Dose Rate 192Ir Source Research Articles

Calibration system correction factor for measuring the reference air kerma rate (RAKR) applied to high dose rate 192Ir sources (HDR)

The aim of this study was to use computer simulation to analyze the impact of the aluminum fixing support on the Reference Air Kerma (RAK), a physical quantity obtained in a calibration system that was experimentally developed in the Laboratory of Radiological Sciences of the University of the State of Rio de Janeiro (LCR-UERJ). Correction factors due to scattered radiation and the geometry of the 192Ir sources were also sought to be determined. The computational simulation was validated by comparing some parameters of the experimental results with the computational results. These parameters were: verification of the inverse square law of distance, determination of (RAKR), analysis of the source spectrum with and without encapsulation, and the sensitivity curve of the Sourcecheck 4PI ionization chamber response, as a function of the distance from the source along the axial axis, using the microSelectron-v2 (mSv2) and GammaMedplus (GMp) sources. Kerma was determined by activity in the Reference air, with calculated values of 1.725 × 10−3 U. Bq−1 and 1.710 × 10−3 U. Bq−1 for the ionization chamber NE 2571 and TN 30001, respectively. The expanded uncertainty for these values was 0.932% and 0.919%, respectively, for a coverage factor (k = 2). The correction factor due to the influence of the aluminum fixing support for measurements at 1 cm and 10 cm from the source was 0.978 and 0.969, respectively. The geometric correction factor of the sources was ksg = 1.005 with an expanded uncertainty of 0.7% for a coverage factor (k = 2). This value has a difference of approximately 0.2% compared to the experimental values.

Dose Distribution of 192Ir HDR Brachytherapy Source Measurement using Gafchromic® EBT3 Film Dosimeter and TLD-100H

This study aims to measure the radial dose function and anisotropy function F(r, θ) of high Dose Rate (HDR) 192Ir source in a fabricated water-equivalent phantom using Gafchromic® EBT3 film and TLD-100H and to compare the results obtained with the MCNP5 calculated values. The phantom was fabricated using Perspex PMMA material. For, the EBT3 films with a required dimension and TLD-100H chips were placed at r=1, 2, 3, 5, and 10 cm from the source. The F(r, θ) measurements were carried out at r=1, 2, 3, 5, and 10 cm with the angle range from 10° to 170°. The result of from EBT3 film and TLD-100H was in good agreement (2.10%±1.99). Compared to MCNP5, the differences are within 0.31% to 11.47% for EBT3 film and 0.08% to 10.58% for TLD-100H. For the F(r, θ), an average deviation with the MCNP5 calculation is 4.94%±2.7. For both and F(r, θ), the effects are prominent at r=10 cm. At this distance, the response of both Gafchromic® EBT3 film and TLD-100H shows less sensitivity as the dose followed the inverse square law. This work demonstrates that Gafchromic® EBT3 film dosimeter and TLD-100H are suitable dosimeters in 192Ir dosimetric measurements at a radial distance of ˂5 cm

Brachytherapy treatment verification using gamma radiation from the internal treatment source combined with an imaging panel—a phantom study

Brachytherapy has an excellent clinical outcome for different treatment sites. However, in vivo treatment verification is not performed in the majority of hospitals due to the lack of proper monitoring systems. This study investigates the use of an imaging panel (IP) and the photons emitted by a high dose rate (HDR) 192Ir source to track source motion and obtain some information related to the patient anatomy. The feasibility of this approach was studied by monitoring the treatment delivery to a 3D printed phantom that mimicks a prostate patient. A 3D printed phantom was designed with a template for needle insertion, a cavity (‘rectum’) to insert an ultrasound probe, and lateral cavities used to place tissue-equivalent materials. CT images were acquired to create HDR 192Ir treatment plans with a range of dwell times, interdwell distances and needle arrangements. Treatment delivery was verified with an IP placed at several positions around the phantom using radiopaque markers on the outer surface to register acquired IP images with the planning CT. All dwell positions were identified using acquisition times ≤0.11 s (frame rates ≥ 9 fps). Interdwell distances and dwell positions (in relation to the IP) were verified with accuracy better than 0.1 cm. Radiopaque markers were visible in the acquired images and could be used for registration with CT images. Uncertainties for image registration (IP and planning CT) between 0.1 and 0.4 cm. The IP is sensitive to tissue-mimicking insert composition and showed phantom boundaries that could be used to improve treatment verification. The IP provided sufficient time and spatial resolution for real-time source tracking and allows for the registration of the planning CT and IP images. The results obtained in this study indicate that several treatment errors could be detected including swapped catheters, incorrect dwell times and dwell positions.

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.

High dose rate 192Ir source calibration: A single institution experience

Measurement of source strength of new high dose rate (HDR) 192Ir supplied by the manufacturer is part of quality assurance recommended by Radiation Safety Section, Ministry of Health of Malaysia. The source strength is determined in reference air kerma rate (RAKR). The purpose of this study was to evaluate RAKR measurement of 192Ir using well-type ionisation chamber with RAKR stated in the certificate provided by the manufacturer. A retrospective study on 19 MicroSelectron HDR 192Ir Classic from 2001 to 2009 and 12 MicroSelectron HDR 192Ir V2 sources from 2009 to 2016 supplied by manufacturer were compared. From the study, the agreement between measured RAKR and RAKR stated in the certificate by manufacturer for all 32 sources supplied were within ±2.5%. As a conclusion, a threshold level of ±2.5% can be used as suitable indicator to spot problems of the brachytherapy system in Department of Nuclear Medicine Radiotherapy and Oncology, Hospital USM.

Dedicated high dose rate 192Ir brachytherapy radiation fields for in vitro cell exposures at variable source-target cell distances: killing of mammalian cells depends on temporal dose rate fluctuation

Afterloading brachytherapy is conducted by the stepwise movement of a radioactive source through surgically implanted applicator tubes where at predefined dwell positions calculated dwell times optimize spatial dose delivery with respect to a planned dose level. The temporal exposure pattern exhibits drastic fluctuations in dose rate at a given coordinate and within a single treatment session because of the discontinuous and repeated source movement into the target volume. This could potentially affect biological response. Therefore, mammalian cells were exposed as monolayers to a high dose rate 192Ir source by utilizing a dedicated irradiation device where the distance between a planar array of radioactive source positions and the plane of the cell monolayer could be varied from 2.5 mm to 40 mm, thus varying dose rate pattern for any chosen total dose.The Gammamed IIi afterloading system equipped with a nominal 370 GBq (10 Ci) 192-Ir source was used to irradiate V79 Chinese hamster lung fibroblasts from both confluent and from exponential growth phase with dose up to 12 Gy (at room temperature, total exposure not exceeding 1 h). For comparison, V79 cells were also exposed to 6 MV x-rays from a clinical linear accelerator (dose rate of 2.5 Gy min−1). As biological endpoint, cell survival was determined by standard colony forming assay. Dose measurements were conducted with a diamond detector (sensitive area 7.3 mm2), calibrated by means of 60Co radiation. Additionally, dose delivery was simulated by Monte Carlo calculations using the EGSnrc code system. The calculated secondary electron fluence spectra at the cell location did not indicate a significant change of radiation quality (i.e. higher linear energy transfer) at the lower distances.Clonogenic cell survival curves obtained after brachytherapy exhibited an altered biological response compared to x-rays which was characterized by a significant reduction of the survival curve shoulder when dose rate fluctuations were high. Therefore, also for the time scale of the present investigation, cellular effects of radiation are not invariant to the temporal pattern in dose rate. We propose that with high dose rate variation the cells activate less efficiently their DNA damage response than after continuous irradiation.

Dosimetric perturbations at high-Z interfaces with high dose rate 192Ir source

PurposeTo investigate dose perturbations created by high-atomic number (Z) materials in high dose rate (HDR) Iridium-192 (192Ir) treatment region. Methods and materialsA specially designed parallel plate ion chamber with 5 μm thick window was used to measure the dose rates from 192Ir source downstream of the high-Z materials. A Monte Carlo (MC) code was employed to calculate the dose rates in both upstream and downstream of the high-Z interfaces at distances ranging from 0.01 to 2 mm. The dose perturbation factor (DPF) was defined as the ratio of dose rate with and without high-Z material in a water phantom. For verifying the Z dependence, both 0.1- and 1.0 mm-thick sheets of Pb, Au, Ta, Sn, Cu, Fe, Ti and Al were used. Results/conclusionsThe DPF depends on the Z and thickness of layer. At the downstream of a 0.1 mm layer of Pb, Au, Ta, Sn, Cu, Fe, Ti and Al, the DPF by MC were 3.73, 3.42, 3.04, 1.71, 1.04, 0.98, 0.92, or 0.94 respectively. When Z is greater than or equal to 50, the MC and experimental results disagree significantly (>20%) due to large DPF gradient but are in agreement for Z less than or equal to 29. Thin layers of Z greater than or equal to 50 near a 192Ir source in water produce significant dose perturbations (i.e. increases) in the vicinity of the medium-high-Z interfaces and may thus cause local over-dose in 192Ir brachytherapy. Conversely, this effect may potentially be used to deliver locally higher doses to targeted tissue.

Dosimetric characterization and output verification for conical brachytherapy surface applicators. Part II. High dose rate 192Ir sources.

Historically, treatment of malignant surface lesions has been achieved with linear accelerator based electron beams or superficial x-ray beams. Recent developments in the field of brachytherapy now allow for the treatment of surface lesions with specialized conical applicators placed directly on the lesion. Applicators are available for use with high dose rate (HDR)(192)Ir sources, as well as electronic brachytherapy sources. Part I of this paper discussed the applicators used with electronic brachytherapy sources. Part II will discuss those used with HDR (192)Ir sources. Although the use of these applicators has gained in popularity, the dosimetric characteristics have not been independently verified. Additionally, there is no recognized method of output verification for quality assurance procedures with applicators like these. This work aims to create a cohesive method of output verification that can be used to determine the dose at the treatment surface as part of a quality assurance/commissioning process for surface applicators used with HDR electronic brachytherapy sources (Part I) and(192)Ir sources (Part II). Air-kerma rate measurements for the (192)Ir sources were completed with several models of small-volume ionization chambers to obtain an air-kerma rate at the treatment surface for each applicator. Correction factors were calculated using MCNP5 and EGSnrc Monte Carlo codes in order to determine an applicator-specific absorbed dose to water at the treatment surface from the measured air-kerma rate. Additionally, relative dose measurements of the surface dose distributions and characteristic depth dose curves were completed in-phantom. Theoretical dose distributions and depth dose curves were generated for each applicator and agreed well with the measured values. A method of output verification was created that allows users to determine the applicator-specific dose to water at the treatment surface based on a measured air-kerma rate. The novel output verification methods described in this work will reduce uncertainties in dose delivery for treatments with these kinds of surface applicators, ultimately improving patient care.

Dosimetric characteristics of a new unit for electronic skin brachytherapy

PurposeBrachytherapy with radioactive high dose rate (HDR) 192Ir source is applied to small skin cancer lesions, using surface applicators, i.e. Leipzig or Valencia type. New developments in the field of radiotherapy for skin cancer include electronic brachytherapy. This technique involves the placement of an HDR X-ray source close to the skin, therefore combining the benefits of brachytherapy with the reduced shielding requirements and targeted energy of low energy X-rays. Recently, the Esteya® Electronic Brachytherapy System (Esteya EBS, Elekta AB-Nucletron, Stockholm, Sweden) has been developed specifically for HDR brachytherapy treatment of surface lesions. The system provides radionuclide free HDR brachytherapy by means of a small 69.5 kV X-ray source. The purpose of this study is to obtain the dosimetric characterization required for clinical implementation, providing the detailed methodology to perform the commissioning.Material and methodsFlatness, symmetry and penumbra, percentage of depth dose (PDD), kV stability, HVL, output, spectrum, linearity, and leakage have been evaluated for a set of applicators (from 10 mm to 30 mm in diameter).ResultsFlatness and symmetry resulted better than 5% with around 1 mm of penumbra. The depth dose gradient is about 7%/mm. A kV value of 68.4 ± 1.0 kV (k = 1) was obtained, in good agreement with manufacturer data (69.5 kV). HVL was 1.85 mm Al. Dose rate for a typical 6 Gy to 7 Gy prescription resulted about 3.3 Gy/min and the leakage value was < 100 µGy/min.ConclusionsThe new Esteya® Electronic Brachytherapy System presents excellent flatness and penumbra as with the Valencia applicator case, combined with an improved PDD, allowing treatment of lesions of up to a depth of 5 mm in combination with reduced treatment duration. The Esteya unit allows HDR brachytherapy superficial treatment within a minimally shielded environment due its low energy.

Direct reconstruction and associated uncertainties of 192Ir source dwell positions in ring applicators using gafchromic film in the treatment planning of HDR brachytherapy cervix patients

This paper details a practical method for the direct reconstruction of high dose rate 192Ir source dwell positions in ring applicators using gafchromic film in the treatment planning of brachytherapy cervix patients. It also details the uncertainties associated with such a process. Eight Nucletron interstitial ring applicators—Ø26 mm (×4), Ø30 mm (×3) and Ø34 mm (×1), and one 60 mm intrauterine tube were used in this study. RTQA2 and XRQA2 gafchromic films were irradiated at pre-programmed dwell positions with three successive 192Ir sources and used to derive the coordinates of the source dwell positions. The source was observed to deviate significantly from its expected position by up to 6.1 mm in all ring sizes. Significant inter applicator differences of up to 2.6 mm were observed between a subset of ring applicators. Also, the measured data were observed to differ significantly from commercially available source path models provided by Nucletron with differences of up to 3.7 mm across all ring applicator sizes. The total expanded uncertainty (k = 2) averaged over all measured dwell positions in the rings was observed to be 1.1 ± 0.1 mm (Ø26 mm and Ø30 mm rings) and 1.0 ± 0.3 mm (Ø34 mm ring) respectively, and when transferred to the treatment planning system, equated to maximum %dose changes of 1.9%, 13.2% and 1.5% at regions representative of the parametrium, lateral fornix and organs at risk respectively.

A standard graphite calorimeter for dosimetry in brachytherapy with high dose rate 192Ir sources

Within the framework of the JRP06 European project ‘Increasing Cancer Treatment Efficacy Using 3D Brachytherapy’, a prototype of a graphite standard calorimeter for the measurement of the absorbed dose rate to water, , for 192Ir sources used in high dose rate (HDR) brachytherapy has been developed at the Italian National Institute of Ionizing Radiation Metrology (ENEA-INMRI). The calorimeter was tested at the Sant'Andrea Hospital in Rome, where measurements were performed in the quasi-adiabatic mode of operation using an 192Ir MicroSelectron® HDR V2 source. The measurements showed a reproducibility of about 1%, while the combined standard uncertainty on the value at the distance of 1 cm from the source was estimated as 1.4%, lower than the uncertainty of determined from the reference air-kerma rate.

Experimental determination of the energy response of alanine pellets in the high dose rate 192Ir spectrum

An experimental determination of the energy correction factor for alanine/paraffin pellets in the 192Ir spectrum at varying distances from the source is presented. Alanine dosimeters were irradiated in water under full scatter conditions with a high dose rate (HDR) 192Ir source (Flexisource), using a dedicated holder. Up to six line sources (catheters) fit in a regular pattern at fixed radial distances from the holder axis, the alanine detector being placed at the centre of the holder. The HDR source was stepping every 0.5 cm within a trocar needle within ± 3.0 cm around the medial plane through the detector in order to achieve dose homogeneity within the detector volume. The energy correction factor of alanine/paraffin pellets in 192Ir relative to 60Co was experimentally determined as the inverse ratio of the dose to water measured in water around the 192Ir source to the dose to water calculated in water using the TG-43 formalism. The pellets were read out with a Bruker EMXmicro spectrometer (X-band). The amplitude of the central line in the alanine absorption spectrum from pellets irradiated within the 192Ir spectrum was directly compared with the amplitude from 60Co-irradiated pellets. The energy correction factors of Harwell pellets irradiated in the 192Ir spectrum are 1.029 ± 0.02, 1.027 ± 0.02 and 1.045 ± 0.02 at a mean weighted source–detector distance of 2.0, 2.9 and 5.3 cm, respectively. The experimentally obtained values for the energy response are 1.3% lower compared to the theoretical values for radial distances smaller than 3 cm.

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 linear source table to determine the treatment times for cylindrical applicators used in high–dose rate brachytherapy

Purpose It is a good practice for quality assurance purposes to check the high–dose rate brachytherapy source dwell times obtained from any computerized treatment planning system (TPS) by independent hand calculations. The aim of this work was to prepare a table that would help to calculate the total dwell times for cylindrical applicators used for high–dose rate brachytherapy. Methods and Materials The results of an earlier investigation of dose rate per unit source strength at different distances from the microSelectron-high–dose rate 192Ir source and the relative weights for dwell times obtained from optimized plans were used to prepare a linear source table. Results This table gives the product of source strength and total dwell times to deliver 1000 cGy at various distances for active lengths ranging from 1 to 8 cm and for treatment distances of 1 to 2.25 cm. One can use this table to determine the total dwell time necessary to deliver the prescribed dose at a specific distance for the source strength at the time of treatment. This can be compared with the total dwell time calculated by the computerized TPS. Conclusion The linear source table makes the determination of total dwell time very easy and would help the clinical physicist to check the accuracy of results of the TPS quickly.

A Monte Carlo dosimetry study using Henschke applicator for cervical brachytherapy

In recent years the Henschke applicator has been widely used for gynecologic patients treated by brachytherapy in Taiwan. However, the commercial brachytherapy planning system did not properly evaluate the dose perturbation caused by the Henschke applicator. Since the European Society for Therapeutic Radiology and Oncology advised that the effect of source shielding should be incorporated into the brachytherapy planning system, it required calculation and comparison of the dose distribution around the applicator. This study used the Monte Carlo MCNP code to simulate the dose distribution in a water phantom that contained the Henschke applicator with one tandem and two ovoids. Three dwell positions of a high dose rate 192Ir source were simulated by including and excluding the applicator. The mesh tally option of the MCNP was applied to facilitate the calculation of a large number of tallies in the phantom. The voxel size effect and the charge particle equilibrium were studied by comparing the results calculated with different tally options. The calculated results showed that the brachytherapy planning system overestimated the rectal dose and that the shielding material in the applicator contributed more than 40% to the rectal dose.

Verification of high dose rate 192Ir source position during brachytherapy treatment

A system for in vivo tracking of 1 Ci 192Ir source during brachytherapy treatment has been built using high resistivity silicon pad detectors as image sensors and knife-edge lead pinholes as collimators. The sensors consist of 256 pads arranged in 32 ×8 grid with pad size 1.4 × 1.4 mm 2 and 1 mm thickness. The sensors have two metal layers, enabling connection of readout electronics (VATAGP3_1 chips) at the edge of the detector. With source self-images obtained from a dual-pinhole system, location of the source can be reconstructed in three dimensions in real time, allowing on-line detection of deviations from planned treatment. The system was tested with 1 Ci 192Ir clinical source in air and plexi-glass phantom. The movements of the source could be tracked in a field of view of approximately 20 × 20 × 20 cm 3 with absolute precision of about 5 mm. Positions of the source, relative to the first measured source position, could be mapped with precision of around 3 mm.

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.

An evaluation of the TSE MR sequence for time efficient data acquisition in polymer gel dosimetry of applications involving high doses and steep dose gradients

The use of magnetic resonance imaging as a readout method for polymer gel dosimetry commonly involves long imaging sessions, particularly when high spatial resolution is required in all three dimensions, for the investigation of dose distributions with steep dose gradients and stringent dose delivery specifications. In this work, a volume selective turbo spin echo (TSE) pulse sequence is compared to the established Carr-Purcell-Meiboom-Gill (CPMG) multiecho acquisition with regard to providing accurate dosimetric results in significantly reduced imaging times. Polyethylene glycol diacrylate based (PABIG) gels were irradiated and subsequently scanned to obtain R2 relaxation rate measurements, using a CPMG multiecho sequence and a dual echo TSE utilizing an acceleration (turbo) factor of 64. R2 values, plotted against corresponding Monte Carlo dose calculations, provided calibration data of PABIG gels dose response over a wide dose range. A linear R2 versus dose relationship was demonstrated for both sequences with TSE results presenting reduced dose sensitivity. Although TSE data were found to deviate from linearity at lower doses compared to CPMG data, a relatively wide dynamic dose range of response extending up to approximately 100 Gy was observed for both sequences. The TSE and CPMG sequences were evaluated with a brachytherapy irradiation using a high dose rate 192Ir source and a gamma knife stereotactic radiosurgery irradiation with a single 4 mm collimator helmet shot. Dosimetric results obtained with the TSE and CPMG are shown to compare equally well with the expected dose distributions for these irradiations. The 60-fold scan time reduction achieved with TSE implies that this sequence could prove to be a useful tool for the introduction of polymer gel dosimetry in clinical radiation therapy applications involving high doses and steep dose gradients.

Does inverse planning applied to Iridium192 high dose rate prostate brachytherapy improve the optimization of the dose afforded by the Paris system?

Background and purposeThe purpose of the work is to analyse for 192Ir prostate brachytherapy (BT) some of the different steps in optimizing the dose delivered to the CTV, urethra and rectum. Materials and methodsBetween 07/1998 and 12/2001, 166 patients were treated with 192Ir wires providing a low dose rate, according to the Paris system philosophy and with the 2D version of the treatment planning IsisR. 40–45Gy were delivered after an external beam radiotherapy of 40Gy. The maximum tolerable doses for BT were 25Gy to the anterior third of the rectum on the whole length of the implant (R dose) and 52Gy to the urethra on a 1cm length (Umax). A Umax/CTV dose ratio >1.3 represented a pejorative value as the planned dose of 40–45Gy could not be achieved. On the other side a ratio ≤1.25 was considered optimal and the intermediate values satisfactory. A R/CTV dose ratio <0.55 which is easily obtained was also stated as an optimal situation. From the CT Scan images performed for these implants, a theoretical study investigated the possibilities of complementary optimization afforded by a 3D treatment planning. This work was based on an inverse planning philosophy and a stepping source technology (SST) for high dose rate 192Ir sources. ResultsAt the end of a learning curve reaching a plateau after the first 71 patients, 90% of the implants with 192Ir wires were stated at least satisfactory for a total rate of 82% for the whole population. When the 3D dosimetry for SST was used, the initial values >1.25 decreased significantly with optimization required on CTV contours and additional constraints on urethra while the R/CTV ratio was maintained under 0.55. For initial Umax/CTV >1.3 or >1.25 but ≤1.3 indeed, the mean respective values of 1.41±0.16 and 1.28±0.01 decreased to 1.28±0.24 and 1.17±0.09 (P<0.001), allowing to increase the total dose to the CTV by 4Gy. ConclusionsThe Paris system which assumes a homogeneous distribution of a minimum number of catheters inside the CTV allowed to anticipate a satisfactory dosimetry in 82% of cases. However, this precision rate could be improved until 95% with an optimization approach based on an inverse planning philosophy. These new 3D optimization methods, ideally based on good quality implants at first allow to deliver the highest doses with minimal probability of creating cold spots inside the CTV or unacceptable hot spots outside.

A standard dosimetry procedure for 192Ir sources used for endovascular brachytherapy

The experimental dosimetry of a high dose rate (HDR) 192Ir source used for the brachytherapy of peripheral vessels is reported. The direct determination of the reference air kerma rate K̇r agrees, within the experimental uncertainty, with the results obtained by a well ionization chamber calibrated at the NIST and the manufacturer's certification. A highly sensitive (HS) radiochromic film (RCF), that presents only one active layer, was used for the source dosimetry in a water phantom. The adopted experimental set-up, with the source in its catheter positioned on the RCF plane, seems to have given better accuracy of the RCF optical density measurements. The agreement between the measurement of the dose rate constant DKr(10 mm, π/2) and the literature data confirmed the coherence of the HS RCF calibration obtained by the kerma in air measurements. The RCF measurements supplied dosimetric information about the dose to water per reference air kerma rate Ḋ(r, θ)/K̇r along the source transverse bisector axis, the radial dose function g(r) and the anisotropy function F(r, θ). The value Ḋ(2 mm, π/2)/K̇r = 22.4 ± 1.2 cGy h−1/(μGy h−1) is supplied with a dose uncertainty that is essentially due to the indeterminacy of the source position in the catheter. The data of the radial and anisotropy functions have been compared with Monte Carlo determinations reported in the literature.