192Ir Brachytherapy Sources Research Articles

Radiochromic film dosimetry of HDR 192Ir source radiation fields

A radiochromic film based dosimetry system for high dose rate (HDR) Iridium-192 brachytherapy source was described. A comparison between calibration curves established in water and Solid Water™ was provided. Pieces of EBT-2 model GAFCHROMIC™ film were irradiated in both water and Solid Water™ with HDR (192)Ir brachytherapy source in a dose range from 0 to 50 Gy. Responses of EBT-2 GAFCHROMIC™ film were compared for irradiations in water and Solid Water™ by scaling the dose between media through Monte Carlo calculated conversion factor for both setups. To decrease uncertainty in dose delivery due to positioning of the film piece with respect to the radiation source, traceable calibration irradiations were performed in a parallel-opposed beam setup. The EBT-2 GAFCHROMIC™ film based dosimetry system described in this work can provide an overall one-sigma dose uncertainty of 4.12% for doses above 1 Gy. The ratio of dose delivered to the sensitive layer of the film in water to the dose delivered to the sensitive layer of the film in Solid Water™ was calculated using Monte Carlo simulations to be 0.9941 ± 0.0007. A radiochromic film based dosimetry system using only the green color channel of a flatbed document scanner showed superior precision if used alone in a dose range that extends up to 50 Gy, which greatly decreases the complexity of work. In addition, Solid Water™ material was shown to be a viable alternative to water in performing radiochromic film based dosimetry with HDR (192)Ir brachytherapy sources.

The impact of uncertainties associated with MammoSite brachytherapy on the dose distribution in the breast

The MammoSite radiation therapy system is a novel technique for treatment of patients with early‐stage breast cancer. It was developed to overcome the longer schedules associated with external‐beam radiation therapy. It consists of a small balloon (4 cm in diameter) connected to an inflation channel and a catheter for the passage of a high dose rate 192Ir brachytherapy source. The device is placed into the tumor resection cavity and inflated with a mixture of saline and radiographic contrast agent to a size that fills the cavity. A high dose rate 192Ir source is driven into the balloon center using a remote afterloader to deliver the prescribed dose at a point 1 cm away from the balloon surface. There are several uncertainties that affect the dose distribution in the MammoSite brachytherapy. They include source position deviation, balloon deformation, and the concentration of the contrast medium inside the balloon. The purpose of this study is to investigate the extent of the dose perturbation for various concentrations of the contrast medium in a MammoSite balloon using Monte Carlo simulations and thermoluminescent dosimetry. This study also combines the impact of these uncertainties on the MammoSite treatment efficacy. The current study demonstrates that the combined uncertainties associated with the MammoSite brachytherapy technique — up to the value of 2 mm balloon deformation, 1 mm source deviation, and 15% contrast concentration — have no impact on the tumor control probability.PACS number: 87.53.‐j

A simplified analytical approach to estimate the parameters required for strength determination of HDR 192Ir brachytherapy sources using a Farmer-type ionization chamber

Measuring the strength of high dose rate (HDR) (192)Ir brachytherapy sources on receipt from the vendor is an important component of a quality assurance program. Owing to their ready availability in radiotherapy departments, the Farmer-type ionization chambers are also used to determine the strength of HDR (192)Ir brachytherapy sources. The use of a Farmer-type ionization chamber requires the estimation of the scatter correction factor along with positioning error (c) and the constant of proportionality (f) to determine the strength of HDR (192)Ir brachytherapy sources. A simplified approach based on a least squares method was developed for estimating the values of f and M(s). The seven distance method was followed to record the ionization chamber readings for parameterization of f and M(s). Analytically calculated values of M(s) were used to determine the room scatter correction factor (K(sc)). The Monte Carlo simulations were also carried out to calculate f and K(sc) to verify the magnitude of the parameters determined by the proposed analytical approach. The value of f determined using the simplified analytical approach was found to be in excellent agreement with the Monte Carlo simulated value (within 0.7%). Analytically derived values of K(sc) were also found to be in good agreement with the Monte Carlo calculated values (within 1.47%). Being far simpler than the presently available methods of evaluating f, the proposed analytical approach can be adopted for routine use by clinical medical physicists to estimate f by hand calculations.

A practical implementation of the 2010 IPEM high dose rate brachytherapy code of practice for the calibration of 192Ir sources

This paper details a practical method for deriving the reference air kerma rate calibration coefficient for Farmer NE2571 chambers using the UK Institute of Physics and Engineering in Medicine (IPEM) code of practice for the determination of the reference air kerma rate for HDR 192Ir brachytherapy sources based on the National Physical Laboratory (NPL) air kerma standard. The reference air kerma rate calibration coefficient was derived using pressure, temperature and source decay corrected ionization chamber response measurements over three successive 192Ir source clinical cycles. A secondary standard instrument (a Standard Imaging 1000 Plus well chamber) and four tertiary standard instruments (one additional Standard Imaging 1000 Plus well chamber and three Farmer NE2571 chambers housed in a perspex phantom) were used to provide traceability to the NPL primary standard and enable comparison of performance between the chambers. Conservative and optimized estimates on the expanded uncertainties (k = 2) associated with chamber response, ion recombination and reference air kerma rate calibration coefficient were determined. This was seen to be 2.3% and 0.4% respectively for chamber response, 0.2% and 0.08% respectively for ion recombination and 2.6% and 1.2% respectively for the calibration coefficient. No significant change in ion recombination with source decay was observed over the duration of clinical use of the respective 192Ir sources.

Consistency of vendor-specified activity values for 192Ir brachytherapy sources

A long-term comparison was done between the manufacturer-stated 192Ir activity and the measured 192Ir activities determined with a well-type ionization chamber. Sources for a Nucletron Micro Selectron high-dose-rate (HDR) unit were used for this purpose. The radioactive sources reference activities were determined using a PTW well-type ionization chamber traceable to the National Institute of Standards and Technology Primary Calibration Laboratory. The measurements were taken in a period of 56 months with 17 different radioactive sources. The manufacturer stated activities were taken from the source calibration certificate provided by the manufacturer. These values were compared with the measured activities. The results have shown that both the percentage deviation of the monthly control measurements with the well-type chamber and the ratio between the measured activities to the manufacturer-stated value lie within ± 2.5%. These results were compared with similar published data and with uncertainty level (3% of the mean and 5% maximum deviation from mean) for brachytherapy sources calibration recommended by the AAPM. It was concluded that a threshold level of ±2.5% can be used as a suitable quality assurance indicator to spot problems in our department. The typical ±5% uncertainty as provided by the manufacturers may be tightened to ±3% to be more in line with published AAPM reports.

MO-G-BRA-06: A Primary Standard for HDR Brachytherapy Calibrations

Purpose: A new Canadian primary standard for high dose rate (HDR) brachytherapycalibrations has been developed, offering improved accuracy through the use of a Pb wedge to determine the scatter component k sc at each measurement position. Methods: The new HDR standard calibrates ,192,Ir brachytherapy sources in terms of air‐kerma strength (S K ). An HDR brachytherapycalibration rig was manufactured and commissioned. This rig utilizes a low (0.5 m) aluminum table to which is mounted a motorized single‐axis linear stage. An afterloader feeds an ,192,Ir source vertically up through a fixed 1 m tall PMMA tube column attached to the table. A second 1 m PMMA tube column is attached to a movable carriage on the linear stage and supports a spherical graphiteionization chamber. A series of 10 measurements are made with the source‐to‐detector distance varying from 10 cm to 55 cm in 5 cm increments. The largest correction factor, k sc (scatter from the stage, walls, floor and ceiling) is determined using a Pb‐wedge technique to block the direct beam and allow a direct measurement of scatter at each distance. The small distance correction is then a simple matter to determine, allowing a more accurate determination of the source strength based on distance squared. Results: : Measurements with a Pb‐wedge technique for determining k sc demonstrated that the scatter component for each position varied, in contradiction to the constant‐scatter supposition of the alternative 7‐distance measurement calculation. These measurements were supported by EGSnrc Monte Carlo simulations. The disagreement between these two techniques for determining k sc represent an (S K ) difference of approximately 0.4%. Conclusions: The new HDR brachytherapycalibration capability demonstrates higher accuracy through a more accurate scatter correction. This standard will participate in a formal international comparison for HDR brachytherapy, tentatively scheduled for 2011.

SU-E-T-117: Absorbed Dose to Water Measurement with a Sandwich Method for High Dose-Rate Ir-192 Brachytherapy Sources

Purpose: This study aimed to establish a standard dosimetry protocol for HDR Ir‐192 sources using an ion chamber calibrated with a Co‐60 beam. We developed a dedicated device for ion chambermeasurements with a sandwich method and examined its measurement accuracy. Methods: A microSelectron‐v2 HDR Ir‐192 source was modeled with the EGSnrc/egs_chamber code. The accuracy of modeling was confirmed by comparing calculated results for gL (r) and F(r, angle) with those of TG‐43. First, an optimal source‐to‐chamber (SCD) separation for Ir‐192 dosimetry was determined from measurements with a PTW 31010 chamber at distances of 1.5–5 cm from the source center in water. The measuredionization chamber reading was corrected with the Monte Carlo‐calculated energy response for Co‐60 and Ir‐192, and was converted to the absorbed dose to water. The measureddoses were compared with TPS values based on TG‐43. We developed a dedicated device for ion chambermeasurements with a sandwich method at the optimal SCD separation. The average dosemeasured with two EXRADIN A1SL chambers was compared with the TPS value. Results: Calculated gL (r) and F(r, angle) values agreed well withthose of TG‐43. The absorbed dose to watermeasured with the PTW31010 chamber was 3% lower than that of TPS at a distance of 5 cm and was 3%– 7% lower at distances less than 5 cm. This was addressed to the uncertainty of the chamber positioning. We made a sandwich measurement device with the separation of 5 cm, considering the uncertainty of positioning and measurement time. The dose to water with the sandwich method was in agreement with that of TG‐43 within −1.2%. Conclusions: The optimal distance for ion chambermeasurements was at 5 cm from the Ir‐192 source. The dose to watermeasurement with the sandwich method is useful for daily dose management for Ir‐192 sources.

Accuracy assessment of the superposition principle for evaluating dose distributions of elongated and curved103Pd and192Ir brachytherapy sources

The current brachytherapy dose calculation formalism determines clinical dose distributions using the superposition principle. However, this approach cannot account for intersource attenuation for extended brachytherapy sources such as elongated and curved sources. The purpose of this study is to determine the line segment length required for optimal accuracy of dose calculations in the vicinity of elongated and curved 103Pd and 192Ir sources using the superposition technique intrinsic to the AAPM TG-43 formalism. Monte Carlo (MC) simulations were performed in water for 103Pd and 192Ir sources with linear, toroidal, and hairpin geometries. Dose distributions for 0.1-8.0 cm line segments from MC simulations were entered into treatment planning system (TPS) using the TG-43 approach with 0.05 cm spatial resolution. Line source dose distributions were benchmarked using MC-to-MC comparisons with the superposition principle, TPS-to-TPS fluence map comparisons, and MC-to-TPS gamma-index comparisons. Toroidal and hairpin geometries were constructed using line segments in the TPS, then TPS-calculated dose distributions were compared to the MC-simulation results. Gamma-index comparisons were performed in the iba(Dosimetry) Omni-Pro I'mRT software using 2 mm distance-to-agreement Ad and 2% dose error deltaD criteria, with a passing rate of > or = 98% of pixels meeting the gamma < or = 1.00 tolerance deemed acceptable. For the MC-to-MC superposition check for line source, the average ratio of the superposition to the solid source length was 1.051 for 103Pd and 1.009 for 192Ir through the whole volume with maximum ratios of 1.34 and 1.32, respectively. TPS-to-TPS comparisons between a solid line source and multiple line segments also provided good agreement. The MC-to-TPS benchmarking indicated where the gamma-index comparison failed were inside the source and within 0.25 cm of the source long-axis. Excluding these regions, 99.6% and 99.9% of the 57 600 in-plane pixels satisfied the gamma-index criteria for 103Pd and 192Ir, respectively. The optimal line segment length for both 103Pd and 192Ir toroidal sources was about 0.5 cm or one-fifth of the torus diameter, whichever was smaller. For all toroidal geometries and all line segment lengths examined, at least 98.9% and 100.0% pixels met the gamma-index criteria for 103Pd and 192Ir, respectively. For both 103Pd and 192Ir hairpin source, all geometric variations had passing rates exceeding 99.2%. However, the best results were obtained from 0.4 cm line segments on the curved part for 103Pd while it was independent of line segment length for 192Ir. A method for using a conventional TPS for brachytherapy treatment planning of elongated and curvilinear brachytherapy sources was developed and benchmarked. This method was evaluated using a gamma-index comparison technique, where appropriate pass-rate criteria were identified. Using a variety of subsegment lengths, the total length for a straight-line source was generally reproduced with accuracy improving as subsegment length increased, approaching the total straight-line length. Toroidal sources were similarly modeled with line segments, and an accuracy tradeoff was found between geometric errors and simulating dose anisotropy along the long-axes. The gamma-index comparison method to analyze the results was shown to be more powerful than point-wise comparison methods, and more versatile than dose ratios on the same plane.

Air kerma standard and measurement comparison on source strength determination for high-dose rate 192Ir brachytherapy in Taiwan

This paper describes the establishment by the Institute of Nuclear Energy Research (INER, Taiwan) of the reference air kerma rate (RAKR) calibration standard for measurement of high-dose rate (HDR) (192)Ir brachytherapy source strength. A bilateral comparison has been made in the RAKR standards for HDR (192)Ir brachytherapy sources at the INER and Physikalisch-Technische Bundesanstalt (PTB, Germany) and the measurement difference was within the overall standard uncertainty and showed good agreement between the two calibration standard systems established at the INER and the PTB. Besides, INER also worked with 20 domestic hospitals to organise an on-site measurement comparison programme to explore the status of HDR (192)Ir brachytherapy source strength determination in Taiwan. The comparison results presented the ratios of RAKR with vendor values, as determined by INER and hospitals from the programme. The ratios fall in all cases within the ± 3 % guaranteed by the vendors for a coverage factor of k = 2 or at 95 % confidence level.

Comparison of high–dose rate prostate brachytherapy dose distributions with iridium-192, ytterbium-169, and thulium-170 sources

Purpose Recent studies have identified that among different available radionuclides, the dose characteristics and shielding properties of ytterbium-169 ( 169Yb) and thulium-170 ( 170Tm) may suit high–dose rate (HDR) brachytherapy needs. The purpose of this work was to compare clinically optimized dose distributions using proposed 169Yb and 170Tm HDR sources with the clinical dose distribution from a standard microSelectron V2 HDR iridium-192 ( 192Ir) brachytherapy source (Nucletron B.V., Veenendaal, The Netherlands). Methods and materials CT-based treatment plans of 10 patients having prostate volumes ranging from 17 to 92 cm 3 were studied retrospectively. Clinical treatment of these patients involved 16 catheters and a microSelectron V2 HDR 192Ir source. All dose plans were generated with inverse planning simulated annealing optimization algorithm. Dose objectives used for the 192Ir radionuclide source were used for the other two radionuclides. The dose objective parameters were adjusted to obtain the same clinical target (prostate) volume coverage as the original 192Ir radionuclide plan. A complete set of dosimetric indices was used to compare the plans from different radionuclides. A pairwise statistical analysis was also performed. Results and conclusions All the dose distributions optimized with specific 192Ir, 169Yb, and 170Tm sources satisfied the standard clinical criteria for HDR prostate implants, such as those for the Radiation Therapy Oncology Group clinical trial 0321, for combined HDR and external beam treatment for prostate adenocarcinoma. For equivalent clinical target volume dose coverage, the specific 169Yb and 170Tm sources resulted in a statistically significant dose reduction to organs at risk compared with microSelectron V2 HDR 192Ir source. This study indicates that a 170Tm or 169Yb radionuclide source may be an alternative to the 192Ir radionuclide sources in HDR brachytherapy.

Evaluation of Gafchromic EBT2 film for the measurement of anisotropy function for high-dose-rate 192Ir brachytherapy source with respect to thermoluminescent dosimetry

Aim The aim of this work was to assess the suitability of the use of a Gafchromic EBT2 film for the measurement of anisotropy function for microSelectron HDR 192Ir (classic) source with a comparative dosimetry method using a Gafchromic EBT2 film and thermoluminescence dosimeters (TLDs). Background Sealed linear radiation sources are commonly used for high dose rate (HDR) brachytherapy treatments. Due to self-absorption and oblique filtration of radiation in the source capsule material, an inherent anisotropy is present in the dose distribution around the source which can be described by a measurable two-dimensional anisotropy function, F( r, θ). Materials and methods Measurements were carried out in a specially designed and locally fabricated PMMA phantom with provisions to accommodate miniature LiF TLD rods and EBT2 film dosimeters at identical radial distances with respect to the 192Ir source. Results The data of anisotropy function generated by the use of the Gafchromic EBT2 film method are in agreement with their TLD measured values within 4%. The produced data are also consistent with their experimental and Monte Carlo calculated results for this source available in the literature. Conclusion Gafchromic EBT2 film was found to be a feasible dosimeter in determining anisotropy in the dose distribution of 192Ir source. It offers high resolution and is a viable alternative to TLD dosimetry at discrete points. The method described in this paper is useful for comparing the performances of detectors and can be applied for other brachytherapy sources as well.

Water equivalency evaluation of PRESAGE® dosimeters for dosimetry of Cs-137 and Ir-192 brachytherapy sources

A major challenge in brachytherapy dosimetry is the measurement of steep dose gradients. This can be achieved with a high spatial resolution three dimensional (3D) dosimeter. PRESAGE® is a polyurethane based dosimeter which is suitable for 3D dosimetry. Since an ideal dosimeter is radiologically water equivalent, we have investigated the relative dose response of three different PRESAGE® formulations, two with a lower chloride and bromide content than original one, for Cs-137 and Ir-192 brachytherapy sources. Doses were calculated using the EGSnrc Monte Carlo package. Our results indicate that PRESAGE® dosimeters are suitable for relative dose measurement of Cs-137 and Ir-192 brachytherapy sources and the lower halogen content PRESAGE® dosimeters are more water equivalent than the original formulation.

Dosimetry in HDR brachytherapy with Fricke-gel layers and Fricke-gel catheters

Fricke-gel layer dosimeters (FGLD) and Fricke gel dosimetric catheters (FGDC) have been designed and tested with the aim of enquiring their suitability for HDR 192Ir brachytherapy source control and for in-vivo dose verification during treatment. Anisotropy function measurements have been carried out with FGLDs in which a thin plastic tube has been placed in for the 192Ir source insertion. FGDCs are constituted by plastic tubes (3 mm of external diameter and 13 cm of length) filled with the dosimeter-gel. Absorbed dose images and profiles were attained by means of optical analysis. Dedicated software has been developed both for achieving anisotropy function values and for obtaining reliable results in visible light absorbance measurements across the thin cylindrical dosimeters. Preparation and analysis procedures have been optimised. The results confirm that the proposed methods are very promising for HDR brachytherapy dosimetry.

SU-GG-T-70: A Dosimetric Study of the MicroSelectron HDR Ir-192 Brachytherapy Source

Purpose: Recently the manufacturer of the mHDR-v2 HDR Ir-192 brachytherapy source clarified design changes that may alter the existing dosimetric data for this source. The object of this study was to obtain new dose rate tables following the TG-43 formalism and to provide these data in close proximity to the source with high spatial resolution including the electron dosimetric contributions from Ir-192 disintegration. Methods and Material: To study the source we used three different Monte Carlo codes: MCNP5, GEANT4, and Penelope. The source was located inside a 40 cm radius water phantom. Dose and kerma were obtained using 0.1 mm thick voxels to provide high-resolution dosimetry near the source. Results: Dose rate distributions obtained with the three Monte Carlo codes were compared, and agreed within the statistical uncertainty. Dosimetric contribution of electrons was important for r < 2 mm. The TG-43 dose-rate constant and radial dose function were similar to prior Monte Carlo studies for the old design. The 2D anisotropy function also coincided with the old design for r > 0.25 cm. Conclusions: Monte Carlo simulations of the mHDR-v2 HDR 192Ir source have been performed with three radiation transport codes showed excellent agreement. Comparison of these results to prior Monte Carlo studies showed good agreement for r > 0.25 cm. If knowledge of dose for r < 0.25 cm is not needed, dosimetric results from the prior Monte Carlo studies will be adequate.

The IPEM code of practice for determination of the reference air kerma rate for HDR 192Ir brachytherapy sources based on the NPL air kerma standard

This paper contains the recommendations of the high dose rate (HDR) brachytherapy working party of the UK Institute of Physics and Engineering in Medicine (IPEM). The recommendations consist of a Code of Practice (COP) for the UK for measuring the reference air kerma rate (RAKR) of HDR 192Ir brachytherapy sources. In 2004, the National Physical Laboratory (NPL) commissioned a primary standard for the realization of RAKR of HDR 192Ir brachytherapy sources. This has meant that it is now possible to calibrate ionization chambers directly traceable to an air kerma standard using an 192Ir source (Sander and Nutbrown 2006 NPL Report DQL-RD 004 (Teddington: NPL) http://publications.npl.co.uk). In order to use the source specification in terms of either RAKR, (ICRU 1985 ICRU Report No 38 (Washington, DC: ICRU); ICRU 1997 ICRU Report No 58 (Bethesda, MD: ICRU)), or air kerma strength, SK (Nath et al 1995 Med. Phys. 22 209–34), it has been necessary to develop algorithms that can calculate the dose at any point around brachytherapy sources within the patient tissues. The AAPM TG-43 protocol (Nath et al 1995 Med. Phys. 22 209–34) and the 2004 update TG-43U1 (Rivard et al 2004 Med. Phys. 31 633–74) have been developed more fully than any other protocol and are widely used in commercial treatment planning systems. Since the TG-43 formalism uses the quantity air kerma strength, whereas this COP uses RAKR, a unit conversion from RAKR to air kerma strength was included in the appendix to this COP. It is recommended that the measured RAKR determined with a calibrated well chamber traceable to the NPL 192Ir primary standard is used in the treatment planning system. The measurement uncertainty in the source calibration based on the system described in this COP has been reduced considerably compared to other methods based on interpolation techniques.

Influence of Photon Energy Spectra from 192Ir and 125I Brachytherapy Sources on Kerma and Dose Rates in Water and Air

Purpose: Several different 192Ir photon energy spectra have been used for both Monte Carlo (MC) simulations of dose deposition and for MC and measured media-energy corrections. However, the dosimetric influence of spectrum choice was not known, and recommendations for the field of medical physics to use a particular 192Ir spectrum are outdated. Furthermore, dosimetric differences between the 125I photon spectrum recommended in the 2004 AAPM TG-43U1 report and that recommended by the National Nuclear Data Center (NNDC) are unknown.

Direct measurement of absorbed dose to water in HDR I192r brachytherapy: Water calorimetry, ionization chamber, Gafchromic film, and TG-43

Gafchromic film and ionometric calibration procedures for HDR 192Ir brachytherapy sources in terms of dose rate to water are presented and the experimental results are compared to the TG-43 protocol as well as with the absolute dose measurement results from a water calorimetry-based primary standard. EBT-1 Gafchromic films, an A1SL Exradin miniature Shonka thimble type chamber, and an SI HDR 1000 Plus well-type chamber (Standard Imaging, Inc., Middleton, WI) with an ADCL traceable Sk calibration coefficient (following the AAPM TG-43 protocol) were used. The Farmer chamber and Gafchromic film measurements were performed directly in water. All results were compared to direct and absolute absorbed dose to water measurements from a 4 degrees C stagnant water calorimeter. Based on water calorimetry, the authors measured the dose rate to water to be 361 +/- 7 microGy/(h U) at a 55 mm source-to-detector separation. The dose rate normalized to air-kerma strength for all the techniques agree with the water calorimetry results to within 0.83%. The overall 1-sigma uncertainty on water calorimetry, ionization chamber, Gafchromic film, and TG-43 dose rate measurement amounts to 1.90%, 1.44%, 1.78%, and 2.50%, respectively. This work allows us to build a more realistic uncertainty estimate for absorbed dose to water determination using the TG-43 protocol. Furthermore, it provides the framework necessary for a shift from indirect HDR 192Ir brachytherapy dosimetry to a more accurate, direct, and absolute measurement of absorbed dose to water.

Study of encapsulated T170m sources for their potential use in brachytherapy

High dose-rate (HDR) brachytherapy is currently performed with 192Ir sources, and 60Co has returned recently into clinical use as a source for this kind of cancer treatment. Both radionuclides have mean photon energies high enough to require specific shielded treatment rooms. In recent years, 169Yb has been explored as an alternative for HDR-brachytherapy implants. Although it has mean photon energy lower than 192Ir, it still requires extensive shielding to deliver treatment. An alternative radionuclide for brachytherapy is 170Tm (Z=69) because it has three physical properties adequate for clinical practice: (a) 128.6 day half-life, (b) high specific activity, and (c) mean photon energy of 66.39 keV. The main drawback of this radionuclide is the low photon yield (six photons per 100 electrons emitted). The purpose of this work is to study the dosimetric characteristics of this radionuclide for potential use in HDR-brachytherapy. The authors have assumed a theoretical 170Tm cylindrical source encapsulated with stainless steel and typical dimensions taken from the currently available HDR 192Ir brachytherapy sources. The dose-rate distribution was calculated for this source using the GEANT4 Monte Carlo (MC) code considering both photon and electron 170Tm spectra. The AAPM TG-43 U1 brachytherapy dosimetry parameters were derived. To study general properties of 170Tm encapsulated sources, spherical sources encapsulated with stainless steel and platinum were also studied. Moreover, the influence of small variations in the active core and capsule dimensions on the dosimetric characteristics was assessed. Treatment times required for a 170Tm source were compared to those for 192Ir and 169Yb for the same contained activity. Due to the energetic beta spectrum and the large electron yield, the bremsstrahlung contribution to the dose was of the same order of magnitude as from the emitted gammas and characteristic x rays. Moreover, the electron spectrum contribution to the dose was significant up to 4 mm from the source center compared to the photon contribution. The dose-rate constant lamda of the cylindrical source was 1.23 cGy h(-1) U(-1). The behavior of the radial dose function showed promise for applications in brachytherapy. Due to the electron spectrum, the anisotropy was large for r <6 mm. Variations in manufacturing tolerances did not significantly influence the final dosimetry data when expressed in cGy h(-1) U(-1). For typical capsule dimensions, maximum reference dose rates of about 0.2, 10, and 2 Gy min(-1) would then be obtained for 170Tm, 192Ir, and 169Yb, respectively, resulting in treatment times greater than those for HDR 192Ir brachytherapy. The dosimetric characteristics of source designs exploiting the low photon energy of 170Tm were studied for potential application in HDR-brachytherapy. Dose-rate distributions were obtained for cylindrical and simplified spherical 170Tm source designs (stainless steel and platinum capsule materials) using MC calculations. Despite the high activity of 170Tm, calculated treatment times were much longer than for 192Ir.

Dosimetric accuracy of a deterministic radiation transport based brachytherapy treatment planning system. Part I: Single sources and bounded homogeneous geometries

The aim of this work is to validate a deterministic radiation transport based treatment planning system (TPS) for single 192Ir brachytherapy source dosimetry in homogeneous water geometries. TPS results were obtained using the deterministic radiation transport option of a BRACHYVISION v. 8.8 system for three characteristic source designs (VS2000, GMPlus HDR, and GMPlus PDR) with each source either centered in a 15 cm radius spherical water phantom, or positioned at varying distance away from the phantom center. Corresponding MC simulations were performed using the MCNPX code v.2.5.0 and source geometry models prepared using information provided by the manufacturers. Comparison in terms of the AAPM TG-43 dosimetric formalism quantities, as well as dose rate distributions per unit air kerma strength with a spatial resolution of 0.1 cm, yielded close agreement between TPS and MC results for the sources centered in the phantom. Besides some regions close to the source longitudinal axes where discrepancies could be characterized as systematic, overall agreement for all three sources studied is comparable to the statistical (type A) uncertainty of MC simulations (1% at the majority of points in the geometry increasing to 2%-3% at points lying both away from the source center and close to the source longitudinal axis). A corresponding good agreement was also found between TPS and MC results for the sources positioned away from the phantom center. Results of this work attest the capability of the TPS to accurately account for the scatter conditions regardless of the size or shape of a given geometry of dosimetric interest, and the position of a source within it. This is important since, as shown in the literature and summarized also in this work, these factors could introduce a significant dosimetric effect that is currently ignored in clinical treatment planning. It is concluded that the implementation of the deterministic radiation transport option of the BRACHYVISION v. 8.8 system for 192Ir brachytherapy dosimetry in homogeneous water geometries yields results of comparable accuracy to the golden standard of Monte Carlo simulation, in clinically viable calculation times.

Dosimetric characterization of an 192Ir brachytherapy source with the Monte Carlo code PENELOPE

Monte Carlo calculations are highly spread and settled practice to calculate brachytherapy sources dosimetric parameters. In this study, recommendations of the AAPM TG-43U1 report have been followed to characterize the Varisource VS2000 192Ir high dose rate source, provided by Varian Oncology Systems. In order to obtain dosimetric parameters for this source, Monte Carlo calculations with PENELOPE code have been carried out. TG-43 formalism parameters have been presented, i.e., air kerma strength, dose rate constant, radial dose function and anisotropy function. Besides, a 2D Cartesian coordinates dose rate in water table has been calculated. These quantities are compared to this source reference data, finding results in good agreement with them. The data in the present study complement published data in the next aspects: (i) TG-43U1 recommendations are followed regarding to phantom ambient conditions and to uncertainty analysis, including statistical (type A) and systematic (type B) contributions; (ii) PENELOPE code is benchmarked for this source; (iii) Monte Carlo calculation methodology differs from that usually published in the way to estimate absorbed dose, leaving out the track-length estimator; (iv) the results of the present work comply with the most recent AAPM and ESTRO physics committee recommendations about Monte Carlo techniques, in regards to dose rate uncertainty values and established differences between our results and reference data. The results stated in this paper provide a complete parameter collection, which can be used for dosimetric calculations as well as a means of comparison with other datasets from this source.