High Dose Rate Source Research Articles

Air-kerma strength determination of a Yb169 high dose rate brachytherapy source

The increased demand for high dose rate (HDR) brachytherapy as an alternative to external beam radiotherapy has led to the introduction of a HDR brachytherapy isotope 169Yb. This source offers a dose rate similar to 192Ir HDR sources, at about one fourth the effective photon energy. This work presents the calibration of this source in terms of air-kerma strength, based on an adaptation of the current, National Institute of Standards and Technology traceable, in air measurement technique currently used for 192Ir HDR sources. Several additional measurement correction factors were required, including corrections for air scatter, air attenuation, and ion recombination. A new method 169Yb is introduced for determining the ion chamber calibration coefficient Nk(169Yb). An uncertainty analysis was also performed, indicating an overall measurement expanded uncertainty in the air-kerma strength (k=2) of 2.2%.

Radiation Dosimetry in Brachytherapy, Introducing A Spherical Diode Dosimeter

Brachytherapy is a cancer treatment modality in which a radiation source is inserted into the tumor. It is done in one of two ways: Low Dose Rate (LDR) treatment where seeds are permanently implanted, or High Dose Rate (HDR), a short time exposure. The isotopes mostly used for LDR treatment are I-125 and Pd-103 of activity about 18.5 MBq per seed. HDR treatment is generally done with Ir-192 sources of activity 370 GBq. We introduce a new source for both HDR and LDR treatment, namely Tm-170. Safety considerations for the medical staff, the patient and people around him include: well shielded sources, shortest handling time and keeping uninvolved people at a safe distance. HDR sources are kept in a lead container and applied by a remote controlled delivery system. Radiation dosimetry and source calibration is done before the sources are applied to the patient. The methods used for dose measurement are: TLD, diode or ion chamber. We introduced a new diode for dosimetry, micro-spherical diode. This tiny diode causes very little perturbation and provides spherically symmetrical dose measurements. Monte Carlo calculations are used to verify the measurements. Source parameters are calculated for source quality comparison.

SU‐GG‐T‐10: Experimental Measurement of Attenuation Factor of a New Type of Shielding Material for a HDR Ir‐192 Source

Purpose: A relatively new type of material, Ledite has been introduced as radiation shielding material for both high dose rate (HDR) brachytherapy and mega voltage external beam treatment rooms. Currently, limited attenuation information is available for this type of material for radionuclides, especially for HDR source such as Ir‐192. Since many of the radiation sources, including Ir‐192 HDR source and Linac beams, are not mono‐energetic and the material is supplied in the form of block, the shielding calculations based on provided TVL value may potentially lead to errors. This study was undertaken to experimentally measure the attenuation parameters of Ledite blocks for an Ir‐192 high dose rate source to ensure adequate shielding design for a HDR treatment room. Method and Materials: Ledite blocks are supplied in dimension of 6 × 6 × 12 inches. A farmer type ionization chamber and a HDR Ir‐192 source (VariSource, Varian Medical Systems, CA) were used in the measurement. The measurements were conducted for thickness of 6 inches. In the measurement, the distance from the source to the chamber was kept the same for the situations of with and without the blocks placed in between, to avoid the inverse‐square effects. The source and chamber were placed at certain distances away from the blocks to account for, to a certain degree, the contributions from both primary and scatter photons. Results: The measurements showed that the attenuation factor for block of thickness of 6 was 0.024. These measured attenuation value was higher than those calculated based on the provided tenth value layer, indicating potential overestimation of radiation attenuation with the provided tenth value layer. Conclusion: Attenuation factor was measured for one type of Ledite Block. It was found that the provided TVL may overestimate radiation attenuation for the HDR source. More investigation is warranted.

MO‐D‐AUD B‐01: An Analytical Approach to Account for Shielding, Anatomical Heterogeneities and Patient Dimensions for 192Ir High Dose Rate Brachytherapy Applications

Purpose: Based on a separate primary and scatter dose calculation technique and pre‐computed Monte Carlo (MC) data, we have developed an analytical approach to account for shielding, anatomical heterogeneities and patient dimensions for high dose rate (HDR) brachytherapy. Method and Materials: Using the PTRAN_CT MC code, primary and scatter dose kernels of an HDR source in water were generated. Separate 3D kernels for rectal treatment with a tungsten‐shielded applicator were also created. Photon attenuation and scatter in tissue heterogeneities were corrected for via ray tracing. To quantify the reduced backscatter close to the skin, MC simulations were performed with an isotropic point source placed at various distances from the center of a 30 cm diameter water sphere. Scatter correction factors were derived, which vary as a function of distances between (1) the source and the surface, (2) the point of interest and the source, and (3) the point of interest and the surface. We applied this analytical method for three clinical cases and compared the results with PTRAN_CT calculations. Results: Our technique accurately accounted for the effects of tungsten shielding and anatomical heterogeneities for a rectal patient plan. The reduced backscatter close to the skin was also calculated correctly for a base of tongue and a breast case. Around bony structures several centimeters away from the active dwell positions, there was a minor discrepancy due to softening of the spectrum. Differences in the lung due to reduced scattering in this low density region were also observed. Conclusion: Making use of pre‐computed 3D scatter dose data, our analytical technique is capable of calculating dose around metal shielding and the patient skin with high accuracy. Its validity and limitations have been studied for HDR applications. Conflict of Interest: Research sponsored by Nucletron BV.

TU‐C‐AUD C‐03: Teletherapy MicroRT Using a Commercial 192Ir Source

The prototype small‐animal irradiator (microRT) developed at Washington University utilizes the commercial 192Ir high‐dose rate (HDR) remote afterloader source in a teletherapy geometry. The system consists of a set of four Tungsten collimators (5.5 mm diameter hole) mounted to an Aluminum support tube. An HDR catheter is used to transport the source to the collimator and pre‐determined dwell positions center the source at the collimator hole. The mouse is placed on a couch that contains a series of drilled holes that act as fiducial localization marks visible on computed tomography (CT) imaging. For most experiments, the mouse is first anesthetized and placed on a couch. The couch is imaged using a commercial CT scanner (spatial resolution approximately 0.6 mm). The couch geometry is automatically registered in the treatment planning software, which was written in the Computational Environment for Radiotherapy Research (CERR) platform. The treatment planner determines the dose and adjusts the source‐to‐target distance to achieve the required field size and the target dose, coupled with the source strength, determines the irradiation time. The couch is mounted to a computer‐controlled three‐dimensional stage that positions the mouse to submillimeter accuracy. Advantages of the system include its relatively high precision, low fabrication cost, and straightforward and robust operation. Disadvantages include the need for having a commercial HDR source and the associated complexities of scheduling experiments, the relatively poor penumbra, and the steep depth‐dose behavior. The system works very well for parallel‐opposed fields to either the whole brain or hemi‐brain, and tumors grown on the mouse flank.

Virtual HDR SM CyberKnife Treatment for Localized Prostatic Carcinoma: Dosimetry Comparison With HDR Brachytherapy and Preliminary Clinical Observations

We tested our ability to approximate the dose (38 Gy), fractionation (four fractions), and distribution of high-dose-rate (HDR) brachytherapy for prostate cancer with CyberKnife (CK) stereotactic body radiotherapy (SBRT) plans. We also report early clinical observations of CK SBRT treatment. Ten patients were treated with CK. For each CK SBRT plan, an HDR plan was designed using common contour sets and simulated HDR catheters. Planning target volume coverage, intraprostatic dose escalation, and urethra, rectum, and bladder exposure were compared. Planning target volume coverage by the prescription dose was similar for CK SBRT and HDR plans, whereas percent of volume of interest receiving 125% of prescribed radiation dose (V125) and V150 values were higher for HDR, reflecting higher doses near HDR source dwell positions. Urethra dose comparisons were lower for CK SBRT in 9 of 10 cases, suggesting that CK SBRT may more effectively limit urethra dose. Bladder maximum point doses were higher with HDR, but bladder dose falloff beyond the maximum dose region was more rapid with HDR. Maximum rectal wall doses were similar, but CK SBRT created sharper rectal dose falloff beyond the maximum dose region. Second CK SBRT plans, constructed by equating urethra radiation dose received by point of maximum exposure of volume of interest to the HDR plan, significantly increased V125 and V150. Clinically, 4-month post-CK SBRT median prostate-specific antigen levels decreased 86% from baseline. Acute toxicity was primarily urologic and returned to baseline by 2 months. Acute rectal morbidity was minimal and transient. It is possible to construct CK SBRT plans that closely recapitulate HDR dosimetry and deliver the plans noninvasively.

Monte Carlo study of the dose rate distributions for the Ir2.A85‐2 and Ir2.A85‐1 Ir‐192 afterloading sources

The two most commonly used modalities of cancer treatment in clinical brachytherapy practice today are high dose rate (HDR) and pulsed dose rate (PDR) brachytherapy. In a clinical treatment, quality dose rate distribution data sets of the brachytherapy sources are required for each source model. The purpose of this study is to obtain detailed dose rate distributions around the new BEBIG HDR and PDR Ir-192 brachytherapy sources. These distributions will then be used as input data in the treatment planning systems dedicated to brachytherapy and its calculations can be verified. The Monte Carlo method was used to obtain the dose rate distributions around the sources studied, taking into account the AAPM-ESTRO recent recommendations. A complete dosimetric data set for the BEBIG Ir-192 HDR and PDR sources, types Ir2.A85-2 and Ir2.A85-1, were obtained. This dosimetric data set is composed of the TG-43 dosimetric functions and parameters and along-away dose rate table to facilitate quality control of treatment planning systems.

In-air calibration of new high dose rate 60Co brachytherapy sources: results of measurements on a GZP6 brachytherapy afterloading unit

Summary Background The air kerma rate of brachytherapy sources should be determined accurately by the manufacturer and medical physicists before clinical use. Aim In the current study the air kerma rate of three new 60Co high dose rate (HDR) brachytherapy sources was obtained by in-air measurements and a Farmer type ionization chamber. Materials/Methods Three 60Co sources of a brachytherapy afterloading unit, GZP6, were calibrated in free air using a Farmer type chamber which was calibrated in terms of air kerma in an external teletherapy 60Co beam. Several correction factors including scatter correction and non-uniformity correction factors were derived and used for in-air calibrations. Results The measured air kerma rates for all sources were in good agreement (less than 2.5%) with manufacturer-provided data, and the reliability of the air kerma rates of sources was validated for clinical application. Conclusion In-air calibration of 60Co HDR sources can be performed using a Farmer type ionization chamber with acceptable accuracy. However, accurate distance measurement and reproducible measurement setup are required.

MicroRT—Small animal conformal irradiator

A novel small animal conformal radiation therapy system has been designed and prototyped: MicroRT. The microRT system integrates multimodality imaging, radiation treatment planning, and conformal radiation therapy that utilizes a clinical 192Ir isotope high dose rate source as the radiation source (teletherapy). A multiparameter dose calculation algorithm based on Monte Carlo dose distribution simulations is used to efficiently and accurately calculate doses for treatment planning purposes. A series of precisely machined tungsten collimators mounted onto a cylindrical collimator assembly is used to provide the radiation beam portals. The current design allows a source-to-target distance range of 1-8 cm at four beam angles: 0 degrees (beam oriented down), 90 degrees, 180 degrees, and 270 degrees. The animal is anesthetized and placed in an immobilization device with built-in fiducial markers and scanned using a computed tomography, magnetic resonance, or positron emission tomography scanner prior to irradiation. Treatment plans using up to four beam orientations are created utilizing a custom treatment planning system-microRTP. A three-axis computer-controlled stage that supports and accurately positions the animals is programmed to place the animal relative to the radiation beams according to the microRTP plan. The microRT system positioning accuracy was found to be submillimeter. The radiation source is guided through one of four catheter channels and placed in line with the tungsten collimators to deliver the conformal radiation treatment. The microRT hardware specifications, the accuracy of the treatment planning and positioning systems, and some typical procedures for radiobiological experiments that can be performed with the microRT device are presented.

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.

On the use of high dose rate Ir192 and Yb169 sources with the MammoSite® radiation therapy system.

Ample literature exists on the dose overestimation by commercially available treatment planning systems in MammoSite® applications using high dose rate Ir192 sources for partial breast brachytherapy as monotherapy, due to their inability to predict the dose reduction caused by the radiographic contrast solution in the balloon catheter. In this work Monte Carlo simulation is used to verify the dose rate reduction in a balloon breast applicator which does not vary significantly with distance and it is 1.2% at the prescription distance for the reference simulated geometry of 10% diluted radiographic contrast media and 2.5cm balloon radius. Based on these findings and the minimal hardening of the initially emitted photon spectrum for Ir192, a simple analytical method is proposed and shown capable for correcting dosimetry planning in clinical applications. Simulations are also performed to assess the corresponding dose reduction in applications of balloon breast applicators using high dose rate Yb169 sources that have recently become available. Results yield a far more significant and distance dependent dose reduction for Yb169 (on the order of 20% at the prescription distance for the abovementioned reference simulation geometry). This dose reduction cannot be accounted for using simple analytical methods as for Ir192 due to the significant hardening of the initially emitted Yb169 photons within the diluted radiographic contrast media. Combined with results of previous works regarding the effect of altered scatter conditions (relative to treatment planning system assumptions) on breast treatment planning accuracy, which is more pronounced for Yb169 relative to Ir192, these findings call for the amendment of dose treatment planning systems before using Yb169 high dose rate sources in balloon breast applicators.

SU-FF-J-24: Toward Real-Time Tracking of HDR Source Position Using Emission Imaging with a Collimator and Flat-Panel Detector

Purpose: High dose rate (HDR) brachytherapy delivers rapidly a large, highly spatially varying dose. Consequently, precise quality assurance of source location and dwell time is important. Here, we propose that, with no additional radiation exposure to the patient, the same flat-panel X-ray detector (FPD) used for anatomical imaging during simulation can also be used for real-time emission imaging of Ir-192 brachytherapy source location. Method and Materials: A standard parallel-hole, low-energy, gamma-camera collimator (weight 34 lbs) was placed on a FPD, in order to preferably accept those Ir-192-emitted photons which are directed perpendicular to the FPD surface. A 10 Ci Ir-192 source was advanced in 2-cm steps through a catheter positioned 20 cm from the collimator/FPD. FPD images where acquired continuously with 6 seconds per frame. Measurements were obtained with 3 different thicknesses (0, 10 cm, and 20 cm) of solid water between the Ir-192 source and the collimator/FPD. Results: A hot spot corresponding to Ir-192 source location was clearly resolved, well above noise, in all images, even with 20 cm solid water. There were penetration patterns corresponding to collimator septal thickness. Imaged source locations tracked the 2 cm steps of the source. Conclusion: A standard gamma-camera collimator provides directional selection of Ir-192-emitted photons that is sufficient to determine source location. The commercial FPD considered provides ample signal-to-noise ratio for imaging Ir-192-emitted HDR photons on the few-second time scale investigated. Thus HDR source location — along the 2 dimensions parallel to the FPD surface — can be determined by positioning a collimator over the same FPD used for transmission anatomical imaging.

Toward endobronchial Ir-192 high-dose-rate brachytherapy therapeutic optimization

A number of patients with lung cancer receive either palliative or curative high-dose-rate (HDR) endobronchial brachytherapy. Up to a third of patients treated with endobronchial HDR die from hemoptysis. Rather than accept hemoptysis as an expected potential consequence of HDR, we have calculated the radial dose distribution for an Ir-192 HDR source, rigorously examined the dose and prescription points recommended by the American Brachytherapy Society (ABS), and performed a radiobiological-based analysis. The radial dose rate of a commercially available Ir-192 source was calculated with a Monte Carlo simulation. Based on the linear quadratic model, the estimated palliative, curative and blood vessel rupture radii from the center of an Ir-192 source were obtained for the ABS recommendations and a series of customized HDR prescriptions. The estimated radius at risk for blood vessel perforation for the ABS recommendations ranges from 7 to 9 mm. An optimized prescription may in some situations reduce this radius to 4 mm. The estimated blood perforation radius is generally smaller than the palliative radius. Optimized and individualized endobronchial HDR prescriptions are currently feasible based on our current understanding of tumor and normal tissue radiobiology. Individualized prescriptions could minimize complications such as fatal hemoptysis without sacrificing efficacy. Fiducial stents, HDR catheter centering or spacers and the use of CT imaging to better assess the relationship between the catheter and blood vessels promise to be useful strategies for increasing the therapeutic index of this treatment modality. Prospective trials employing treatment optimization algorithms are needed.

Helmet Mold-Based Surface Brachytherapy for Homogeneous Scalp Treatment: a Case Report

BACKGROUND/CASE REPORT: External-beam radiotherapy of complex-shaped areas is sometimes difficult to realize. In a patient with chronic lymphatic leukemia (CLL) infiltrates of the skin of the whole scalp, conventional external-beam radiotherapy with electrons or photons was not able to treat the target sufficiently. Thus, the authors developed a brachytherapy moulage technique using a customized helmet (polyethylene) with flexible interstitial plastic applicators and irradiated the target very homogeneously with a microselectron (192)Ir HDR (high-dose-rate) source (2.0 Gy daily fractionated; 36.0 Gy total dose related to the reference points in 3 mm focus depth). Technical difficulties, CT-supported three-dimensional conformal treatment planning, and verification with TLD probe measurements are described. The treatment was well tolerated and resulted in complete local remission of the CLL infiltrates within a follow-up of 30 months. The presented treatment of lymphoma infiltrates at the scalp by HDR moulage techniques is exceptional but safe and practicable to achieve local tumor control.

Design of a phantom for the quality control of high dose rate 192Ir source used in brachytherapy

Background and purpose A new phantom is proposed for measuring the strength of 192Ir high dose rate sources and for verification of the dose calculated by the treatment planning system. The complete formalism and measurement procedure for this phantom is described, as well as the preliminary results obtained in a number of centers around Brazil. Materials and methods The measurements are performed using powder thermoluminescent dosimeter capsules; the source strength is measured in air and the verification of the dose calculation algorithm in water phantom. The correction factors required to take into account the specificities related to the geometry and the phantom materials have been assessed using the PENELOPE Monte Carlo code and experimental methods. The dedicated phantom, constructed to use as part of a QA program, in this case specifically for high dose rate 192Ir brachytherapy sources, allows simultaneous irradiation of three thermoluminescent dosimeter capsules, requiring only one source stop (dwell positions). Results The phantom was mailed to seven radiotherapy institutions in Brazil, and the results show its usefulness in verifying the source air kerma and correctness of treatment planning dose calculation in water phantom. Conclusions The comparison made between the phantom measurements, the well-type ionization chamber, and source specifications stated by the hospital (most of the times provided by the source manufacturer) agreed within 3% showing the quality in the HDR dose delivery in Brazilian radiotherapy centers.

A dosimetric comparison of and for HDR brachytherapy of the breast, accounting for the effect of finite patient dimensions and tissue inhomogeneities

Monte Carlo simulation dosimetry is used to compare 169Yb to 192Ir for breast high dose rate (HDR) brachytherapy applications using multiple catheter implants. Results for bare point sources show that while 169Yb delivers a greater dose rate per unit air kerma strength at the radial distance range of interest to brachytherapy in homogeneous water phantoms, it suffers a greater dose rate deficit in missing scatter conditions relative to 192Ir. As a result of these two opposing factors, in the scatter conditions defined by the presence of the lung and the finite patient dimensions in breast brachytherapy the dose distributions calculated in a patient equivalent mathematical phantom by Monte Carlo simulations for the same implant of either 169Yb or 1921r commercially available sources are found comparable. Dose volume histogram results support that 169Yb could be at least as effective as 192Ir delivering the same dose to the lung and slightly reduced dose to the breast skin. The current treatment planning systems' approach of employing dosimetry data precalculated in a homogeneous water phantom of given shape and dimensions, however, is shown to notably overestimate the delivered dose distribution for 169Yb. Especially at the skin and the lung, the treatment planning system dose overestimation is on the order of 15%-30%. These findings do not undermine the potential of 169Yb HDR sources for breast brachytherapy relative to the most commonly used 192Ir HDR sources. They imply, however, that there could be a need for the amendment of dose calculation algorithms employed in clinical treatment planning of particular brachytherapy applications, especially for intermediate photon energy sources such as 169Yb.

A dosimetric study on the high dose rate Flexisource

In this work, the dose rate distribution of a new Ir-192 high dose rate source (Flexisource used in the afterloading Flexitron system, Isodose Control, Veenendaal, The Netherlands) is studied by means of Monte Carlo techniques using the GEANT4 code. The dosimetric parameters of the Task Group No. 43 Report (TG43) formalism and two-dimensional rectangular look-up tables have been obtained.

Dosimetric characterization of a novel intracavitary mold applicator for high dose rate endorectal brachytherapy treatment

The dosimetric properties of a novel intracavitary mold applicator for 192Ir high dose rate (HDR) endorectal cancer treatment have been investigated using Monte Carlo (MC) simulations and experimental methods. The 28 cm long applicator has a flexible structure made of silicone rubber for easy passage into cavities with deep-seated tumors. It consists of eight source catheters arranged around a central cavity for shielding insertion, and is compatible for use with an endocavitary balloon. A phase space model of the HDR source has been validated for dose calculations using the GEANT4 MC code. GAFCHROMIC EBT model film was used to measure dose distributions in water around shielded and unshielded applicators with two loading configurations, and to quantify the shielding effect of a balloon injected with an iodine solution (300 mg I/mL). The film calibration procedure was performed in water using an 192Ir HDR source. Ionization chamber measurements in a Lucite phantom show that placing a tungsten rod in the applicator attenuates the dose in the shielded region by up to 85%. Inserting the shielded applicator into a water-filled balloon pushes the neighboring tissues away from the radiation source, and the resulting geometric displacement reduces the dose by up to 53%; another 8% dose reduction can be achieved when the balloon is injected with an iodine solution. All experimental results agree with the GEANT4 calculations within measurement uncertainties.

2339: Accelerated Hyperfractionated Radiotherapy for Cervical Cancer: A Multi-Institutional Prospective Study Among 8 Asian Countries (FNCA CERVIX-II)

2339: Accelerated Hyperfractionated Radiotherapy for Cervical Cancer: A Multi-Institutional Prospective Study Among 8 Asian Countries (FNCA CERVIX-II)

Comparison of radiation shielding requirements for HDR brachytherapy using and sources

has received a renewed focus lately as an alternative to sources for high dose rate (HDR) brachytherapy. Following the results of a recent work by our group which proved to be a good candidate for HDR prostate brachytherapy, this work seeks to quantify the radiation shielding requirements for HDR brachytherapy applications in comparison to the corresponding requirements for the current HDR brachytherapy standard. Monte Carlo simulation (MC) is used to obtain and broad beam transmission data through lead and concrete. Results are fitted to an analytical equation which can be used to readily calculate the barrier thickness required to achieve a given dose rate reduction. Shielding requirements for a HDR brachytherapy treatment room facility are presented as a function of distance, occupancy, dose limit, and facility workload, using analytical calculations for both and HDR sources. The barrier thickness required for is lower than that for by a factor of 4–5 for lead and 1.5–2 for concrete. Regarding HDR brachytherapy applications, the lead shielding requirements do not exceed , even in highly conservative case scenarios. This allows for the construction of a lead door in most cases, thus avoiding the construction of a space consuming, specially designed maze. The effects of source structure, attenuation by the patient, and scatter conditions within an actual treatment room on the above‐noted findings are also discussed using corresponding MC simulation results.