Low Dose Rate Sources Research Articles

Validation of the TOPAS Monte Carlo toolkit for LDR brachytherapy simulations

Purpose:This work has the purpose of validating the Monte Carlo toolkit TOol for PArticle Simulation (TOPAS) for low-dose-rate (LDR) brachytherapy uses. Methods and Materials:Simulations of 12 LDR sources and 2 COMS eye plaques (10 mm and 20 mm in diameter) and comparisons with published reference data from the Carleton Laboratory for Radiotherapy Physics (CLRP), the TG-43 consensus data and the TG-129 consensus data were performed. Sources from the IROC Houston Source Registry were modeled. The OncoSeed 6711 and the SelectSeed 130.002 were also modeled for historical reasons. For each source, the dose rate constant, the radial dose function and the anisotropy functions at 0.5, 1 and 5 cm were extracted. For the eye plaques (loaded with 125I sources), dose distribution maps, dose profiles along the central axis and transverse axis were calculated. Results:Dose rate constants for 11 of the 12 sources are within 4% of the consensus data and within 2% of the CLRP data. The radial dose functions and anisotropy functions are mostly within 2% of the CLRP data. In average, 92% of all voxels are within 1% of the CLRP data for the eye plaques dose distributions. The dose profiles are within 0.5% (central axis) and 1% (transverse axis) of the reference data. Conclusion:The TOPAS MC toolkit was validated for LDR brachytherapy applications. Single-seed and multi-seed results agree with the published reference data. TOPAS has several benefits such as a simplified approach to MC simulations and an accessible brachytherapy package including comprehensive learning resources.

Assessment of combined modality therapy for non-small-cell lung carcinoma: A simulation study concerning concurrent chemo-brachytherapy.

Although surgery is the treatment of choice for early-stage non-small-cell lung carcinoma, almost two-thirds of patients do not have acceptable pulmonary function for extensive surgeries. The alternative approach for this large group of patients is sublobar resection along with low-dose-rate (LDR) brachytherapy (BT). However, patients with resected lungs have a high risk of recurrence and are often treated with platinum-based (Pt-based) chemotherapy (CT). In this study, we aimed to evaluate the absorbed doses of lung and other thoracic organs, considering concurrent chemo-BT with LDR sources in two modalities: conventional vs. unconventional Pt-based CT. We used the MCNPX code for simulations and to obtain the lung absorbed dose, dose enhancement factor (DEF), and Pt threshold concentration for the abovementioned modalities. Our results indicate that DEF correlates directly with Pt concentration at prescription point and is inversely correlated with depth. Dose enhancement for conventional CT concurrent with BT is <2%, while it is >2% in case of unconventional Pt-based CT wherein the Pt concentration exceeds 0.2 mg/g lung tissue. Also, the absorbed dose of healthy thoracic organs decreased by 2-11% in the latter approach. In conclusion, the concurrent chemo-BT in the lung environment could enhance the therapeutic doses merely by using unconventional CT methods, while lung Pt accumulation exceeds 0.2 mg/g.

Update of the CLRP TG-43 parameter database for low-energy brachytherapy sources.

To update the Carleton Laboratory for Radiotherapy Physics (CLRP) TG-43 dosimetry database for low-energy (≤50keV) photon-emitting low-dose rate (LDR) brachytherapy sources utilizing the open-source EGSnrc application egs_brachy rather than the BrachyDose application used previously for 27 LDR sources in the 2008 CLRP version (CLRPv1). CLRPv2 covers 40 sources ( Pd, I, and Cs). A comprehensive set of TG-43 parameters is calculated, including dose-rate constants, radial dose functions with functional fitting parameters, 1D and 2D anisotropy functions, along-away dose-rate tables, Primary-Scatter separation dose tables (for some sources), and mean photon energies at the surface of the sources. The database also documents the source models which will become part of the egs_brachy distribution. Datasets are calculated after a systematic recoding of the source geometries using the egs++ geometry package and its egs_brachy extensions. Air-kerma strength per history is calculated for models of NIST's Wide-Angle Free-Air chamber (WAFAC) and for a point detector located at 10cm on the source's transverse axis. Full scatter water phantoms with varying voxel resolutions in cylindrical coordinates are used for dose calculations. New statistical uncertainties of source volume corrections for phantom voxels which overlap with brachytherapy sources are implemented in egs_brachy, and all CLRPv2 data include these uncertainties. For validation, data are compared to CLRPv1 and other data in the literature. Data are available at https://physics.carleton.ca/clrp/egs_brachy/seed_database_v2, http://doi.org/10.22215/clrp/tg43v2. As well as being presented graphically in comparisons to previous calculations, data are available in Excel (.xlsx) spreadsheets for each source. The database has applications in research, dosimetry, and brachytherapy treatment planning. This comprehensive update provides the medical physics community with more accurate TG-43 dose evaluation parameters, as well as fully benchmarked and described source models which are distributed with egs_brachy.

Characterisation of a cobalt-60 small-beam animal irradiator using a realtime silicon pixelated detector

The paper presents a study performed by the Centre for Medical Radiation Physics (CMRP) using a high spatial and temporal resolution silicon pixelated detector named MagicPlate-512. The study focuses on the characterisation of three pencil beams from a low-dose rate, 6 TBq, cobalt-60 source, in terms of percentage depth dose, beam profiles, output factor and shutter timing. Where applicable, the findings were verified against radiochromic EBT3 film and ionization chambers. It was found that the results of the MagicPlate-512 and film agreed within 0.9 mm for penumbra and full-width at half-maximum measurements of the beam profiles, and within 0.75% for percentage depth dose study. The dose rate of the cobalt-60 source was determined to be (10.65 ± 0.03) cGy/min at 1.5 cm depth in Solid Water. A significant asymmetry of the small pencil beam profile was found, which is due to the irregular machining of the small collimator. The average source shutter speed was calculated to be 26 cm/s. The study demonstrates that the MagicPlate-512 dosimetry system, developed at CMRP, is capable of beam characterisation even in cases of very low dose rate sources.

Magnetic Resonance Imaging-Guided High–Dose Rate Brachytherapy for Cervical Cancer

Introduction Cervical cancer represents 0.8% of all new cancer cases and in 2012 there was an estimated 249,512 women living with cervical cancer in the United States. Concurrent chemoradiation followed by brachytherapy represents the standard of care in patients with International Federation of Gynecology and Obstetrics stage IB2 to IVA tumors (tumors larger than 4 cm or with parametrial involvement). During the first phase of treatment, patients receive external beam radiotherapy (EBRT) to the pelvis specifically targeting the cervix, uterus, and regional lymphatics. Patients typically receive 45-50.4 Gy with concurrentweekly cisplatin over 5weeks. This is followed by brachytherapy where applicators are placed in close proximity to the tumor typically via an intracavitary approach for temporary loading of a radioactive source. Brachytherapy plays a crucial role in the management of invasive cervical cancer. It has a rapid dose fall off allowing the tumor to receive a high dose while relatively sparing nearby structures such as the bladder, sigmoid, and rectum. Brachytherapy delivery options have evolved over time and can be delivered through multiple (typically 5) outpatient procedures using a high–dose rate source or during 2 inpatient stays using a low–dose rate source. By taking advantage of the geometric andphysical properties of brachytherapy, the central principle is to escalate the tumor dose to a curative rangewhileminimizing the dose and toxicity to the surrounding organs at risk (rectum, bladder, and sigmoid). Over time with advances in imaging, brachytherapy treatment planning has evolved tomeet these goals. In the early era, brachytherapy was planned using 2-dimensional images (Fig. 1). Defined points in space were used to evaluate the quality of brachytherapy implants and to estimate the tumor dose and dose to organs at risk. With the widespread use of computed tomography (CT) within radiation oncology departments, CT-based planning (Fig. 2) is now feasible. The

Absolute measurement of LDR brachytherapy source emitted power: Instrument design and initial measurements.

Energy-based source strength metrics may find use with model-based dose calculation algorithms, but no instruments exist that can measure the energy emitted from low-dose rate (LDR) sources. This work developed a calorimetric technique for measuring the power emitted from encapsulated low-dose rate, photon-emitting brachytherapy sources. This quantity is called emitted power (EP). The measurement methodology, instrument design and performance, and EP measurements made with the calorimeter are presented in this work. A calorimeter operating with a liquid helium thermal sink was developed to measure EP from LDR brachytherapy sources. The calorimeter employed an electrical substitution technique to determine the power emitted from the source. The calorimeter's performance and thermal system were characterized. EP measurements were made using four (125)I sources with air-kerma strengths ranging from 2.3 to 5.6 U and corresponding EPs of 0.39-0.79 μW, respectively. Three Best Medical 2301 sources and one Oncura 6711 source were measured. EP was also computed by converting measured air-kerma strengths to EPs through Monte Carlo-derived conversion factors. The measured EP and derived EPs were compared to determine the accuracy of the calorimeter measurement technique. The calorimeter had a noise floor of 1-3 nW and a repeatability of 30-60 nW. The calorimeter was stable to within 5 nW over a 12 h measurement window. All measured values agreed with derived EPs to within 10%, with three of the four sources agreeing to within 4%. Calorimeter measurements had uncertainties ranging from 2.6% to 4.5% at the k = 1 level. The values of the derived EPs had uncertainties ranging from 2.9% to 3.6% at the k = 1 level. A calorimeter capable of measuring the EP from LDR sources has been developed and validated for (125)I sources with EPs between 0.43 and 0.79 μW.

Matched Fields Technique for &lt;formula formulatype="inline"&gt; &lt;tex Notation="TeX"&gt; $^{90}{\rm Sr}/^{90}{\rm Y}$ &lt;/tex&gt;&lt;/formula&gt; Low-Dose Rate Source for Keloid Treatments

<sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">90</sup> Sr/ <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">90</sup> Y low-dose rate sources are commonly used in pterygium, keloid, and skin postsurgery treatments to avoid reincidence. The beta radioactive materials are deposited on metal plates, in geometries, such as square, rectangular, and circular, to treat lesions up to 2 mm depth from the skin surface. Although it has a limitation for deeper lesion treatments, beta application presents an advantage of minor healthy tissue damage. The chemical dosimeter Fricke Xylenol Gel (FXG), which presents high spatial resolution and effective atomic number (Z <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">eff</sub> ) similar to the soft tissue, can determine the depth dose distribution generated under a source and simultaneously simulate the soft tissue. In this paper, the FXG was used to evaluate the depth dose distributions, along two adjacent square <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">90</sup> Sr/ <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">90</sup> Y sources, to verify the radiation homogeneity at the interface region. From the results, one could infer a subdose region under the interface area, when the metal plates are matched (ordinary procedure for treatments using adjacent metal plates). Absorbed dose percentages of 53% and 32% were found at the surface and at 1 mm depth, respectively, which are lower than those expected for the interface region. In this sense, the results suggest that the radioactive areas must be adjacent instead of the physical ones, which means that the metal plates should be overlapped by 5 mm, approximately.

Development of an adjoint sensitivity field-based treatment-planning technique for the use of newly designed directional LDR sources in brachytherapy

The development and application of an automated 3D greedy heuristic (GH) optimization algorithm utilizing the adjoint sensitivity fields for treatment planning to assess the advantage of directional interstitial prostate brachytherapy is presented. Directional and isotropic dose kernels generated using Monte Carlo simulations based on Best Industries model 2301 I-125 source are utilized for treatment planning. The newly developed GH algorithm is employed for optimization of the treatment plans for seven interstitial prostate brachytherapy cases using mixed sources (directional brachytherapy) and using only isotropic sources (conventional brachytherapy). All treatment plans resulted in V100 > 98% and D90 > 45 Gy for the target prostate region. For the urethra region, the D10Ur, D90Ur and V150Ur and for the rectum region the V100cc, D2cc, D90Re and V90Re all are reduced significantly when mixed sources brachytherapy is used employing directional sources. The simulations demonstrated that the use of directional sources in the low dose-rate (LDR) brachytherapy of the prostate clearly benefits in sparing the urethra and the rectum sensitive structures from overdose. The time taken for a conventional treatment plan is less than three seconds, while the time taken for a mixed source treatment plan is less than nine seconds, as tested on an Intel Core2 Duo 2.2 GHz processor with 1GB RAM. The new 3D GH algorithm is successful in generating a feasible LDR brachytherapy treatment planning solution with an extra degree of freedom, i.e. directionality in very little time.

Investigation of the dose rate dependency of the PAGAT gel dosimeter at low dose rates

Medical physicists need dosimeters such as gel dosimeters capable of determining three-dimensional dose distributions with high spatial resolution. To date, in combination with magnetic resonance imaging (MRI), polyacrylamide gel (PAG) polymers are the most promising gel dosimetry systems. The purpose of this work was to investigate the dose rate dependency of the PAGAT gel dosimeter at low dose rates. The gel dosimeter was used for measurement of the dose distribution around a Cs-137 source from a brachytherapy LDR source to have a range of dose rates from 0.97 Gy h−1 to 0.06 Gy h−1. After irradiation of the PAGAT gel, it was observed that the dose measured by gel dosimetry was almost the same at different distances (different dose rates) from the source, although the points nearer the source had been expected to receive greater doses. Therefore, it was suspected that the PAGAT gel is dose rate dependent at low dose rates. To test this further, three other sets of measurements were performed by placing vials containing gel at different distances from a Cs-137 source. In the first two measurements, several plastic vials were exposed to equal doses at different dose rates. An ionization chamber was used to measure the dose rate at each distance. In addition, three TLD chips were simultaneously irradiated in order to verify the dose to each vial. In the third measurement, to test the oxygen diffusion through plastic vials, the experiment was repeated again using plastic vials in a nitrogen box and glass vials. The study indicates that oxygen diffusion through plastic vials for dose rates lower than 2 Gy h−1 would affect the gel dosimeter response and it is suggested that the plastic vials or (phantoms) in an oxygen free environment or glass vials should be used for the dosimetry of low dose rate sources using PAGAT gel to avoid oxygen diffusion through the vials.

SU‐DD‐A2‐05: A Conversion Method of Low Dose Rate to Pulsed Dose Rate Intracavitary Brachytherapy Prescription for the Treatment of Cervical Carcinoma

Purpose: To report on a retrospective clinical dosimetry analysis of the conversion of the low‐dose rate (LDR) to pulsed dose rate (PDR) intracavitary brachytherapy (ICBT). Methods and Materials: A tool was developed that converts an LDR prescription to PDR. This method incorporates the differences between LDR and PDR radial dose functions and includes perturbation factors of the different ICBT applicators as derived from MCNP Monte Carlo simulations. Absolute dwell time per PDR source position is calculated and entered into the Nucletron Plato TPS. A cohort of 67 LDR patients was randomly selected from our clinical GYN database. Plans were generated with Varian BrachyVision v6.5 for Selectron LDR sources, and with Nucletron Plato v14.1 for the PDR source. Doses were compared at the ICRU 38 patient points: Bladder, Rectum, A right and left, B right and left, 3 o'clock and 9 o'clock. A 5% difference between LDR and PDR doses at these points was considered significant. Results: In every case, the difference in dose between the LDR plan and the PDR plan was less than 5%. In addition, the differences in dose appeared to be randomly distributed about zero, implying that there was no systematic difference in the LDR and PDR dose points. Conclusions: A transfer method of our traditional LDR clinical program to PDR was validated. This insures our history with ICBT LDR experience is consistent for future ICBT patients undergoing PDR treatments.

Long-Term Survival and Risk of Second Cancers After Radiotherapy for Cervical Cancer

To evaluate the risk of second cancers after cervical cancer treated with radiotherapy for Asian populations. We reviewed 2,167 patients with cervical cancer undergoing radiotherapy between 1961 and 1986. Intracavitary brachytherapy was performed with high-dose rate source (82%) or low-dose rate source (12%). Relative risk (RR), absolute excess risk (AR), and cumulative risk of second cancer were calculated using the Japanese disease expectancy table. For 1,031 patients, the impact of smoking habit on the increasing risk of second cancer was also evaluated. The total number of person-years of follow-up was 25,771, with 60 patients being lost to follow-up. Among the 2,167 patients, 1,063 (49%) survived more than 10 years. Second cancers were observed in 210 patients, representing a significant 1.2-fold risk (95% confidence interval [CI], 1.1-1.4) of developing second cancer compared with the general population, 1.6% excess risk per person per decade of follow-up, and elevating cumulative risk up to 23.8% (95% CI, 20.3-27.3) at 30 years after radiotherapy. The RR of second cancer was 1.6-fold for patients with the smoking habit and 1.4-fold for those without. Small but significant increased risk of second cancer was observed among Japanese women with cervical cancer mainly treated with high-dose rate brachytherapy. Considering the fact that about half of the patients survived more than 10 years, the benefit of radiotherapy outweighs the risk of developing second cancer.

MO‐E‐AUD‐07: Calculating the Air‐Kerma Strength and Dose‐Rate Constant of 125I and 103Pd Low Dose Rate Brachytherapy Sources Using Spectra Measured With a High‐Purity Germanium Spectrometer

Purpose: To calculate spectroscopic analogs of the air‐kerma strength and dose‐rate constant using the fully corrected energy spectra of and low dose‐rate (LDR) brachytherapy sources measured with a high‐purity germanium (HPGe) spectrometer. Methods and Materials: A high‐purity germanium spectrometer (CANBERRA, GR2519), driven by CANBERRA Genie2000™ gamma analysis software, was used to measure spectra of Theragenics model 200 and Best Medical model 2301 sources. The thin beryllium entrance window (0.05 cm) allows high transmittance of low energy photons making it ideal for LDR applications. An iterative deconvolution algorithm was used to correct each measured spectrum for detector effects such as efficiency and germanium fluorescence. The deconvolved spectrum represents the photon spectrum incident on the entrance window of the detector. The spectrum was corrected to vacuum to represent the spectrum emitted from the encapsulated source. The spectroscopic air‐kerma strength, SK,Spect, was calculated from the fully corrected emitted spectrum of each source. A spectroscopic dose‐rate constant, ΛSpect, was also calculated for both the models 200 and 2301 using the method developed by Chen and Nath (Med. Phys. 28, 86–96, 2001). Results: SK,Spect for model 200 and model 2301 sources were calculated. The values were compared with standard SK measurements showing an agreement of 3.2–3.4% for most sources, while one source exhibited a 12% difference that is being investigated further. ΛSpect was calculated to be 0.699 cGy h−1U−1 for the model 200 and 1.033 cGy h−1U−1 for the model 2301. These values are within the experimental uncertainty of the consensus dose‐rate constant values, which are 0.686 cGy h−1U−1 and 1.025 cGy h−1U−1, respectively. Conclusions: The spectroscopic method for calculating the air‐kerma strength and dose‐rate constant of LDR sources has proven to be a promising technique that can be extended to additional source models and new designs entering the market.

Calculated and measured brachytherapy dosimetry parameters in water for the Xoft Axxent X‐Ray Source: An electronic brachytherapy sourcea)

A new x-ray source, the model S700 Axxent X-Ray Source (Source), has been developed by Xoft Inc. for electronic brachytherapy. Unlike brachytherapy sources containing radionuclides, this Source may be turned on and off at will and may be operated at variable currents and voltages to change the dose rate and penetration properties. The in-water dosimetry parameters for this electronic brachytherapy source have been determined from measurements and calculations at 40, 45, and 50 kV settings. Monte Carlo simulations of radiation transport utilized the MCNP5 code and the EPDL97-based mcplib04 cross-section library. Inter-tube consistency was assessed for 20 different Sources, measured with a PTW 34013 ionization chamber. As the Source is intended to be used for a maximum of ten treatment fractions, tube stability was also assessed. Photon spectra were measured using a high-purity germanium (HPGe) detector, and calculated using MCNP. Parameters used in the two-dimensional (2D) brachytherapy dosimetry formalism were determined. While the Source was characterized as a point due to the small anode size, < 1 mm, use of the one-dimensional (1D) brachytherapy dosimetry formalism is not recommended due to polar anisotropy. Consequently, 1D brachytherapy dosimetry parameters were not sought. Calculated point-source model radial dose functions at gP(5) were 0.20, 0.24, and 0.29 for the 40, 45, and 50 kV voltage settings, respectively. For 1<r<7 cm, measured point-source model radial dose functions were typically within 4% of calculated results. Calculated values for F(r, theta) for all operating voltages were within 15% of unity along the distal end (theta=0 degree), and ranged from F(1 cm, 160 degrees) = 0.2 to F(15 cm, 175 degrees) = 0.4 towards the catheter proximal end. For all three operating voltages using the PTW chamber, measured dependence of output as a function of azimuthal angle, psi, was typically on average +/-3% for 0 degree < or = psi < or = 360 degrees. Excluding an energy response function, measurements of normalized photon energy spectra were made for three operating voltages, and were typically within 2% agreement with the normalized Monte Carlo calculated spectra. In general, the model S700 Source exhibited depth dose behavior similar to low-energy photon-emitting low dose rate sources 125I and l03Pd, yet with capability for variable and much higher dose rates and subsequently adjustable penetration capabilities. This paper presents the calculated and measured in-water brachytherapy dosimetry parameters for the model S700 Source at the aforementioned three operating voltages.

SU‐FF‐T‐312: Liquid Helium Calorimeter for Brachytherapy Source Strength Measurement

Purpose: To construct and demonstrate the utility of a new liquid helium calorimeter for the determination of contained and emitted power of low dose rate (LDR) and beta emitting brachytherapy sources Method and Materials: A liquid helium adaptation of the UWMRRC Stump calorimeter (Rev. Sci. Instrum., 76, 2005) was constructed for use with LDR and beta emitting encapsulated brachytherapy sources. Silver absorber construction provides 4π measurement geometry for contained power measurements, eliminating the need for geometric or solid‐angle corrections and removing uncertainty stemming from source anisotropy. Temperature of the absorbers as well as the detector housing is actively controlled, and an electrical substitution measurement technique is used. This technique does not require corrections for thermal coefficients that change with temperature, thus eliminating the uncertainty of the absorber mass from the measurement uncertainty. Temperature is monitored using germanium resistance thermometers, and overall system temperature stability is improved by placing the liquid helium reservoir inside a vacuum chamber that is in turn surrounded by liquid nitrogen. Results: The decreased noise floor of the liquid helium calorimeter will permit more accurate measurements of low activity sources than was possible with the Stump calorimeter. Preliminary MCNP5 Monte Carlo modeling indicates that the contained power collectors will be 99.99% efficient for the LDR sources for which the calorimeter was designed, and that for the 90Sr/90Y beta emitting source, the efficiency decreases to 98.13% as a result of the more energetic bremsstrahlung photons. Conclusion: Knowledge of the contained and emitted power for brachytherapy sources can yield a direct source strength value for use as a benchmark in Monte Carlo statistical models. The emitted fraction, which is the quotient of the emitted power and the total contained power, can also be utilized in determining the validity of geometric and atomic source composition used in statistical models.

Polymer gel dosimetry close to an 125I interstitial brachytherapy seed

Despite its advantages, the polymer gel–magnetic resonance imaging (MRI) method has not, as yet, been successfully employed in dosimetry of low energy/low dose rate photon-emitting brachytherapy sources such as 125I or 103Pd interstitial seeds. In the present work, two commercially available 125I seed sources, each of approximately 0.5 U, were positioned at two different locations of a polymer gel filled vial. The gel vial was MR scanned with the sources in place 19 and 36 days after seed implantation. Calibration curves were acquired from the coupling of MRI measurements with accurate Monte Carlo dose calculations obtained simulating the exact experimental setup geometry and materials. The obtained gel response data imply that while linearity of response is sustained, sensitivity (calibration curve slope) is significantly increased (approximately 60%) compared to its typical value for the 192Ir (or 60Co and 6 MV LINAC) photon energies. Water equivalence and relative energy response corrections of the gel cannot account for more than 3–4% of this increase, which, therefore, has to be mainly attributed to physicochemical processes related to the low dose rate of the sources and the associated prolonged irradiation time. The calibration data obtained from one 125I source were used to provide absolute dosimetry results for the other 125I source, which were found to agree with corresponding Monte Carlo calculations within experimental uncertainties. It is therefore suggested that, regardless of the underlying factors accounting for the gel dose response to 125I irradiations, polymer gel dosimetry of new 125I or 103Pd sources should be carried out as originally proposed by Heard and Ibbot (2004 J. Phys.: Conf. Ser. 3 221–3), i.e., by irradiating the same gel sample with the new low dose rate source, as well as with a well-characterized low dose rate source which will provide the dose calibration curve for the same irradiation conditions.

Calibration of the photon component of 198Au stents

Purpose 198Au has promising characteristics for radioactive stent material, having properties as a mixed β-particle and γ emitter. Calibration of these radioactive stents is required to provide accurate clinical dosimetry. Methods and materials We have developed an electroplating technique to incorporate stable gold onto stents followed by activation to 198Au in the University of Wisconsin nuclear reactor. The calibration method is a modification of the NIST traceable, in-air calibration technique for high-dose-rate (HDR) 192Ir sources. Results The air-kerma strength of HDR and low-dose-rate (LDR) sources was measured for proof of principle and found to agree to within 3% of values obtained with other NIST traceable calibration techniques. The photon component of two 198Au radioactive stents was measured over a period of 3 days. Conclusion The air-kerma strength of HDR and LDR sources was measured for proof of principle and found to agree to within 3% of values obtained with other NIST traceable calibration techniques.

Low vs high dose-rate effects on the acute reaction of pig skin to cobalt-60 gamma rays

The acute moist reaction of pig skin to regimens of low and high dose-rate irradiation with plaques containing a single, small 60Co source was studied in order to determine the application time for a continuous, low dose-rate exposure which is equivalent to a fractionated, high dose-rate course. The dose distributions for the high and low dose rate sources were identical. All irradiations were carried out without anesthesia. The visual threshold for moist desquanwtion, i.e., the margin of the initial peak reaction, served as the biological endpoint. Ten young Chester White female pigs were irradiated—eight areas on the high dose-rate (control) side, and eight corresponding areas on the low dose-rate (experimental) side. During the subsequent 60-day observation period the longitudinal and transverse diameters of the acute moist reactions were measured three times weekly (Mon.-Wed.-Fri.). For the purpose of analysis the average initial peak diameter of the moist reaction was plotted against application time for the low dose-rate (experimental) side of each pig. linear regression was employed to determine the isoeffect low dose-rate application time. It was found that a continuous, low dose rate application of 63.8 hours duration at a dose-rate of 88.6 ran per hour was equivalent to four 1100 ran high dose-rate fractions separated by 24-hour intervals. The present experimental data for 60Co gamma rays are useful in calculating the parameters of time-dose formulae desinged to equate low and high dose-rate radiotherapy.