Calibration of multiple LDR brachytherapy sources

Dosimetric characterization of a new directional low-dose rate brachytherapy source

PURPOSE

CivaTech Oncology Inc. (Durham, NC) has developed a novel low-dose rate (LDR) brachytherapy source called the CivaSheet.TM The source is a planar array of discrete elements ("CivaDots") which are directional in nature. The CivaDot geometry and design are considerably different than conventional LDR cylindrically symmetric sources. Thus, a thorough investigation is required to ascertain the dosimetric characteristics of the source. This work investigates the repeatability and reproducibility of a primary source strength standard for the CivaDot and characterizes the CivaDot dose distribution by performing in-phantom measurements and Monte Carlo (MC) simulations. Existing dosimetric formalisms were adapted to accommodate a directional source, and other distinguishing characteristics including the presence of gold shield x-ray fluorescence were addressed in this investigation.

METHODS

Primary air-kerma strength (SK ) measurements of the CivaDots were performed using two free-air chambers namely, the Variable-Aperture Free-Air Chamber (VAFAC) at the University of Wisconsin Medical Radiation Research Center (UWMRRC) and the National Institute of Standards and Technology (NIST) Wide-Angle Free-Air Chamber (WAFAC). An intercomparison of the two free-air chamber measurements was performed along with a comparison of the different assumed CivaDot energy spectra and associated correction factors. Dose distribution measurements of the source were performed in a custom polymethylmethacrylate (PMMA) phantom using GafchromicTM EBT3 film and thermoluminescent dosimeter (TLD) microcubes. Monte Carlo simulations of the source and the measurement setup were performed using MCNP6 radiation transport code.

RESULTS

The CivaDot SK was determined using the two free-air chambers for eight sources with an agreement of better than 1.1% for all sources. The NIST measured CivaDot energy spectrum intensity peaks were within 1.8% of the MC-predicted spectrum intensity peaks. The difference in the net source-specific correction factor determined for the CivaDot free-air chamber measurements for the NIST WAFAC and UW VAFAC was 0.7%. The dose-rate constant analog was determined to be 0.555 cGy h-1 U-1 . The average difference observed in the estimated CivaDot dose-rate constant analog using measurements and MCNP6-predicted value (0.558 cGy h-1 U-1 ) was 0.6% ± 2.3% for eight CivaDot sources using EBT3 film, and -2.6% ± 1.7% using TLD microcube measurements. The CivaDot two-dimensional dose-to-water distribution measured in phantom was compared to the corresponding MC predictions at six depths. The observed difference using a pixel-by-pixel subtraction map of the measured and the predicted dose-to-water distribution was generally within 2-3%, with maximum differences up to 5% of the dose prescribed at the depth of 1 cm.

CONCLUSION

Primary SK measurements of the CivaDot demonstrated good repeatability and reproducibility of the free-air chamber measurements. Measurements of the CivaDot dose distribution using the EBT3 film stack phantom and its subsequent comparison to Monte Carlo-predicted dose distributions were encouraging, given the overall uncertainties. This work will aid in the eventual realization of a clinically viable dosimetric framework for the CivaSheet based on the CivaDot dose distribution.

Guidelines by the AAPM and GEC-ESTRO on the use of innovative brachytherapy devices and applications: Report of Task Group 167

Although a multicenter, Phase III, prospective, randomized trial is the gold standard for evidence-based medicine, it is rarely used in the evaluation of innovative devices because of many practical and ethical reasons. It is usually sufficient to compare the dose distributions and dose rates for determining the equivalence of the innovative treatment modality to an existing one. Thus, quantitative evaluation of the dosimetric characteristics of innovative radiotherapy devices or applications is a critical part in which physicists should be actively involved. The physicist's role, along with physician colleagues, in this process is highlighted for innovative brachytherapy devices and applications and includes evaluation of (1) dosimetric considerations for clinical implementation (including calibrations, dose calculations, and radiobiological aspects) to comply with existing societal dosimetric prerequisites for sources in routine clinical use, (2) risks and benefits from a regulatory and safety perspective, and (3) resource assessment and preparedness. Further, it is suggested that any developed calibration methods be traceable to a primary standards dosimetry laboratory (PSDL) such as the National Institute of Standards and Technology in the U.S. or to other PSDLs located elsewhere such as in Europe. Clinical users should follow standards as approved by their country's regulatory agencies that approved such a brachytherapy device. Integration of this system into the medical source calibration infrastructure of secondary standard dosimetry laboratories such as the Accredited Dosimetry Calibration Laboratories in the U.S. is encouraged before a source is introduced into widespread routine clinical use. The American Association of Physicists in Medicine and the Groupe Européen de Curiethérapie-European Society for Radiotherapy and Oncology (GEC-ESTRO) have developed guidelines for the safe and consistent application of brachytherapy using innovative devices and applications. The current report covers regulatory approvals, calibration, dose calculations, radiobiological issues, and overall safety concerns that should be addressed during the commissioning stage preceding clinical use. These guidelines are based on review of requirements of the U.S. Nuclear Regulatory Commission, U.S. Department of Transportation, International Electrotechnical Commission Medical Electrical Equipment Standard 60601, U.S. Food and Drug Administration, European Commission for CE Marking (Conformité Européenne), and institutional review boards and radiation safety committees.

Source strength verification and quality assurance of preloaded brachytherapy needles using a CMOS flat panel detector

PURPOSE

Current methods of low dose rate brachytherapy source strength verification for sources preloaded into needles consist of either assaying a small number of seeds from a separate sample belonging to the same lot used to load the needles or performing batch assays of a subset of the preloaded seed trains. Both of these methods are cumbersome and have the limitations inherent to sampling. The purpose of this work was to investigate an alternative approach that uses an image-based, autoradiographic system capable of the rapid and complete assay of all sources without compromising sterility.

METHODS

The system consists of a flat panel image detector, an autoclavable needle holder, and software to analyze the detected signals. The needle holder was designed to maintain a fixed vertical spacing between the needles and the image detector, and to collimate the emissions from each seed. It also provides a sterile barrier between the needles and the imager. The image detector has a sufficiently large image capture area to allow several needles to be analyzed simultaneously.Several tests were performed to assess the accuracy and reproducibility of source strengths obtained using this system. Three different seed models (Oncura 6711 and 9011 (125)I seeds, and IsoAid Advantage (103)Pd seeds) were used in the evaluations. Seeds were loaded into trains with at least 1 cm spacing.

RESULTS

Using our system, it was possible to obtain linear calibration curves with coverage factor k = 1 prediction intervals of less than ±2% near the centre of their range for the three source models. The uncertainty budget calculated from a combination of type A and type B estimates of potential sources of error was somewhat larger, yielding (k = 1) combined uncertainties for individual seed readings of 6.2% for (125)I 6711 seeds, 4.7% for (125)I 9011 seeds, and 11.0% for Advantage (103)Pd seeds.

CONCLUSIONS

This study showed that a flat panel detector dosimetry system is a viable option for source strength verification in preloaded needles, as it is capable of measuring all of the sources intended for implantation. Such a system has the potential to directly and efficiently estimate individual source strengths, the overall mean source strength, and the positions within the seed-spacer train.

Commissioning of a well type chamber for HDR and LDR brachytherapy applications: a review of methodology and outcomes

For safe and accurate dose delivery in brachytherapy, associated equipment is subject to commissioning and ongoing quality assurance (QA). Many centres depend on the use of a well-type chamber ('well chamber') for performing brachytherapy dosimetry. Documentation of well chamber commissioning is scarce despite the important role the chamber plays in the whole brachytherapy QA process. An extensive and structured commissioning of the HDR 1000 plus well chamber (Standard Imaging Inc, Middleton WI) for HDR and LDR dosimetry was undertaken at Sir Charles Gairdner Hospital. The methodology and outcomes of this commissioning is documented and presented as a guideline to others involved in brachytherapy. The commissioning tests described include mechanical integrity, leakage current, directional dependence, response, length of uniform response, the influence of insert holders, ion collection efficiency, polarity effect, accuracy of measured air kerma strength (S(K)) or reference air kerma rate (K(R)) and baseline setting (for ongoing constancy checks). For the HDR 1000 plus well chamber, some of the insert holders modify the response curve. The measured sweet length was 2.5 cm which is within 0.5% of that specified by the manufacturer. Correction for polarity was negligible (0.9999) and ion recombination was small (0.9994). Directional dependence was small (less than 0.2%) and leakage current was negligible. The measured K(R) for (192)Ir agreed within 0.11% compared with a second well chamber of similar model and was within 0.5% of that determined via a free-in-air measurement method. Routine constancy checks over a year agreed with the baseline within 0.4%.

103Pd strings: Monte Carlo assessment of a new approach to brachytherapy source design

PURPOSE

A new type of (103)Pd source (CivaString and CivaThin by CivaTech Oncology, Inc.) is examined. The source contains (103)Pd and Au radio-opaque marker(s), all contained within low-Zeff organic polymers that permit source flexibility. The CivaString source is available in lengths L of 10, 20, 30, 40, 50, and 60 mm, and referred to in the current study as CS10-CS60, respectively. A thinner design, CivaThin, has sources designated as CT10-CT60, respectively. The CivaString and CivaThin sources are 0.85 and 0.60 mm in diameter, respectively. The source design is novel and offers an opportunity to examine its interesting dosimetric properties in comparison to conventional (103)Pd seeds.

METHODS

The MCNP5 radiation transport code was used to estimate air-kerma rate and dose rate distributions with polar and cylindrical coordinate systems. Doses in water and prostate tissue phantoms were compared to determine differences between the TG-43 formalism and realistic clinical circumstances. The influence of Ti encapsulation and 2.7 keV photons was examined. The accuracy of superposition of dose distributions from shorter sources to create longer source dose distributions was also assessed.

RESULTS

The normalized air-kerma rate was not highly dependent on L or the polar angle θ, with results being nearly identical between the CivaString and CivaThin sources for common L. The air-kerma strength was also weakly dependent on L. The uncertainty analysis established a standard uncertainty of 1.3% for the dose-rate constant Λ, where the largest contributors were μen/ρ and μ/ρ. The Λ values decreased with increasing L, which was largely explained by differences in solid angle. The radial dose function did not substantially vary among the CivaString and CivaThin sources for r ≥ 1 cm. However, behavior for r < 1 cm indicated that the Au marker(s) shielded radiation for the sources having L = 10, 30, and 50 mm. The 2D anisotropy function exhibited peaks and valleys that corresponded to positions adjacent to (103)Pd wells and Au markers, respectively. Dose distributions of both source types had minimal anisotropy in comparison to conventional (103)Pd seeds. Contributions by 2.7 keV photons comprised ≤ 0.1% of the dose from all photons at positions farther than 0.13 mm from the polymer source surface. Differences between absorbed dose to water and prostate became more substantial as distance from the sources increased, with prostate dose being about 13% lower for r = 5 cm. Using a cylindrical coordinate system, dose superposition of small length sources to replicate the dose distribution for a long length source proved to be a robust technique; a 2.0% tolerance compared with the reference dose distribution did not exceed 0.1 cm(3) for any of the examined source combinations.

CONCLUSIONS

By design, the CivaString and CivaThin sources have novel dosimetric characteristics in comparison to Ti-encapsulated (103)Pd seeds. The dosimetric characterization has determined the reasons for these differences through analysis using Monte Carlo-based radiation transport simulations.

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

Clinical practice and quality assurance challenges in modern brachytherapy sources and dosimetry

Modern brachytherapy has led to effective treatments through the establishment of broadly applicable dosimetric thresholds for maximizing survival with minimal morbidity. Proper implementation of recent dosimetric consensus statements and quality assurance procedures is necessary to maintain the established level of safety and efficacy. This review classifies issues as either "systematic" or "stochastic" in terms of their impact on large groups or individual patients, respectively. Systematic changes affecting large numbers of patients occur infrequently and include changes in source dosimetric parameters, prescribing practice, dose calculation formalism, and improvements in calculation algorithms. The physicist must be aware of how incipient changes accord with previous experience. Stochastic issues involve procedures that are applied to each patient individually. Although ample guidance for quality assurance of brachytherapy sources exists, some ambiguities remain. The latest American Association of Physicists in Medicine guidance clarifies what is meant by independent assay, changes source sampling recommendations, particularly for sources in sterile strands and sterile preassembled needles, and modifies action level thresholds. The changing environment of brachytherapy has not changed the fact that the prime responsibility for quality assurance in brachytherapy lies with the institutional medical physicist.

Primary calibration of coiled 103Pd brachytherapy sources

Coiled 103Pd brachytherapy sources have been developed by RadioMed Corporation for use as low-dose-rate (LDR) interstitial implants. The coiled sources are provided in integer lengths from 1 to 6 cm and address many common issues seen with traditional LDR brachytherapy sources. The current standard for determining the air-kerma strength (SK) of low-energy LDR brachytherapy sources is the National Institute of Standards and Technology's Wide-Angle Free-Air Chamber (NIST WAFAC). Due to geometric limitations, however, the NIST WAFAC is unable to determine the S(K) of sources longer than 1 cm. This project utilized the University of Wisconsin's Variable-Aperture Free-Air Chamber (UW VAFAC) to determine the S(K) of the longer coiled sources. The UW VAFAC has shown agreement in S(K) values of 1 cm length coils to within 1% of those determined with the NIST WAFAC, but the UW VAFAC does not share the same geometric limitations as the NIST WAFAC. A new source holder was constructed to hold the coiled sources in place during measurements with the UW VAFAC. Correction factors for the increased length of the sources have been determined and applied to the measurements. Using the new source holder and corrections, the S(K) of 3 and 6 cm coiled sources has been determined. Corrected UW VAFAC data and ionization current measurements from well chambers have been used to determine calibration coefficients for use in the measurement of 3 and 6 cm coiled sources in well chambers. Thus, the UW VAFAC has provided the first transferable, primary measurement of low-energy LDR brachytherapy sources with lengths greater than 1 cm.