Comparisons of a proposed five-seed assay method with the single-seed and batch assay methods for I-125 seeds in ultrasound-guided prostate implants

A method for confirming a third-party assay of I-125 seeds used for prostate implants

The purpose of this work is to describe a method and apparatus that can be used to confirm the source strength of a large number of I-125 seeds while maintaining sterility, accuracy, reproducibility, and time efficiency. Source strengths ranging from 0.395 to 0.504 U/seed were available for this study. Three different seed configurations were measured: loose, linked, and loaded needles. A third-party 10% assay (NIST traceable) was provided. A custom stand was built out of aluminum to hold an exposure meter [Inovision (Fluke) 451P pressurized ion chamber] at 25 cm above the I-125 sources to measure the exposure rate. The measurements were made in an operating room, and a sterile sheet was placed under the nonsterile aluminum stand on a sterile loading table. Seeds and needles were placed in a sterile tray for these measurements. Two hundred and six loose seeds in six batches (0.395, 0.395, 0.409, 0.444, 0.444, and 0.444 U/seed) and 1434 seeds in 10 batches containing various strands (0.444, 0.444, 0.444, 0.444, .0444, 0.466, 0.466, 0.504, and 0.504 U/seed) were measured. For the loose and stranded seeds, the average exposure rate per unit activity was measured to be 0.589 mR/h·U with a standard deviation of 0.017. Loaded needles were measured with an average exposure rate per unit activity to be 0.269 mR/h·U with a standard deviation of 0.014. We conclude that the method described here is capable of confirming a third-party assay when performed on a large number of loose or stranded seeds in bulk. It is less reliable for preloaded needles.

Source strength assay of iodine-125 seeds sealed within sterile packaging

Early‐stage prostate cancer is widely treated by iodine‐125 (I‐125) seed implantation. While quality assurance methods are in place to assure consistency in I‐125 seed source strength, current methods involve the breaking of the sterilization package, raising issues concerning sterility and time limitations. The purpose of this study was to develop a method of characterizing the total source strength of I‐125 seeds within a cartridge that has been sealed within a sterilization package and to evaluate the probability of detecting an out‐of‐calibration seed (aberrant seed). We defined a protocol to determine the ability of a well‐type ionization chamber to detect aberrant I‐125 seeds within a cartridge sealed in the sterilization package. A novel jig for a well‐type ionization chamber was designed to accommodate the sterilization package. One seed was chosen randomly from two cartridges containing five or 15 seeds (0.544 U source strength) and was exchanged with aberrant seeds of six different source strengths. The source strength was measured at each position within the cartridge. The results indicated that the response of the well chamber was sensitive to changes in the aberrant seed position within the cartridge and the source strength of the aberrant seed. The correlation coefficient between single seed and batch assay results was high (0.998). A novel jig and a measurement method using a well ionization chamber were developed, which allowed for a batch assay characterization of the total source strength of I‐125 seeds within a cartridge sealed within sterilization package. This method is simple, time‐saving, and offers greater practical application.

PACS number: D6.20.Dk

Verification of air-kerma strength of 125I seed for permanent prostate implants in Japan

BACKGROUND

To assure the physical quality of brachytherapy, we investigated the difference between measured and manufacturer's stated source strengths in a single model SourceTech Medical (STM)1251 (125)I seed.

METHODS

A well-type ionization chamber with a single-seed holder was used to measure the source strength of 2412 (125)I seeds before implant in 34 patients. The air-kerma strength was 0.450 U for all cases. The mean source strength for each patient was measured and compared with the manufacturer's stated value. The deviation from the measured value was compared with the tolerance range of the American Association of Physicists in Medicine (AAPM) TG-56 report's recommendation.

RESULTS

The measured source strength was higher than the manufacturer's stated value, with a median difference of 1% (range, 2% to 5%). Sixteen of the total of 2412 seeds (0.7%) were more than 5% different from the manufacturer's stated value. The median SD from the mean value was 2.2% (range, 1.1% to 2.5%) for all patients.

CONCLUSION

This is the first report of a single-seed assay performed for the model STM1251 (125)I seed. In this study the manufacturer's stated strength agreed well with the measured value. Nevertheless, the advisability of performing a single-seed assay at every institution should be considered, by referring to the appropriate regulations; for example, those used in the United States.

Dosimetric consideration of individual 125I source strength measurement and a large-scale comparison of that measured with a nominal value in permanent prostate implant brachytherapy

PURPOSE

We investigated the difference between measured and manufacturer's nominal source strength in a large sample of a single model of (125)I seeds. Physical characteristics of single seed measurement by the well-type ionization chamber were also investigated to provide dosimetric data.

MATERIALS AND METHODS

A well-type ionization chamber with a single seed holder was used to measure source strength of all 1935 (125)I seeds implanted in the initial 28 patients in our hospital. Physical characteristics including linearity of readings for different integral time intervals, reproducibility, isotropy, and axial positional sensitivity were assessed. To calculate the source strength, the integral charge during 30 s was measured and converted to air kerma strength. The nominal activity stated by the manufacturer was compared with the measured value.

RESULTS

Linearity, reproducibility, and isotropy of the well-type ionization chamber were within 0.2%. Measured source strength was on average 2.1% (range -7.6% to +7.2%), lower than the nominal value. Standard deviation of all measured seeds was 2.0%. The maximum difference between the measured and the manufacturer's nominal source strength in each patient was -3.7%. The standard deviation averaged 1.6%.

CONCLUSION

The nominal source strength of the (125)I seeds agreed well with the measured value. Our study can be helpful as guidance for individual (125)I seed source strength measurement.

Quality assurance of I-125 seeds for prostate brachytherapy using an imaging plate

PURPOSE

OncoSeed is delivered in a sterile environment in the form of a cartridge, so it is impractical to resterilize and reload seeds after calibration. We investigated a new method using an imaging plate dosimetry system to characterize all seeds in the OncoSeed cartridge in a sterile environment.

METHODS AND MATERIALS

Seeds within the cartridge were placed on an imaging plate, and the imaging plate irradiated. To remove scatter radiation, and improve spatial resolution of seed images, we used X-ray parallel cross grids. The irradiated imaging plate was scanned using a Bio-imaging Analyzer System, and radioactivity intensities of seed images were given in counts. Counts could be translated to profiles, and each seed within the cartridge was analyzed.

RESULTS

Results showed a good correlation between counts and total radioactivity of the seeds within the cartridge. Thus, using a least-squares line, it was possible to characterize a cartridge with unknown apparent activity. By analyzing the profiles, it was possible not only to detect a miscalibrated seed in the cartridge from its relative difference in counts, but also to identify its position in the cartridge. No significant changes in counts were seen between sterile and nonsterile environments.

CONCLUSION

Using an imaging plate dosimetry system, all seeds in a cartridge could be characterized in a sterile environment.

Calibration of multiple LDR brachytherapy sources

A trend is underway toward the use of prepackaged low dose rate brachytherapy sources, which come in the form of strands, coiled line sources, preloaded needles, and sterile cartridge packs. Since the medical physicist is responsible for verification of source strength prior to patient treatment, development of prepackaged source strength verification methods is needed. Existing guidelines are reviewed to establish the situation that medical physicists find with respect to prepackaged sources. This investigation presents an experimental evaluation of the effect of some of these multiseed geometries on source strength measurements. Multiseed strands and coils, whether 125I, 103Pd, or 192Ir can be measured in a chamber with a long, sensitive axial length with a uniform response. Sterile seed cartridge packs can also be measured but require a correction factor to be applied. Sources in needles, however, cannot be measured in the needle since there is too great a variation in needle composition and needle tolerance thickness. Removing these seeds from the needle into a sterile measurement insert, which maintains sterility is a practical source strength verification method, similar to those done for multiple seed configurations in a well chamber with adequate axial uniformity. Values are compared with individual air kerma strength calibrations, and correction factors, are presented' where needed. In each case, care must be taken to maintain sterility as multiple seeds are measured in well chamber inserts.

Calibration procedures for seeds preloaded in cartridges

Radioactive seeds preloaded in sterilized cartridges or needles are commonly obtainable from manufacturers. Under the US regulations for control of radioactive materials, seed users are required to account for all seeds and independently verify their air kerma strength (SK). As a result, the viability of inspection schemes that rely on measurement of aggregate seeds is of interest. In this paper we consider the conditions (if any) under which cartridge inspection can satisfy regulatory requirements and still provide practical benefit (i.e., time savings) against the regular single-seed assay. The standards for comparison are the recommendations of AAPM TG40, AAPM TG56, and ACR's "Standard for the Performance of Manually Loaded Brachytherapy Sources." The practical benefit is judged in comparison to the effort required to apply the 10% assay recommendation of TG40 to seeds in cartridges. Two specific cartridge inspection schemes are considered: (a) measuring the SK of each cartridge in a batch; (b) measuring a single cartridge sampled at random from the batch. Unlike the 10% assay, which is defined (imperfectly, in our view) without reference to the prevalence of in-calibration seeds, the estimation of the relative merits of cartridge inspection methods must necessarily include such information and, as such, is manufacturer specific. In this paper results are provided for Oncura model 6711 125I seeds in shielded and unshielded Mick cartridges. We show that the only practically useful cartridge inspection scheme is the batch scheme applied to unshielded cartridges. The false positive rates associated with the other schemes are such that we expect to open a cartridge (and perform the 10% assay) at least 80% of the time. While anything less than 100% of the time is theoretically an improvement, this neglects the additional effort required to assay the cartridges.