saline. Sodium thiosulfate is a reducing agent that prevents the conversion of iodide to iodate so as to minimize I-125 evaporation. As a result, significantly shortened leakage half-lives were observed in this group. Seed agitation was also performed and no significant deviations of the leakage rates were observed. Considering the body fluid is more complex than saline, the in vivo leakage half-life, in case a source leak is encountered, may vary significantly from what is presented in this paper due to chemical reactions. In vivo measurements thus may produce a more accurate estimation of leakage half-life and thyroid uptake dose.

Radiation safety with use of I-125 seeds for localization of nonpalpable breast lesions

RATIONALE AND OBJECTIVES

To describe radiation safety procedures for limiting exposures during localizing placement of iodine-125 (I-125) seeds in nonpalpable breast lesions. Radiation safety tasks included seed receipt, assay, sterilization, transfer, and placement; surgical localization and retrieval; and extraction of seeds by pathologists. Additional regulatory aspects included institutional review board approval, physician credentialing, off-label use, and governmental licensing.

MATERIALS AND METHODS

Titanium seeds were assayed to ensure strength (1.85-5.55 MBq). Radiologists credentialed in mammography placed seeds with an 18-gauge needle under ultrasound or mammographic guidance. Surgeons located seeds with a hand-held, solid-state radiation detector. I-125 seeds were extracted from excised tissue and secured by pathologists.

RESULTS

After the investigational phase, state permission was obtained for the institution's Radiation Safety Committee to oversee the clinical application of the procedure. In more than 300 procedures, all seeds and targeted lesions were removed successfully; more than 98% of patients had seeds removed within 24 hours. Mean diameter of excised specimens was about 4 cm, for a maximum dose to residual breast tissue of 2 cGy (approximately that of a two-view mammogram). Badge monitoring showed no loss of seeds and no increase in physician or technologist exposure. Radiation safety precautions facilitated safe handling.

CONCLUSION

Clinically, the procedure has been accepted as routine, with standardized steps for safe and secure handling of radioactive seeds. Compared with conventional methods, use of I-125 seeds offers an improved method for localizing before surgical excision and has essentially replaced wire localization in our tertiary-care academic medical center.

Real-time tumor tracking using implanted positron emission markers: Concept and simulation study

The delivery accuracy of radiation therapy for pulmonary and abdominal tumors suffers from tumor motion due to respiration. Respiratory gating should be applied to avoid the use of a large target volume margin that results in a substantial dose to the surrounding normal tissue. Precise respiratory gating requires the exact spatial position of the tumor to be determined in real time during treatment. Usually, fiducial markers are implanted inside or next to the tumor to provide both accurate patient setup and real-time tumor tracking. However, current tumor tracking systems require either substantial x-ray exposure to the patient or large fiducial markers that limit the value of their application for pulmonary tumors. We propose a real-time tumor tracking system using implanted positron emission markers (PeTrack). Each marker will be labeled with low activity positron emitting isotopes, such as 124I, 74As, or 84Rb. These isotopes have half-lives comparable to the duration of radiation therapy (from a few days to a few weeks). The size of the proposed PeTrack marker will be 0.5-0.8 mm, which is approximately one-half the size of markers currently employed in other techniques. By detecting annihilation gammas using position-sensitive detectors, multiple positron emission markers can be tracked in real time. A multimarker localization algorithm was developed using an Expectation-Maximization clustering technique. A Monte Carlo simulation model was developed for the PeTrack system. Patient dose, detector sensitivity, and scatter fraction were evaluated. Depending on the isotope, the lifetime dose from a 3.7 MBq PeTrack marker was determined to be 0.7-5.0 Gy at 10 mm from the marker. At the center of the field of view (FOV), the sensitivity of the PeTrack system was 240-320 counts/s per 1 MBq marker activity within a 30 cm thick patient. The sensitivity was reduced by 45% when the marker was near the edge of the FOV. The scatter fraction ranged from 12% (124I, 74As) to 16% (84Rb). In addition, four markers (labeled with 124I) inside a 30 cm diameter water phantom were simulated to evaluate the feasibility of the multimarker localization algorithm. Localization was considered successful if a marker was localized to within 2 mm from its true location. The success rate of marker localization was found to depend on the number of annihilation events used and the error in the initial estimate of the marker position. By detecting 250 positron annihilation events from 4 markers (average of 62 events per marker), the marker success rates for initial errors of +/-5, +/-10, and +/-15 mm were 99.9%, 99.6%, and 92.4%, respectively. Moreover, the average localization error was 0.55 (+/-0.27) mm, which was independent of initial error. The computing time for localizing four markers was less than 20 ms (Pentium 4, 2.8 GHz processor, 512 MB memory). In conclusion, preliminary results demonstrate that the PeTrack technique can potentially provide real-time tumor tracking with low doses associated with the marker's activity. Furthermore, the small size of PeTrack markers is expected to facilitate implantation and reduce patient risk.

Studies on the production and quality assurance of miniature 125I radioactive sources suitable for treatment of ocular and prostate cancers

(125)I sources were prepared by adsorption of (125)I on palladium-coated silver wires. The effect of reducing agent on percentage adsorption of (125)I was studied and the amount of adsorbed activity on source core was studied by repeated adsorption cycles. The activity per source in the sources produced from the same batch varied with coefficient of variation (i.e. the ratio of standard deviation to mean multiplied by 100) less than 10%. The unencapsulated source exhibited low leachability (< 0.01%). The laser parameters were optimized to obtain quality welds with negligible leak rate. The sources were laser-encapsulated in titanium capsules of 0.8 mm (OD) x 4.5mm (l). The release of radioactivity from encapsulated sources in an immersion test at 50 degrees C for 5 h was <5 nCi (185 Bq). The surface contamination on the sealed capsules was found to be<0.05 nCi or 1.85 Bq per source. The sources were used in the treatment of a child having retinoblastoma.

Evaluation of an automated seed loader for seed calibration in prostate brachytherapy

Automated seed loaders for permanent prostate implants are now commercially available. Besides improved radiation safety, these systems offer seed assay capability and ease of needle loading, making preplanned as well as intra‐operative implant procedures more time‐efficient. The Isoloader (Mentor Corp., CA) uses individual I125 seeds (SL‐125 ProstaSeed) loaded in up to 199 chambers inside a shielded cartridge. The unit performs seed counting and calibration using a built‐in solid‐state detector. In order to evaluate the reproducibility and accuracy of the calibration process, two test cartridges were measured with the Isoloader itself and compared with a well‐type ionization chamber (HDR‐1000Plus, Standard Imaging).

The air kerma strength measurements for all seeds using the Isoloader had a standard deviation of about 2.7%. For the eight seeds assayed more intensively using both the Isoloader and well chamber, the standard deviations of the measurements for each seed were in the range of 0.8% to 2.8% and 0.6% to 1.3%, respectively. The variation in the Isoloader calibration is attributed to small detector solid angle and bead geometry within seed capsules (verified by radiographs). The reproducibility of the air kerma strength measured by the Isoloader was comparable to that from the well chamber and was clinically acceptable. Seed strength measured with the Isoloader was on average 1%~2% larger than that measured with the well chamber, indicating that the accuracy of the Isoloader was clinically acceptable.

PACS numbers: 87.53.Jw, 87.53.Xd, 87.56.Fc

Prostatic edema in I125 permanent prostate implants: Dynamical dosimetry taking volume changes into account

The purpose of this study is to determine the impact of edema on the dose delivered to the target volume. An evaluation of the edema characteristics was first made, and then a dynamical dosimetry algorithm was developed and used to compare its results to a standard clinical (static) dosimetry. Source positions and prostate contours extracted from 66 clinical cases on images taken at different points in time (planning, implant day, post-implant evaluation) were used, via the mean interseed distance, to characterize edema [initial increase (deltar0), half-life (tau)]. An algorithm was developed to take into account the edema by summing a time series of dose-volume histograms (DVHs) with a weight based on the fraction of the dose delivered during the time interval considered. The algorithm was then used to evaluate the impact of edema on the dosimetry of permanent implants by comparing its results to those of a standard clinical dosimetry. The volumetric study yielded results as follows: the initial prostate volume increase was found to be 1.58 (ranging from 1.15 to 2.48) and the edema half-life, approximately 30 days (range: 3 to 170 days). The dosimetric differences in D90 observed between the dynamic dosimetry and the clinical one for a single case were up to 15 Gy and depended on the edema half-life and the initial volume increase. The average edema half-life, 30 days, is about 3 times longer than the previously reported 9 days. Dosimetric differences up to 10% of the prescription dose are observed, which can lead to differences in the quality assertion of an implant. The study of individual patient edema resorption with time might be necessary to extract meaningful clinical correlation or biological parameters in permanent implants.

Treatment planning consideration for prostate implants with the new linear RadioCoil 103Pd brachytherapy source

Recently, various linear source models, for example, 103Pd RadioCoil, have been introduced to overcome the shortcomings of traditional "seed" type interstitial prostate brachytherapy implants, such as migration and clumping of the seeds. However, the existing prostate treatment-planning systems have not been updated to perform dose calculation for implants with linear sources greater than 1.0 cm in length. In these investigations, two new models are developed for 3D dose calculation for a prostate implant with linear brachytherapy sources using the commercially available treatment-planning systems. The proposed models are referred to as the linear-segmented source (LSS) model and the point-segmented source (PSS) model. The calculated dose distributions obtained by these models for a single linear source have been validated by their comparison with the Monte Carlo-simulated data. Moreover, these models were used to calculate the dose distributions for a multilinear source prostate implant, and the results were compared to "seed" type implants. The results of these investigations show that the LSS model better approximates the linear sources than the PSS model. Moreover, these models have shown a better approximation of the dose distribution from a linear source for 0.5 cm source segments as compared to 1.0 cm source segments. However, for the points close to the longitudinal axis of the source located outside the region bounded by the active length, both models show differences of approximately +/-15%. These deficiencies are attributed to the limitations of the TG43 formalism for elongated sources.

Dosimetry parameters of BARC OcuProsta I-125 seed source

A new model of 125I seed source, named OcuProsta seed, was designed and fabricated by Radiopharmaceuticals Division of Bhabha Atomic Research Centre for ophthalmic and interstitial applications. AAPM TG 43 recommended dosimetry parameters for this seed source were determined experimentally using TLD as well as by Monte Carlo (MC) simulation. Measured and MC calculated values of the dose rate constant (DRC) are 0.95 +/- 0.065 cGyh(-1)U(-1) and 0.972 +/- 0.005 cGyh(-1)U(-1), respectively. The mean of measured and calculated DRC (lambda = 0.96 cGyh(-1)U(-1)) was recommended for the clinical dosimetry of OcuProsta seed. Measured and MC calculated radial dose function, g(r), anisotropy function, F(r,theta), anisotropy factor and anisotropy constants are also found to be in good agreement to each other. Dosimetry parameters of OcuProsta seed were compared with the published values of similar in-design 125I seed sources. The DRC of BARC OcuProsta seed is very close to Amersham 6711 seed and is also comparable to the DRC of Best model 2301, Syncor PharmaSeed and Isotron selectSeed within the uncertainty of measurement/calculation. The g(r) of OcuProsta seed shows a difference of up to 10% in comparison to the g(r) values of the similar in-design seed sources. The values of anisotropy function of OcuProsta are 7-13% different from the anisotropy function of Amersham 6711 and Syncor PharmaSeed. The anisotropy constant of OcuProsta is close to Amersham 6711 seed while it is about 9% smaller than the anisotropy constant of Best model 2301 and Synchor PharmaSeed.

144Ce as a potential candidate for interstitial and intravascular brachytherapy

PURPOSE

To investigate the suitability of (144)Ce for both interstitial and intravascular brachytherapy applications.

METHODS AND MATERIALS

Monte Carlo calculations of radial dose rate distributions in water were performed for (144)Ce in a spring-shaped source and compared with two commonly used interstitial and intravascular sources, (192)Ir and (32)P. The numeric simulations were checked experimentally with a calibrated ionization chamber in a water phantom. Other source characteristics, such as half-life and specific activity, were also compared.

RESULTS

For interstitial brachytherapy, (144)Ce presents dosimetric advantages over (192)Ir in terms of higher dose rate at shorter distances and lower irradiation of organs outside the tumor. The source size and shape reduce the anisotropy and the number of dwell positions necessary. The longer half-life of (144)Ce might also be advantageous over (192)Ir. For intravascular brachytherapy, (144)Ce permits the treatment of larger arteries as compared with (32)P, compensates centering errors more effectively, and has a more suitable half-life. The experimental validation showed good agreement (within 10%) with the Monte Carlo simulated dose rate distributions.

CONCLUSIONS

There are certain potential advantages of using (144)Ce as a source for both interstitial and intravascular brachytherapy. The basis for this finding is provided by the Monte Carlo radial dose rate comparisons with (192)Ir and (32)P, as well as by such characteristics as half-life and specific activity.

Evaluation of the new cesium-131 seed for use in low-energy x-ray brachytherapy

Characterization measurements and calculations were performed on a new medical seed developed by IsoRay Inc. in Richland, Washington, that utilizes the short-lived isotope 131Cs. This model has recently received FDA 510(k) clearance. The objective of this work was to characterize the dosimetric properties of the new seed according to the AAPM Task Group 43 recommendations. Cesium-131 is a low-energy x-ray emitter, with the most prominent peaks in the 29 keV to 34 keV region. The intended application is brachytherapy for treating cancers in prostate, breast, head and neck, lung, and pancreas. The evaluations performed included air-kerma strength, radial dose function, anisotropy in phantom, half-life, energy spectra, and internal activity. The results indicate the CS-1 seeds have a dose-rate constant of 0.915 cGy hr(-1) U(-1) in water, dose penetration characteristics similar to 125I and 103Pd, anisotropy function values on the order of 0.71 at short distances and small angles, and an average anisotropy factor of 0.964. The overall dosimetric characteristics are similar to 125I and 103Pd seeds with the exception of half-life, which is 9.7 days, as compared to 17 days for 103Pd and 60 days for 125I. The shorter half-life may offer significant advantages in biological effectiveness.

Distortions induced by radioactive seeds into interstitial brachytherapy dose distributions

In a previous article, we presented development and verification of an integral transport equation-based deterministic algorithm for computing three-dimensional brachytherapy dose distributions. Recently, we have included fluorescence radiation physics and parallel computation to the standing algorithms so that we can compute dose distributions for a large set of seeds without resorting to the superposition methods. The introduction of parallel computing capability provided a means to compute the dose distribution for multiple seeds in a simultaneous manner. This provided a way to study strong heterogeneity and shadow effects induced by the presence of multiple seeds in an interstitial brachytherapy implant. This article presents the algorithm for computing fluorescence radiation, algorithm for parallel computing, and display results for an 81-seed implant that has a perfect and imperfect lattice. The dosimetry data for a single model 6711 seeds is presented for verification and heterogeneity factor computations using simultaneous and superposition techniques are presented.

The effect of the radial function on I-125 seeds used for permanent prostate implantation

The purpose of this study was to evaluate the integrity of eight commercially-available low-activity Iodine-125 (125I) seeds for their radial function g(r) and its effect on the dose delivered to the adjacent critical structures when used in permanent prostate implants (PPI). Ten previously treated patients were retrospectively used in this comparison. The Amersham Health Oncura seed was used to peripherally design an isodose distribution with urethral and anterior rectal wall sparing. Plan criteria included minimum coverage of 144 Gy to the planning target volume (PTV), < or = 70% dose to 150% of the PTV volume (V150-PTV), and the quantity of needles < or = 70% of the size of the PTV, in cc. Upon completion of the Oncura plan, the seed type was changed and the activity was adjusted until the V100-PTV for each of the other 7 seed types matched the V100-PTV defined by the Oncura seed. Computed tomography (CT)-based postimplant dosimetry was used to determine the dose to 40% (D40) of the bulb of the penis (in Gy). Dose-volume histograms (DVH) were used to evaluate the differences to V100 (in %) and D40 (in Gy) of the anterior rectal wall and bulb of the penis, and V100 (in %) of the urethra. The data was tabulated. Radioactive 125I sources included in this study were 125I Source 2301 (Best); I-Plant (MedTech), IoGold (Mentor), Oncura (Amersham Health), ProstaSeed (UroCor), SelectSeed (Nucletron), SourceTech (Bard), and Symmetra (UroMed). The sizes of the PTV for the 10 patients ranged from 18.82 cc to 48.99 cc. The Oncura seed was used as the reference seed and all other seed types were normalized to it for data comparison. It was determined that the dose rate constant (Delta) and anisotropy factor (phi) contribute to the activity needed to achieve comparable V100-PTV doses, but a strong dependence on the radial function g(r) was found to effect the doses to the critical structures studied. Values of g(r) at 4 cm were calculated and the IoGold and SourceTech seeds were determined to have the highest g(r) values, with ProstaSeed and SelectSeed having the lowest values. 125I Source 2301 and IoGold required less activity per seed to achieve the same dose to the V100-PTV due to the higher dose rate and anisotrophy constants (Delta.phi). The seed types with silver were less penetrating and resulted in the production of characteristic x-rays that modified the energy spectrum and influenced the radial function. The seeds requiring the lowest activity showed the highest dose to the anterior rectal wall, a posterior adjacent structure; the urethra, an interior structure; and the bulb, an inferior structure. This study was designed to investigate the integrity of eight different commercially-available seed types, and their dependence on the g(r) in seed choice. It was determined that the dose rate constant and anisotropy factor determine the activity needed for implantation but a strong dependence on the radial function was found to effect the doses to the adjacent structures.

The robustness of dose distributions to displacement and migration of 125I permanent seed implants over a wide range of seed number, activity, and designs

PURPOSE

To investigate the robustness of permanent prostate implant dosimetry for various (125)I seed activities and various seed models. The dosimetric impact of seed misplacement and seed migration (seed loss) is also taken into account using various standard dose indices.

METHODS AND MATERIALS

A dose-based inverse planning algorithm is used for automated dosimetric plan creation (45-60 s per plan) and provides an unbiased way to compare the robustness of various optimal dosimetric plans. Seed misplacement and seed migration are simulated by way of Monte Carlo, based on the measured displacement distributions from clinical postimplant cases. Plans were generated for seed activities between 0.2 and 1.4 mCi (0.25 to 1.78 U) and for 11 different seed models.

RESULTS

The numbers of seeds and needles are shown to decrease rapidly for a seed activity between 0.3 mCi and 0.6 mCi (0.38 and 0.76 U). The loss in V100, from 100%, because of seed misplacement is below 10% for an apparent activity ranging from 0.2 to 0.9 mCi (0.25 to 1.14 U). A minimum degradation in V100 is observed around 0.6-0.7 mCi (0.76-0.89 U). D90 increases from 150 to 170 Gy between 0.3 and 0.7 mCi (0.38 and 0.89 U) and decreases afterward to fall below 140 Gy at higher activity. V200 and D10 to the target volume both show an increase in hot spots up to 0.7 mCi, and then decrease linearly at higher activities for all seed models. V200 and D10 to the urethra remain about constant for all seed activities up to 0.8 mCi (1.02 U), at which point they start to decrease. All seed models follow this general trend.

CONCLUSIONS

Plans were shown to be robust to misplacement and migration of seeds over a wide range of seed activity and for various seed models. With a properly tuned inverse planning algorithm able to ensure the dose coverage and protection for the organs at risk in the presence of placement errors (displacement and migration), the choice of a preferred seed activity, in a range up to about 0.7 mCi (0.89 U), is open. The upper part of this range offers the opportunity to significantly reduce the number of seeds and needles, thus reducing surgical trauma to the patient, saving time in an operating room planning setting, and reducing the cost of a permanent prostate implant procedure.

Improving the use of vibro-acoustography for brachytherapy metal seed imaging: a feasibility study

Vibro-acoustography method is explored for detecting and imaging brachytherapy metal seeds in gel phantoms. In a previous paper, we have shown that some immersed objects' resonance frequencies could be detected by vibro-acoustography. Here, we use this idea to optimize the vibro-acoustic excitation of two different sized brass seeds implanted in an agar gel phantom. In the experiments, the best excitation vibration frequencies were determined either by calculating fundamental resonance frequencies for each of the seeds or the experimental optimal resonance frequency of the gel. The resulting vibro-acoustography images demonstrate remarkable contrast in acoustic emission amplitude compared with images obtained at nonresonance frequencies. Results suggest the possible application of vibro-acoustography for directing prostate brachytherapy seed implantation treatment.

Measurement of the ultrasound backscatter signal from three seed types as a function of incidence angle: application to permanent prostate brachytherapy

PURPOSE

To measure the relative ultrasound backscatter of different seed types as a function of seed orientation and to evaluate the corresponding images of these seeds.

METHODS AND MATERIALS

Three seed types were evaluated: OncoSeed (standard), EchoSeed (corrugated), and RAPID Strand(RS). Ultrasound images for angles of incidence varying from 90 degrees (perpendicular) to 20 degrees at 5MHz and 7.5MHz were produced by raster scanning the seeds in a degassed water bath. Seed images were visually inspected and analyzed using the integrated-optical-density (IOD) method.

RESULTS

Corrugated seeds appear as contiguous objects over the range of frequencies and orientations examined, whereas standard seeds appear as contiguous objects from 90 degrees to 80 degrees only. The ranges and means of the backscattered IOD ratio of the seeds from 85 degrees to 20 degrees were: (corrugated vs. standard) 1.48 to 3.72 (2.32 +/- 0.62) for 5 MHz and 1.26 to 3.77 (2.19 +/- 0.84) for 7.5 MHz and (corrugated vs. RS) 1.21 to 9.53 (2.98 +/- 2.48) for 5 MHz and 1.008 to 10.86 (2.79 +/- 3.08) for 7.5 MHz. Backscattered signal increase ranged from 1.66 dB to 20.7 dB for the corrugated seed as compared to the other seeds.

CONCLUSIONS

Corrugated seeds produce greater backscatter signal and a more readily identifiable seed image over a large range of seed orientation as compared with standard brachytherapy seeds.

Comparison of methods for preparation of 125I brachytherapy source cores for the treatment of eye cancer

Two procedures for fixing the 125I activity on different matrices were explored. In the first method, anodic deposition of 125I on silver wire was used. In the second, 125I was adsorbed as iodate (IO3-) on alumina microspheres by using a solid-solution interface technique. While the 125I uptake was approximately 84% on the silver wires by electrodeposition method, the microspheres showed approximately 97% by the solid-solution interface technique. The low leachable (0.05%) wires, and comparatively high leachable (approximately 4%) spheres were trial encapsulated and laser welded. The encapsulated sources were measured for source strength and were found to be suitable for the treatment of eye cancer.

Brachytherapy dosimetry of 125I and 103Pd sources using an updated cross section library for the MCNP Monte Carlo transport code

Permanent implantation of low energy (20-40 keV) photon emitting radioactive seeds to treat prostate cancer is an important treatment option for patients. In order to produce accurate implant brachytherapy treatment plans, the dosimetry of a single source must be well characterized. Monte Carlo based transport calculations can be used for source characterization, but must have up to date cross section libraries to produce accurate dosimetry results. This work benchmarks the MCNP code and its photon cross section library for low energy photon brachytherapy applications. In particular, we calculate the emitted photon spectrum, air kerma, depth dose in water, and radial dose function for both 125I and 103Pd based seeds and compare to other published results. Our results show that MCNP's cross section library differs from recent data primarily in the photoelectric cross section for low energies and low atomic number materials. In water, differences as large as 10% in the photoelectric cross section and 6% in the total cross section occur at 125I and 103Pd photon energies. This leads to differences in the dose rate constant of 3% and 5%, and differences as large as 18% and 20% in the radial dose function for the 125I and 103Pd based seeds, respectively. Using a partially updated photon library, calculations of the dose rate constant and radial dose function agree with other published results. Further, the use of the updated photon library allows us to verify air kerma and depth dose in water calculations performed using MCNP's perturbation feature to simulate updated cross sections. We conclude that in order to most effectively use MCNP for low energy photon brachytherapy applications, we must update its cross section library. Following this update, the MCNP code system will be a very effective tool for low energy photon brachytherapy dosimetry applications.

Integral-transport-based deterministic brachytherapy dose calculations

We developed a transport-equation-based deterministic algorithm for computing three-dimensional brachytherapy dose distributions. The deterministic algorithm has been based on the integral transport equation. The algorithm provided us with the capability of computing dose distributions for multiple isotropic point and/or volumetric sources in a homogenous/heterogeneous medium. The algorithm results have been benchmarked against the results from the literature and MCNP results for isotropic point sources and volumetric sources.

Model 3500 125I brachytherapy source dosimetric characterization

Low-energy gamma-emitting isotopes encapsulated for permanent implant are routinely applied in brachytherapy, most notably for prostate cancer. Before clinical use of a new source design, a full dosimetric analysis and standardized calibration are essential. Results of experimental measurement and analysis are reported here for the I-Plant (Implant Sciences Corporation) 125I source, model 3500. Dose measurements were made using standard methods employing thermoluminscent dosimeters in a water equivalent plastic phantom. Precision machined bores in the phantom located dosimeters and source(s) in a reproducible fixed geometry providing for transverse-axis and angular dose profiles over a range of distances from 0.17 to 10 cm. The data were analyzed in terms of parameters recommended by AAPM TG-43. The dose-rate constant, delta = 1.01 cGy/h U (+/-6%) (1 U = 1 cGy cm2 h(-1)), was evaluated with reference to a TG-51 calibrated 60Co standard, accounting for dosimeter response differences between 60Co and 125I photons. The radial dose function, g(r), the anisotropy function, F(r, theta), the anisotropy factor, phi(an)(r), and the point-source approximation anisotropy constant, phi(an), were derived from one- and two-dimensional dose distribution data measured in the phantom, accounting for finite dosimeter volume and with attention to inter-chip effects. The results confirm prior dosimetric characterization of the model 3500, and indicate that the new source is comparable to the MED3631-A/M and 6702 source designs and may substitute for model 6711 in permanent implants for the treatment of prostate cancer.