Air Kerma Strength Research Articles

Determination of task group 43 dosimetric parameters for CSM40 137Cs source for use in brachytherapy.

The CSM40 137Cs source model is currently being used in clinical brachytherapy. According to the recommendations of task group No. 43 (TG-43) of the American Association of Physicists in Medicine, dosimetry parameters of brachytherapy sources should be determined by two independent investigators before their clinical use. The aim of this study was to determine the TG-43 dosimetry parameters for a medium-dose-rate CSM40 137Cs source. The determined dosimetric parameters included the air kerma strength, dose rate constant, radial dose function, and anisotropy function. To determine the source's dosimetric parameters, the CSM40 source was stimulated by the Monte Carlo N-Particle MCNP code. The TG-43 parameters were compared with the data of Vijande et al. on this source. The results showed that the dosimetry parameters for this source had good agreement with the results of Vijande et al. The dosimetric parameters of the CSM40 source can be used in treatment-planning systems incorporating this source model. The data can also be used for the quality assurance of treatment-planning systems.

Risk Assessment in Permanent Prostate Brachytherapy Using AAPM TG 100 Methodology

to analyze major risks associated with permanent prostate brachytherapy based on reported medical events and to provide a reasonable estimate of O (Occurrence), S (Severity) and D (lack of detectability) factors for major failure modes (FM) in permanent prostate brachytherapy. Between January 1999 and December 2016, 384 patients received prostate seed implants that were reported to NRC as medical events. The majority of those events are excluded from the current study because they were reported solely based on a dose-volume parameter from post-implant dosimetry, e.g. , D90less than the prescribed dose, what could well be the results of prostate expansions before and after the procedures. Incidents related to leaking sources, lost seed or shipment, or aborted procedures due to technical difficulties are also excluded. The present study focuses on the remaining 85 events, as sufficient details were supplied in the reports. For each FM, the Occurrence (O) factor is determined as the frequency of occurrence relative to the most common failure mode(O=10). The Severity (S) factor of each event, on a scale of 1-10, was given mainly based on the magnitude of dose deviation from the prescribed dose or seed displacement relative to the planned position, or the percentage of source strength implanted outside the target volume. The Severity of each FM is then taken as the average of the severity factors of all medical events in the very category. The Lack of Detectability (D), also on a scale of 1-10, is assigned to each FM based on the nature of the failure, number of patients involved in the same error and author’s experience. The Risk Priority Number or RPN is calculated as RPN=OxSxD. Ten(10) major failure modes (FM) are identified in the study, and they are (from highest to lowest RPN): 1. Significant portion of source strength or activity implanted outside of target volume; 2. Incorrect activity used because of unit error (mCi or air kerma strength) ; 3. Incorrect dose rate constant used in planning system; 4. Incorrect prescription dose used in planning; 5. Shipping error; 6. Incorrect calculation in planning; 7. Wrong plan used in back-to-back implants; 8. Wrong seed package used in back-to back implants; 9. Wrong activity entered in computer planning; and (10) implant rescheduled but old set of seed package used for implant. Incidents reported to regulatory agencies can be used to estimate RPN values for errors in permanent prostate brachytherapy. AAPM TG 100 provides an effective tool for developing a QM program by prioritizing the use of QA resources with priorities given to the most hazardous failures.

Gamma dosimetric parameters in some skeletal muscle relaxants

We have studied the attenuation of gamma radiation of energy ranging from 84 keV to 1330 keV ( $$^{170}\hbox {Tm}, ^{22}\!\hbox {Na},^{137}\!\hbox {Cs}$$ , and $$^{60}\hbox {Co}$$ ) in some commonly used skeletal muscle relaxants such as tubocurarine chloride, gallamine triethiodide, pancuronium bromide, suxamethonium bromide and mephenesin. The mass attenuation coefficient is measured from the attenuation experiment. In the present work, we have also proposed the direct relation between mass attenuation coefficient $$(\mu /\rho )$$ and mass energy absorption coefficient $$(\mu _{\mathrm{en}}/\rho )$$ based on the nonlinear fitting procedure. The gamma dosimetric parameters such as mass energy absorption coefficient $$(\mu _{\mathrm{en}}/\rho )$$ , effective atomic number $$(Z_{\mathrm{eff}})$$ , effective electron density $$(N_{\mathrm{el}})$$ , specific $${\upgamma }$$ -ray constant, air kerma strength and dose rate are evaluated from the measured mass attentuation coefficient. These measured gamma dosimetric parameters are compared with the theoretical values. The measured values agree with the theoretical values. The studied gamma dosimetric values for the relaxants are useful in medical physics and radiation medicine.

Technical Note: Empirical altitude correction factors for well chamber measurements of permanent prostate and breast seed implant sources.

Previous studies in the literature have measured an altitude effect for low-energy brachytherapy seeds; a correction factor applied in addition to PTP to account for the breakdown of Bragg-Gray cavity theory at low energies in well-type ionization chambers. In clinical practice, many centers use altitude correction factors that are not seed-model-specific. The purpose of this work is to present altitude correction factors for several seed models without documented factors in the literature. An in-house constructed pressure vessel was used with a well-type ionization chamber to measure the air-kerma strength of the IsoAid Advantage (Pd-103), Theragenics AgX100 (I-125), and Nucletron selectSeed (I-125) at a pressure range representative of those encountered worldwide. The TheraSeed 200 (Pd-103) was also measured for comparison to the originally published correction factor for validation of the experimental process. When correction factors derived in this work were within experimental uncertainties of those published, no new correction factors were proposed. The three seed models measured herein all demonstrated a similar response to change in pressure as previously documented in the literature with the HDR 1000 Plus well-type ionization chamber. Correction factors of the functional form PA=k1(P[torr])k2, consistent with those previously published, were found to be appropriate for these seed models. A new correction factor is proposed for the Theragenics AgX100 and Nucletron selectSeed (k1 =0.0417, k2 =0.479). The IsoAid Advantage, however, agreed to within uncertainty with the published altitude correction factor for the TheraSeed200; thus the application of the same correction factor is appropriate (k1 =0.0241, k2 =0.562). This work presents altitude correction factors for three permanent implant brachytherapy seed models in clinical use. This will allow clinics to utilize model-specific factors, reducing systematic errors in their air-kerma strength verifications.

Monte Carlo dosimetric characterization of the Flexisource Co-60 high-dose-rate brachytherapy source using PENELOPE

Purpose60Co sources have been commercialized as an alternative to 192Ir sources for high-dose-rate (HDR) brachytherapy. One of them is the Flexisource Co-60 HDR source manufactured by Elekta. The only available dosimetric characterization of this source is that of Vijande et al. [J Contemp Brachytherapy 2012; 4:34–44], whose results were not included in the AAPM/ESTRO consensus document. In that work, the dosimetric quantities were calculated as averages of the results obtained with the Geant4 and PENELOPE Monte Carlo (MC) codes, though for other sources, significant differences have been quoted between the values obtained with these two codes. The aim of this work is to perform the dosimetric characterization of the Flexisource Co-60 HDR source using PENELOPE. Methods and MaterialsThe MC simulation code PENELOPE (v. 2014) has been used. Following the recommendations of the AAPM/ESTRO report, the radial dose function, the anisotropy function, the air-kerma strength, the dose rate constant, and the absorbed dose rate in water have been calculated. ResultsThe results we have obtained exceed those of Vijande et al. In particular, the absorbed dose rate constant is ∼0.85% larger. A similar difference is also found in the other dosimetric quantities. The effect of the electrons emitted in the decay of 60Co, usually neglected in this kind of simulations, is significant up to the distances of 0.25 cm from the source. ConclusionsThe systematic and significant differences we have found between PENELOPE results and the average values found by Vijande et al. point out that the dosimetric characterizations carried out with the various MC codes should be provided independently.

Online pretreatment verification of high-dose rate brachytherapy using an imaging panel

Brachytherapy is employed to treat a wide variety of cancers. However, an accurate treatment verification method is currently not available. This study describes a pre-treatment verification system that uses an imaging panel (IP) to verify important aspects of the treatment plan.A detailed modelling of the IP was only possible with an extensive calibration performed using a robotic arm. Irradiations were performed with a high dose rate (HDR) 192Ir source within a water phantom. An empirical fit was applied to measure the distance between the source and the detector so 3D Cartesian coordinates of the dwell positions can be obtained using a single panel. The IP acquires 7.14 fps to verify the dwell times, dwell positions and air kerma strength (Sk). A gynecological applicator was used to create a treatment plan that was registered with a CT image of the water phantom used during the experiments for verification purposes. Errors (shifts, exchanged connections and wrong dwell times) were simulated to verify the proposed verification system.Cartesian source positions (panel measurement plane) have a standard deviation of about 0.02 cm. The measured distance between the source and the panel (z-coordinate) have a standard deviation up to 0.16 cm and maximum absolute error of ≈0.6 cm if the signal is close to sensitive limit of the panel. The average response of the panel is very linear with Sk. Therefore, Sk measurements can be performed with relatively small errors. The measured dwell times show a maximum error of 0.2 s which is consistent with the acquisition rate of the panel. All simulated errors were clearly identified by the proposed system.The use of IPs is not common in brachytherapy, however, it provides considerable advantages. It was demonstrated that the IP can accurately measure Sk, dwell times and dwell positions.

Determination of TG-43 Dosimetric Parameters for Photon Emitting Brachytherapy Sources.

Objective:Brachytherapy sources are widely used for the treatment of cancer. The report of Task Group No. 43 (TG-43) of American Association of Physicists in Medicine is known as the most common method for the determination of dosimetric parameters for brachytherapy sources. The aim of this study is to obtain TG-43 dosimetric parameters for 60Co, 137Cs, 192Ir and 103Pd brachytherapy sources by Monte Carlo simulation. Methods:In this study, 60Co (model Co0.A86), 137Cs (model 6520-67), 192Ir (model BEBIG) and 103Pd (model OptiSeed) brachytherapy sources were simulated using MCNPX Monte Carlo code. To simulate the sources, the exact geometric characterization of each source was defined in Monte Carlo input programs. Dosimetric parameters including air kerma strength, dose rate constant, radial dose function and anisotropy function were calculated for each source. Each input program was run with sufficient number of particle histories. The maximum type A statistical uncertainty in the simulation of the 60Co, 137Cs, 192Ir and 103Pd sources, were equal to 4%, 4%, 3.19% and 6.50%, respectively. Results:The results for dosimetry parameters of dose rate constant, radial dose function and anisotropy function for the 60Co, 137Cs, 192Ir and 103Pd sources in this study demonstrated good agreement with other studies. Conclusion:Based on the good agreement between the results of this study and other studies, the TG-43 results for Co0.A86 60Co, 67-65200 137Cs, BEBIG 192Ir and OptiSeed 103Pd sources are validated and can be used as input data in treatment planning systems (TPSs) and to validate the TPS calculations.

Verification of SuperMC for simulation of a high-dose-rate brachytherapy source

In this study, SuperMC (Super Monte Carlo simulation program for nuclear and radiation simulation) was tested and verified for simulation of a high-dose-rate brachytherapy source. The Monte Carlo simulation includes calculations of the air kerma strength, dose rate constant, radial dose function and anisotropy function as recommended by the American Association of Physicists in Medicine (AAPM) in Task Group reports 43 and 43U1 (TG-43, TG-43U1). The air kerma strength, dose rate constant, radial dose function and anisotropy function were compared with previously published Monte Carlo simulation results and experimental data. The calculated parameters were found to be in good agreement with published Monte Carlo and measured data. The value obtained from the SuperMC simulation for the air kerma strength was 9.779 × 10−8 U·Bq−1 and the dose rate constant was 1.1092 ± 0.02% cGy·h−1·U−1. The time to transport 5 × 107 photons showed SuperMC to be relatively faster than MCNP. The results show that SuperMC can be used for fast and accurate simulations and dosimetric calculations of HDR brachytherapy sources.

Determination of dosimetric parameters for shielded 153Gd source in prostate cancer brachytherapy.

BackgroundInterstitial rotating shield brachytherapy (I-RSBT) is a recently developed method for treatment of prostate cancer. In the present study TG-43 dosimetric parameters of a 153Gd source were obtained for use in I-RSBT.Materials and methodsA 153Gd source located inside a needle including a Pt shield and an aluminum window was simulated using MCNPX Monte Carlo code. Dosimetric parameters of this source model, including air kerma strength, dose rate constant, radial dose function and 2D anisotropy function, with and without the shields were calculated according to the TG-43 report.ResultsThe air kerma strength was found to be 6.71 U for the non-shielded source with 1 GBq activity. This value was found to be 0.04 U and 6.19 U for the Pt shield and Al window cases, respectively. Dose rate constant for the non-shielded source was found to be 1.20 cGy/(hU). However, for a shielded source with Pt and aluminum window, dose rate constants were found to be 0.07 cGy/(hU) and 0.96 cGy/(hU), on the shielded and window sides, respectively. The values of radial dose function and anisotropy function were tabulated for these sources. Additionally, isodose curves were drawn for sources with and without shield, in order to evaluate the effect of shield on dose distribution.ConclusionsExistence of the Pt shield may greatly reduce the dose to organs at risk and normal tissues which are located toward the shielded side. The calculated air kerma strength, dose rate constant, radial dose function and 2D anisotropy function data for the 153Gd source for the non-shielded and the shielded sources can be used in the treatment planning system (TPS).

Air-kerma strength determination of an HDR 192 Ir source including a geometric sensitivity study of the seven-distance method.

To perform an in-air air-kerma strength (SK) calibration of the Bebig model Ir2.A85-2 192Ir high-dose rate (HDR) brachytherapy source manufactured by Mallinckrodt Medical (Westerduinweg, Germany) with the NIST-traceable seven-distance technique established by the University of Wisconsin. A comparison was made between the reference air-kerma rate (RAKR) reported on a certificate from the Physikalisch-Technische Bundesanstalt (Berlin, Germany) (PTB) primary laboratory and the SK determined at the University of Wisconsin Madison Radiation Research Center (UWMRRC). A theoretical sensitivity study was performed to investigate the impact that variations in the experimental setup have on the computed SK from the seven-distance algorithm in order to determine if the uncertainty budget for the seven-distance method should be expanded. The manufacturer-reported SK for the source was compared to the SK determined from the University of Wisconsin Accredited Dosimetry Calibration Laboratory (UWADCL) transfer standard well chambers and the seven-distance technique. Monte Carlo techniques (MCNP6) were employed to compare the theoretical SK calibration coefficients of a Standard Imaging (Middleton, WI) HDR1000 Plus well chamber using Standard Imaging model 70010 and model 70110 source holders to determine if a holder dependence was present. Radiochromic film (EBT3, Ashland) exposures were performed to assess the dose distribution of the source in phantom. The seven-distance algorithm was coded in MATLAB®(R2013b) and benchmarked with MCNP6 with the capacity to model distance offset behaviors among nominal positions. This offset model was used in a Monte Carlo simulation coded in MATLAB to determine the average uncertainty in the SK calculations from the seven-distance algorithm. The measured SK using the seven-distance technique at the UWMRRC agreed with the RAKR reported on the PTB source certificate and the SK on the Mallinckrodt source certificate to within 0.28% and -0.79%, respectively. It was found that the difference between the SK measured from the transfer standard well chambers at the UWADCL and the seven-distance method was between 0.13% and 0.30% at the 95% confidence level. Monte Carlo results showed negligible differences between the simulated SK calibration coefficient for an HDR1000 Plus well chamber using the model 70010 or model 70110 source holder. The autoradiographs from the source in Virtual WaterTM showed that the dose distribution is symmetric. Additionally, the sensitivity study performed in MATLAB showed that the SK calculated with the seven-distance algorithm could deviate by 0.24% from randomly generated distance offsets within 1mm in magnitude. The differences between the SK measurements determined from the seven-distance technique and the accredited UWADCL measurement results were within the k=2 uncertainty reported for an accredited calibration. Excellent agreement was found between the measured SK and RAKR methods used at the UWMRRC and PTB, respectively. Additionally, the sensitivity study has shown that the seven-distance algorithm accurately determines the SK of a source while having a variable chamber offset among nominal positions; the uncertainty budget for the seven-distance method does not need to be expanded at this time. It has been determined that the current standard used by the UWADCL for well chamber calibrations is valid for the Bebig model Ir2.A85-2 192Ir brachytherapy source.

Focal application of low-dose-rate brachytherapy for prostate cancer: a pilot study.

PurposeTo evaluate the feasibility and to report the early outcomes of focal treatment of prostate cancer using low-dose-rate brachytherapy (LDR-PB).Material and methodsSeventeen patients were screened with multi-parametric magnetic resonance imaging (mpMRI), 14 of whom proceeded to receive trans-perineal template mapping biopsy (TTMB). Focal LDR-PB was performed on five eligible patients using dual air kerma strength treatment plans based on planning target volumes derived from cancer locations and determined by TTMB. Patient follow-up includes prostate specific antigen (PSA) measurements, urinary and sexual function questionnaires, repeated imaging and TTMB at specific intervals post-treatment.ResultsFeasibility of focal LDR-PB was shown and short-term outcomes are promising. While the detection rate of tumors, a majority of which were low grade GS 3 + 3, was found to be low on mpMRI (sensitivity of 37.5%), our results suggest the potential of mpMRI in detecting the presence of higher grade (GS ≥ 3 + 4), and bilateral disease indicating its usefulness as a screening tool for focal LDR-PB.ConclusionsLow-dose-rate brachytherapy is a favorable ablation option for focal treatment of prostate cancer, requiring minimal modification to the standard (whole gland) LDR-PB treatment, and appears to have a more favorable side effect profile. Further investigation, in the form of a larger study, is needed to assess the methods used and the long-term outcomes of focal LDR-PB.

A directional 103Pd brachytherapy device: Dosimetric characterization and practical aspects for clinical use

PurposeA brachytherapy (BT) device has been developed with shielding to provide directional BT for preferentially irradiating malignancies while sparing healthy tissues. The CivaSheet is a flexible low-dose-rate BT device containing CivaDots with 103Pd shielded by a thin Au disk. This is the first report of a clinical dosimetric characterization of the CivaSheet device. Methods and MaterialsRadiation dose distributions near a CivaDot were estimated using the MCNP6 radiation transport code. CivaSheet arrays were also modeled to evaluate the dose superposition principle for treatment planning. The resultant data were commissioned in a treatment planning system (TPS) (VariSeed 9.0), and the accuracy of the dose superposition principle was evaluated for summing individual elements comprising a planar CivaSheet. ResultsThe dose-rate constant (0.579 cGy/h/U) was lower than for 103Pd seeds due to Au L-shell x-rays increasing the air-kerma strength. Radial dose function values at 0.1, 0.5, 2, 5, and 10 cm were 1.884, 1.344, 0.558, 0.088, and 0.0046, respectively. The two-dimensional anisotropy function exhibited dramatic reduction between the forward (0°) and rearward (180°) directions by a factor of 276 at r = 0.1 cm, 24 at r = 1 cm, and 5.3 at r = 10 cm. This effect diminished due to increasingly scattered radiation. The largest gradient in the two-dimensional anisotropy function was in contact with the device at 92° due to the Au disk shielding. TPS commissioning and dose superposition accuracies were typically within 2%. ConclusionsSimulations of the CivaDot yielded comprehensive dosimetry parameters that were entered into a TPS and deemed acceptable for clinical use. Dosimetry measurements of the CivaSheet are also of interest to the BT community.

Transit dose comparisons for 60Co and 192Ir HDR sources

The goal of this study is to evaluate the ambient dose due to the transit of high dose rate (HDR) 60Co sources along a transfer tube as compared to 192Ir ones in a realistic clinical scenario.This goal is accomplished by evaluating air-kerma differences with Monte Carlo calculations using PENELOPE2011. Scatter from both the afterloader and the patient was not taken into account. Two sources, mHDR-v2 and Flexisource Co-60, (Elekta Brachytherapy, Veenendaal, the Netherlands) have been considered. These sources were simulated within a standard transfer tube located in an infinite air phantom. The movement of the source was included by displacing their positions along the connecting tube from z = −75 cm to z = +75 cm and combining them. Since modern afterloaders like Flexitron (Elekta) or Saginova (BEBIG GmbH) are able to use equally 192Ir and 60Co sources, it was assumed that both sources are displaced with equal speed. Typical HDR source activity content values were provided by the manufacturer.2D distributions were obtained with type-A uncertainties (k = 2) less than 0.01%. From those, the air-kerma ratio 60Co/192Ir was evaluated weighted by their corresponding typical activities. It was found that it varies slowly with distance (less than 10% variation at 75 cm) but strongly in time due to the shorter half-life of the 192Ir (73.83 d). The maximum ratio is located close to the tube. It reaches a value of 0.57 when the typical activity of the sources at the time when they were installed by the vendor was used. Such ratio increases up to 1.28 at the end of the recommended working life (90 d) of the 192Ir source.60Co/192Ir air-kerma ratios are almost constant (0.51–0.57) in the vicinity of the source-tube with recent installed sources. Nevertheless, air-kerma ratios increase rapidly (1.15–1.29) whenever the 192Ir is approaching the end of its life. In case of a medical event requiring the medical staff to access the treatment room, these ratios indicate that the dosimetric impact on the medical team will be lower, with a few exceptions, in the case of 60Co-based HDR brachytherapy as compared to 192Ir-based one when typical air-kerma strength values are considered.

SU-G-201-15: Nomogram as an Efficient Dosimetric Verification Tool in HDR Prostate Brachytherapy

Purpose: Nomogram as a simple QA tool for HDR prostate brachytherapy treatment planning has been developed and validated clinically. Reproducibility including patient-to-patient and physician-to-physician variability was assessed. Methods: The study was performed on HDR prostate implants from physician A (n=34) and B (n=15) using different implant techniques and planning methodologies. A nomogram was implemented as an independent QA of computer-based treatment planning before plan execution. Normalized implant strength (total air kerma strength Sk*t in cGy cm2 divided by prescribed dose in cGy) was plotted as a function of PTV volume and total V100. A quadratic equation was used to fit the data with R2 denoting the model predictive power. Results: All plans showed good target coverage while OARs met the dose constraint guidelines. Vastly different implant and planning styles were reflected on conformity index (entire dose matrix V100/PTV volume, physician A implants: 1.27±0.14, physician B: 1.47±0.17) and PTV V150/PTV volume ratio (physician A: 0.34±0.09, physician B: 0.24±0.07). The quadratic model provided a better fit for the curved relationship between normalized implant strength and total V100 (or PTV volume) than a simple linear function. Unlike the normalized implant strength versus PTV volume nomogram which differed between physicians, a unique quadratic model based nomogram (Sk*t)/D=−0.0008V2+0.0542V+1.1185 (R2=0.9977) described the dependence of normalized implant strength on total V100 over all the patients from both physicians despite two different implant and planning philosophies. Normalized implant strength - total V100 model also generated less deviant points distorting the smoothed ones with a significantly higher correlation. Conclusion: A simple and universal, excel-based nomogram was created as an independent calculation tool for HDR prostate brachytherapy. Unlike similar attempts, our nomogram is insensitive to implant style and does not rely on reproducing dose calculations using TG-43 formalism, thus making it a truly independent check.

SU‐F‐T‐12: Monte Carlo Dosimetry of the 60Co Bebig High Dose Rate Source for Brachytherapy

Purpose:The purpose of this work is to obtain the dosimetry parameters in accordance with the AAPM TG‐43U1 formalism with Monte Carlo calculations regarding the BEBIG 60Co high‐dose‐rate brachytherapy. The geometric design and material details of the source was provided by the manufacturer and was used to define the Monte Carlo geometry.Methods:The dosimetry studies included the calculation of the air kerma strength Sk, collision kerma in water along the transverse axis with an unbounded phantom, dose rate constant and radial dose function. The Monte Carlo code system that was used was EGSnrc with a new cavity code, which is a part of EGS++ that allows calculating the radial dose function around the source. The XCOM photon cross‐section library was used. Variance reduction techniques were used to speed up the calculation and to considerably reduce the computer time. To obtain the dose rate distributions of the source in an unbounded liquid water phantom, the source was immersed at the center of a cube phantom of 100 cm3.Results:The obtained dose rate constant for the BEBIG 60Co source was 1.108±0.001 cGyh‐1U‐1, which is consistent with the values in the literature. The radial dose functions were compared with the values of the consensus data set in the literature, and they are consistent with the published data for this energy range.Conclusion:The dose rate constant is consistent with the results of Granero et al. and Selvam and Bhola within 1%. Dose rate data are compared to GEANT4 and DORZnrc Monte Carlo code. However, the radial dose function is different by up to 10% for the points that are notably near the source on the transversal axis because of the high‐energy photons from 60Co, which causes an electronic disequilibrium at the interface between the source capsule and the liquid water for distances up to 1 cm.

SU‐F‐T‐05: Dosimetric Evaluation and Validation of Newlydeveloped Well Chamber for Use in the Calibration of Brachytherapy Sources

Purpose:To evaluate the dosimetric characteristics of newly developed well type ionization chamber and to validate the results with the commercially available calibrated well chambers that are being used for the calibration of brachytherapy sources.Methods:The newly developed well type ionization chamber (BDS 1000) has been designed for the convenient use in brachytherapy which is open to atmospheric condition. The chamber has a volume of 240 cm3 and weight of 2.5 Kg. The calibration of the radioactive source with activities from 0.01 mCi to 20 Ci can be carried out using this chamber. The dosimetric parameters such as leakage current, stability, scattering effect, ion collection efficiency, reference air kerma rate and nominal response with energy were carried out with the BDS 1000 well type ion chamber. The evaluated dosimetric characteristics of BDS1000 well chamber were validated with two other commercially available well chambers (HDR 1000 plus and BTC/3007).Results:The measured leakage current observed was negligible for the newly developed BDS 1000 well type ion chamber. The ion collection efficiency was close to 1 and the response of the chamber was found to be very stable. The determined sweet spot was at 42 mm from bottom of the chamber insert. The reference air kerma rate was found to be 4.634 × 105 Gym2hr‐1A‐1 for the BDS 1000 well chamber. The overall dosimetric characteristics of BDS 1000 well chamber was in good agreement with the dosimetric properties of other two well chambers.Conclusion:The dosimetric study shows that the newly developed BDS 1000 well type ionization chamber is high sensitive and reliable chamber for reference air kerma strength calibration. The results obtained confirm that this chamber can be used for the calibration of HDR and LDR brachytherapy sources.

SU‐F‐T‐09: In Phantom Full‐Implant Validation of Plastic Scintillation Detectors for in Vivo Dosimetry During Low Dose Rate Brachytherapy

Purpose:To validate in a water phantom the use of plastic scintillation detectors to measure dose to the urethra and the rectal wall during a clinically realistic low dose rate (LDR) brachytherapy implant.Methods:A template was designed to replicate a clinically realistic LDR brachytherapy prostate implant inside a water phantom. Twenty‐two catheters were inserted, including one mimicking the urethra and another the rectal wall. The needles inserted in the remaining 20 catheters were composed of thin‐walled nylon tubes in which I‐125 radioactive seeds (Air Kerma Strengths of (0.328±0.020)U) were abutted together with plastic spacers to replicate a typical loading. A plastic scintillation detector (PSD) with a 5‐mm long × 1‐mm diameter sensitive element was first placed inside the urethra and 1‐second measurements were performed for 60s after each needle implant. Measurements were also performed at multiple positions along the urethra once all the needles were inserted. The procedure was then repeated with the PSD placed at the rectal wall.Results:Individual dose‐rates ranging from 0.07µGy/s to 1.5µGy/s were measured after each needle implant. The average absolute relative differences were (6.2±3.6)% and (6.9±6.5)% to the values calculated with the TG‐43 formalism, for the urethra and rectal wall respectively. These results are within expectations from the error uncertainty budget once accounting for uncertainties in seeds’ strength and positioning. Interestingly, the PSD allowed for unplanned error detection as the study was performed. Finally, the measured dose after the full implant at different positions along the mimicked organs at risk were in agreement with TG‐43 values for all of the positions tested.Conclusion:Plastic scintillation detectors could be used as in vivo detectors for LDR brachytherapy as they would provide accurate dose information after each needle implant as well as along the organs at risk at the end of the implant.

SU-G-201-06: Directional Low-Dose Rate Brachytherapy: Determination of the TG-43 Dose-Rate Constant Analog for a New Pd-103 Source

Purpose: The aim of this work is to determine the TG-43 dose-rate constant analog for a new directional low-dose rate brachytherapy source based on experimental methods and comparison to Monte Carlo simulations. The CivaSheet™ is a new commercially available planar source array comprised of a variable number of discrete directional source elements called “CivaDots”. Given the directional nature and non-conventional design of the source, modifications to the AAPM TG-43 protocol for dosimetry are required. As a result, various parameters of the TG-43 dosimetric formalism have to be adapted to accommodate this source. This work focuses on the dose-rate constant analog determination for a CivaDot. Methods: Dose to water measurements of the CivaDot were performed in a polymethyl methacrylate phantom (20×20×12 cm3) using thermoluminescent dosimeters (TLDs) and Gafchromic EBT3 film. The source was placed in the center of the phantom, and nine TLD micro-cubes were irradiated along its central axis at a distance of 1 cm. For the film measurements, the TLDs were substituted by a (3×3) cm2 EBT3 film. Primary air-kerma strength measurements of the source were performed using a variable-aperture free-air chamber. Finally, the source was modeled using the Monte Carlo N-Particle Transport Code 6. Results: Dose-rate constant analog observed for a total of eight CivaDots using TLDs and five CivaDots using EBT3 film was within ±7.0% and ±2.9% of the Monte Carlo predicted value respectively. The average difference observed was −4.8% and −0.1% with a standard deviation of 1.7% and 2.1% for the TLD and the film measurements respectively, which are both within the comparison uncertainty. Conclusion: A preliminary investigation to determine the doserate constant analog for a CivaDot was conducted successfully with good agreement between experimental and Monte Carlo based methods. This work will aid in the eventual realization of a clinically-viable dosimetric framework for the CivaSheet. This work was partially supported by NCI contract (HHSN261201200052C) through CivaTech Oncology Inc.

Automating Position and Timing Quality Assurance for High Dose Rate Brachytherapy Using Radioluminescent Phosphors and Optical Imaging

procedures for both systems are entirely different. The Axxent system uses a well-type chamber calibrated in terms of Air Kerma Strength (AKS). The calibration system for the Intrabeam device is performed in terms of absorbed dose to water utilizing a soft X-ray ionization chamber in a water. The calibration factor of the chamber is derived from Air Kerma calibrations and a conversion into Dw according to ICRU 17. For the development of the primary standard device and investigations of the calibration chain characterizations of both sources were required: X-ray emission spectra were measured at various azimuthal and radial angles, relative 3Ddose distributions in distances below 3 cm were determined with radiochromic gels. Scintillator and X-ray storage film measurements were used for complement measurements. Monte-Carlo-simulations were performed to mimic the characteristics of the sources, for the characterization of the utilized measuring devices, and to calculate conversionand correction factors for the primary standard. The German (PTB) and Czech (CMI) national collaborated in this project, founded in the framework of the European Metrology Research Programme (EMRP) (project: MetrExtRT, http://radiotherapy-emrp.eu/). The prototype of the primary standard is based on the ‘‘Grovex II’’, an extrapolation chamber in a phantom of water-equivalent material, which is already utilized at PTB as a primary standard device for I-125 seeds. However, the phantom material, RW1, is less suitable for the radiation fields of EBXs as due to the higher amount of low-energy photons much more disturbing lowenergy fluorescence-photons of the ingredients of RW1, Mg and Ca, are induced. Therefore, the phantom material needed to be substituted and after rebuilding, the chamber needed to be recommissioned. Results: The results of the measurements for the characterization of the sources and the complementary Monte Carlo simulations will be presented with a special emphasis on the information required for the realization and dissemination of the desired quantity absorbed dose to water. The procedure of the realization will be exemplified and compared to that of the realization of the absorbed dose to water for I-125 seeds including a discussion of the uncertainties. Conclusions: The realization of the absorbed dose to water for EBXs is ongoing. In a further step consistent procedures or protocols for the dissemination of the quantity needs to be established. Thereby, fundamental issues are the establishment of criteria to be met by devices defines as EBX and a consistent protocol for all EBX devices is desirable or practicable.

Effect of photon energy spectrum on dosimetric parameters of brachytherapy sources.

AimThe aim of this study is to quantify the influence of the photon energy spectrum of brachytherapy sources on task group No. 43 (TG-43) dosimetric parameters.BackgroundDifferent photon spectra are used for a specific radionuclide in Monte Carlo simulations of brachytherapy sources.Materials and methodsMCNPX code was used to simulate 125I, 103Pd, 169Yb, and 192Ir brachytherapy sources. Air kerma strength per activity, dose rate constant, radial dose function, and two dimensional (2D) anisotropy functions were calculated and isodose curves were plotted for three different photon energy spectra. The references for photon energy spectra were: published papers, Lawrence Berkeley National Laboratory (LBNL), and National Nuclear Data Center (NNDC). The data calculated by these photon energy spectra were compared.ResultsDose rate constant values showed a maximum difference of 24.07% for 103Pd source with different photon energy spectra. Radial dose function values based on different spectra were relatively the same. 2D anisotropy function values showed minor differences in most of distances and angles. There was not any detectable difference between the isodose contours.ConclusionsDosimetric parameters obtained with different photon spectra were relatively the same, however it is suggested that more accurate and updated photon energy spectra be used in Monte Carlo simulations. This would allow for calculation of reliable dosimetric data for source modeling and calculation in brachytherapy treatment planning systems.