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Multi-needle localization in ultrasound (US) images is a crucial step of treatment planning for US-guided prostate brachytherapy. However, current computer-aided technologies are mostly focused on single-needle digitization, while manual digitization is labor intensive and time consuming. In this paper, we proposed a deep learning-based workflow for fast automatic multi-needle digitization, including needle shaft detection and needle tip detection. The major workflow is composed of two components: a large margin mask R-CNN model (LMMask R-CNN), which adopts the lager margin loss to reformulate Mask R-CNN for needle shaft localization, and a needle based density-based spatial clustering of application with noise algorithm which integrates priors to model a needle in an iteration for a needle shaft refinement and tip detections. Besides, we use the skipping connection in neural network architecture to improve the supervision in hidden layers. Our workflow was evaluated on 23 patients who underwent US-guided high-dose-rate (HDR) prostrate brachytherapy with 339 needles being tested in total. Our method detected 98% of the needles with 0.091 ± 0.043 mm shaft error and 0.330 ± 0.363 mm tip error. Compared with only using Mask R-CNN and only using LMMask R-CNN, the proposed method gains a significant improvement on both shaft error and tip error. The proposed method automatically digitizes needles per patient with in a second. It streamlines the workflow of transrectal ultrasound-guided HDR prostate brachytherapy and paves the way for the development of real-time treatment planning system that is expected to further elevate the quality and outcome of HDR prostate brachytherapy.

External-beam radiotherapy followed by high dose rate (HDR) brachytherapy is the standard-of-care for treating gynecologic cancers. The enhanced soft-tissue contrast provided by magnetic resonance imaging (MRI) makes it a valuable imaging modality for diagnosing and treating these cancers. However, in contrast to computed tomography (CT) imaging, the appearance of the brachytherapy catheters, through which radiation sources are inserted to reach the cancerous tissue later on, is often variable across images. This paper reports, for the first time, a new deep-learning-based method for fully automatic segmentation of multiple closely spaced brachytherapy catheters in intraoperative MRI. Represented in the data are 50 gynecologic cancer patients treated by MRI-guided HDR brachytherapy. For each patient, a single intraoperative MRI was used. 826 catheters in the images were manually segmented by an expert radiation physicist who is also a trained radiation oncologist. The number of catheters in a patient ranged between 10 and 35. A deep 3D convolutional neural network (CNN) model was developed and trained. In order to make the learning process more robust, the network was trained 5 times, each time using a different combination of shown patients. Finally, each test case was processed by the five networks and the final segmentation was generated by voting on the obtained five candidate segmentations. 4-fold validation was executed and all the patients were segmented. An average distance error of 2.0  ±  3.4 mm was achieved. False positive and false negative catheters were 6.7% and 1.5% respectively. Average Dice score was equal to 0.60  ±  0.17. The algorithm is available for use in the open source software platform 3D Slicer allowing for wide scale testing and research discussion. In conclusion, to the best of our knowledge, fully automatic segmentation of multiple closely spaced catheters from intraoperative MR images was achieved for the first time in gynecological brachytherapy.

For the purpose of magnetic resonance imaging (MRI) guidance of prostate high-dose-rate (HDR) brachytherapy, this paper presents a study on the potential of clinically relevant MRI sequences to facilitate tracking or localization of brachytherapy devices (HDR source/titanium needle), and which could simultaneously be used to visualize the anatomy. The tracking or localization involves simulation of the MRI artifact in combination with a template matching algorithm. Simulations of the MRI artifacts induced by an HDR brachytherapy source and a titanium needle were implemented for four types of sequences: spoiled gradient echo, spin echo, balanced steady-state free precession (bSSFP) and bSSFP with spectral attenuated inversion recovery (SPAIR) fat suppression. A phantom study was conducted in which mentioned sequences (in 2D) as well as the volumetric MRI sequences of the current clinical scan protocol were applied to obtain the induced MRI artifacts for an HDR source and a titanium needle. Localization of the objects was performed by a phase correlation based template matching algorithm. The simulated images demonstrated high correspondences with the acquired MR images, and allowed localization of the objects. A comparison between the object positions obtained for all applied MRI sequences showed deviations (from the average position) of 0.2-0.3 mm, proving that all MRI sequences were suitable for localization of the objects, irrespective of their 2D or volumetric nature. This study demonstrated that the MRI artifact induced by an HDR source or a titanium needle could be simulated for the four investigated types of MRI sequences (spoiled gradient echo, spin echo, bSSFP and bSSFP-SPAIR), valuable for real-time object localization in clinical practice. This leads to more flexibility in the choice of MRI sequences for guidance of HDR brachytherapy, as they are suitable for both object localization and anatomy visualization.

Low dose rate (LDR) brachytherapy is a minimally invasive form of radiation therapy, used to treat prostate cancer, and it involves permanent implantation of radioactive sources (seeds) inside of the prostate gland. Treatment planning in brachytherapy involves a decision making process for the placement of the sources in order to deliver an effective dose of radiation to cancerous tissue in the prostate while sparing the surrounding healthy tissue. Such a decision making process can be modeled as a mixed-integer linear programming (MILP) problem. In this paper, we introduce a novel MILP optimization model framework for interstitial LDR prostate brachytherapy designed to explicitly mimic the qualities of treatment plans produced manually by expert planners. Our approach involves incorporating a unique set of clinically important constraints, called spatial constraints, into the optimization model. Computational results for an initial model reflecting clinical practice at our cancer center show that the treatment plans produced largely capture the spatial and dosimetric characteristics of manual plans created by expert planners.

This paper presents a simulation study of BrachyShade, a proposed internal source-tracking system for real time quality assurance in high dose rate prostate brachytherapy. BrachyShade consists of a set of spherical tungsten occluders located above a pixellated silicon photodetector. The source location is estimated by minimising the mean squared error between a parametric model of the shadow image and acquired images of the shadows projected on the detector plane. A novel algorithm is finally employed to correct the systemic error resulting from Compton scattering in the medium. The worst-case error obtained with BrachyShade for a 13.5 ms image acquisition is less than 1.3 mm in the most distant part of the treatment volume, while for 75% of source locations an error of less than 0.42 mm was achieved.

We respond to the comments by Dr Yegin by identifying the source of an error in a fit in our original paper but arguing that the lack of a fit does not affect the conclusion based on the raw data that [Formula: see text] is an accurate code and we provide further benchmarking data to demonstrate this point.

High dose rate (HDR) brachytherapy treatment planning is conventionally performed manually and/or with aids of preplanned templates. In general, the standard of care would be elevated by conducting an automated process to improve treatment planning efficiency, eliminate human error, and reduce plan quality variations. Thus, our group is developing AutoBrachy, an automated HDR brachytherapy planning suite of modules used to augment a clinical treatment planning system. This paper describes our proof-of-concept module for vaginal cylinder HDR planning that has been fully developed. After a patient CT scan is acquired, the cylinder applicator is automatically segmented using image-processing techniques. The target CTV is generated based on physician-specified treatment depth and length. Locations of the dose calculation point, apex point and vaginal surface point, as well as the central applicator channel coordinates, and the corresponding dwell positions are determined according to their geometric relationship with the applicator and written to a structure file. Dwell times are computed through iterative quadratic optimization techniques. The planning information is then transferred to the treatment planning system through a DICOM-RT interface. The entire process was tested for nine patients. The AutoBrachy cylindrical applicator module was able to generate treatment plans for these cases with clinical grade quality. Computation times varied between 1 and 3 min on an Intel Xeon CPU E3-1226 v3 processor. All geometric components in the automated treatment plans were generated accurately. The applicator channel tip positions agreed with the manually identified positions with submillimeter deviations and the channel orientations between the plans agreed within less than 1 degree. The automatically generated plans obtained clinically acceptable quality.

This paper demonstrates that optimization strategies derived from the field of compressed sensing (CS) improve computational performance in inverse treatment planning (ITP) for high-dose-rate (HDR) brachytherapy. Following an approach applied to low-dose-rate brachytherapy, we developed a reformulation of the ITP problem with the same mathematical structure as standard CS problems. Two greedy methods, derived from hard thresholding and subspace pursuit are presented and their performance is compared to state-of-the-art ITP solvers. Applied to clinical prostate brachytherapy plans speed-up by a factor of 56-350 compared to state-of-the-art methods. Based on a Wilcoxon signed rank-test the novel method statistically significantly decreases the final objective function value (p  <  0.01). The optimization times were below one second and thus planing can be considered as real-time capable. The novel CS inspired strategy enables real-time ITP for HDR brachytherapy including catheter optimization. The generated plans are either clinically equivalent or show a better performance with respect to dosimetric measures.

Monte Carlo (MC) particle transport simulation on a graphics-processing unit (GPU) platform has been extensively studied recently due to the efficiency advantage achieved via massive parallelization. Almost all of the existing GPU-based MC packages were developed for voxelized geometry. This limited application scope of these packages. The purpose of this paper is to develop a module to model parametric geometry and integrate it in GPU-based MC simulations. In our module, each continuous region was defined by its bounding surfaces that were parameterized by quadratic functions. Particle navigation functions in this geometry were developed. The module was incorporated to two previously developed GPU-based MC packages and was tested in two example problems: (1) low energy photon transport simulation in a brachytherapy case with a shielded cylinder applicator and (2) MeV coupled photon/electron transport simulation in a phantom containing several inserts of different shapes. In both cases, the calculated dose distributions agreed well with those calculated in the corresponding voxelized geometry. The averaged dose differences were 1.03% and 0.29%, respectively. We also used the developed package to perform simulations of a Varian VS 2000 brachytherapy source and generated a phase-space file. The computation time under the parameterized geometry depended on the memory location storing the geometry data. When the data was stored in GPU's shared memory, the highest computational speed was achieved. Incorporation of parameterized geometry yielded a computation time that was ~3 times of that in the corresponding voxelized geometry. We also developed a strategy to use an auxiliary index array to reduce frequency of geometry calculations and hence improve efficiency. With this strategy, the computational time ranged in 1.75-2.03 times of the voxelized geometry for coupled photon/electron transport depending on the voxel dimension of the auxiliary index array, and in 0.69-1.23 times for photon only transport.

The development of magnetic resonance (MR) guided high dose rate (HDR) brachytherapy for prostate cancer has gained increasing interest for delivering a high tumor dose safely in a single fraction. To support needle placement in the limited workspace inside the closed-bore MRI, a single-needle MR-compatible robot is currently under development at the University Medical Center Utrecht (UMCU). This robotic device taps the needle in a divergent way from a single rotation point into the prostate. With this setup, it is warranted to deliver the irradiation dose by successive insertions of the needle. Although robot-assisted needle placement is expected to be more accurate than manual template-guided insertion, needle positioning errors may occur and are likely to modify the pre-planned dose distribution.In this paper, we propose a dose plan adaptation strategy for HDR prostate brachytherapy with feedback on the needle position: a dose plan is made at the beginning of the interventional procedure and updated after each needle insertion in order to compensate for possible needle positioning errors. The introduced procedure can be used with the single needle MR-compatible robot developed at the UMCU. The proposed feedback strategy was tested by simulating complete HDR procedures with and without feedback on eight patients with different numbers of needle insertions (varying from 4 to 12). In of the cases tested, the number of clinically acceptable plans obtained at the end of the procedure was larger with feedback compared to the situation without feedback. Furthermore, the computation time of the feedback between each insertion was below 100 s which makes it eligible for intra-operative use.

High-dose-rate brachytherapy is a tumor treatment method where a highly radioactive source is brought in close proximity to the tumor. In this paper we develop a simulated annealing algorithm to optimize the dwell times at preselected dwell positions to maximize tumor coverage under dose-volume constraints on the organs at risk. Compared to existing algorithms, our algorithm has advantages in terms of speed and objective value and does not require an expensive general purpose solver. Its success mainly depends on exploiting the efficiency of matrix multiplication and a careful selection of the neighboring states. In this paper we outline its details and make an in-depth comparison with existing methods using real patient data.

Well-type ionization chambers are used for measuring the source strength of radioactive brachytherapy sources before clinical use. Initially, the well chambers are calibrated against a suitable national standard. For high dose rate (HDR) (192)Ir, this calibration is usually a two-step process. Firstly, the calibration source is traceably calibrated against an air kerma primary standard in terms of either reference air kerma rate or air kerma strength. The calibrated (192)Ir source is then used to calibrate the secondary standard well-type ionization chamber. Calibration laboratories are usually only equipped with one type of HDR (192)Ir source. If the clinical source type is different from that used for the calibration of the well chamber at the standards laboratory, a source geometry factor, k(sg), is required to correct the calibration coefficient for any change of the well chamber response due to geometric differences between the sources. In this work we present source geometry factors for six different HDR (192)Ir brachytherapy sources which have been determined using Monte Carlo techniques for a specific ionization chamber, the Standard Imaging HDR 1000 Plus well chamber with a type 70010 HDR iridium source holder. The calculated correction factors were normalized to the old and new type of calibration source used at the National Physical Laboratory. With the old Nucletron microSelectron-v1 (classic) HDR (192)Ir calibration source, ksg was found to be in the range 0.983 to 0.999 and with the new Isodose Control HDR (192)Ir Flexisource k(sg) was found to be in the range 0.987 to 1.004 with a relative uncertainty of 0.4% (k = 2). Source geometry factors for different combinations of calibration sources, clinical sources, well chambers and associated source holders, can be calculated with the formalism discussed in this paper.

HDR BrachyView is a novel in-body dosimetric imaging system for real-time monitoring and verification of the source position in high dose rate (HDR) prostate brachytherapy treatment. It is based on a high-resolution pixelated detector array with a semi-cylindrical multi-pinhole tungsten collimator and is designed to fit inside a compact rectal probe, and is able to resolve the 3D position of the source with a maximum error of 1.5 mm. This paper presents an evaluation of the additional dose that will be delivered to the patient as a result of backscatter radiation from the collimator. Monte Carlo simulations of planar and cylindrical collimators embedded in a tissue-equivalent phantom were performed using Geant4, with an (192)Ir source placed at two different source-collimator distances. The planar configuration was replicated experimentally to validate the simulations, with a MOSkin dosimetry probe used to measure dose at three distances from the collimator. For the cylindrical collimator simulation, backscatter dose enhancement was calculated as a function of axial and azimuthal displacement, and dose distribution maps were generated at three distances from the collimator surface. Although significant backscatter dose enhancement was observed for both geometries immediately adjacent to the collimator, simulations and experiments indicate that backscatter dose is negligible at distances beyond 1 mm from the collimator. Since HDR BrachyView is enclosed within a 1 mm thick tissue-equivalent plastic shell, all backscatter radiation resulting from its use will therefore be absorbed before reaching the rectal wall or other tissues. dosimetry, brachytherapy, HDR.

This paper describes the development and characterization of a fully automated in vitro cell irradiator using an electronic brachytherapy source to perform radiation sensitivity bioassays. This novel irradiator allows complex variable dose and dose rate schemes to be delivered to multiple wells of 96-well culture plates used in standard biological assays. The Xoft Axxent® eBx™ was chosen as the x-ray source due to its ability to vary tube current up to 300 µA for a 50 kVp spectrum using clinical surface applicators. Translation of the multiwell plate across the fixed radiation field is achieved using a precision motor driven computer controlled positioning system. A series of measurements was performed to characterize dosimetric performance of the system. Measurements have shown that the radiation output measured with an end window ionization chamber is stable between operating currents of 50-300 µA. In addition, radiochromic film was used to characterize the field flatness and symmetry. The average field flatness in the in-plane and cross-plane direction was 2.9 ± 1.0% and 4.0 ± 1.7%, respectively. The average symmetry in the in-plane and cross-plane direction was 1.8 ± 0.9% and 1.6 ± 0.5%, respectively. The optimal focal spot resolution at the cellular plane was determined by measuring sequential irradiations on radiochromic film for three different well spacing schemes. It was determined that the current system can irradiate every other well with negligible impact on the radiation field characteristics. Finally, a performance comparison between this system and a common cabinet irradiator is presented.

This paper details a practical method for the direct reconstruction of high dose rate (192)Ir 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 (192)Ir 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.

The aim of this paper is to determine the dosimetric impact of trace elements in human tissues for low-energy photon sources used in brachytherapy. Monte Carlo dose calculations were used to investigate the dosimetric effect of trace elements present in normal or cancerous human tissues. The effect of individual traces (atomic number Z = 11-30) was studied in soft tissue irradiated by low-energy brachytherapy sources. Three other tissue types (prostate, adipose and mammary gland) were also simulated with varying trace concentrations to quantify the contribution of each trace to the dose distribution. The dose differences between cancerous and healthy prostate tissues were calculated in single- and multi-source geometries. The presence of traces in a tissue produces a difference in the dose distribution that is dependent on Z and the concentration of the trace. Low-Z traces (Na) have a negligible effect (<0.3%) in all tissues, while higher Z (K) had a larger effect (>3%). There is a potentially significant difference in the dose distribution between cancerous and healthy prostate tissues (4%) and even larger if compared to the trace-free composition (15%) in both single- and multi-sourced geometries. Trace elements have a non-negligible (up to 8% in prostate D(90)) effect on the dose in tissues irradiated with low-energy photon sources. This study underlines the need for further investigation into accurate determination of the trace composition of tissues associated with low-energy brachytherapy. Alternatively, trace elements could be incorporated as a source of uncertainty in dose calculations.

Task group 43 (TG43)-based dosimetry algorithms are efficient for brachytherapy dose calculation in water. However, human tissues have chemical compositions and densities different than water. Moreover, the mutual shielding effect of seeds on each other (interseed attenuation) is neglected in the TG43-based dosimetry platforms. The scientific community has expressed the need for an accurate dosimetry platform in brachytherapy. The purpose of this paper is to present ALGEBRA, a Monte Carlo platform for dosimetry in brachytherapy which is sufficiently fast and accurate for clinical and research purposes. ALGEBRA is based on the GEANT4 Monte Carlo code and is capable of handling the DICOM RT standard to recreate a virtual model of the treated site. Here, the performance of ALGEBRA is presented for the special case of LDR brachytherapy in permanent prostate and breast seed implants. However, the algorithm is also capable of handling other treatments such as HDR brachytherapy.

This paper details a practical method for deriving the reference air kerma rate calibration coefficient for Farmer NE2571 chambers using the U.K. Institute of Physics and Engineering in Medicine (IPEM) code of practice for the determination of the reference air kerma rate for HDR (192)Ir 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 (192)Ir 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.

The success of prostate brachytherapy critically depends on delivering adequate dose to the prostate gland, and the capability of intraoperatively localizing implanted seeds provides potential for dose evaluation and optimization during therapy. REDMAPS is a recently reported algorithm that carries out seed localization by detecting, matching and reconstructing seeds in only a few seconds from three acquired x-ray images (Lee et al 2011 IEEE Trans. Med. Imaging 29 38-51). In this paper, we present an automatic pose correction (APC) process that is combined with REDMAPS to allow for both more accurate seed reconstruction and the use of images with relatively large pose errors. APC uses a set of reconstructed seeds as a fiducial and corrects the image pose by minimizing the overall projection error. The seed matching and APC are iteratively computed until a stopping condition is met. Simulations and clinical studies show that APC significantly improves the reconstructions with an overall average matching rate of ⩾99.4%, reconstruction error of ⩽0.5 mm, and the matching solution optimality of ⩾99.8%.

In this paper, we present an anatomy-based three-dimensional dose optimization approach for HDR brachytherapy using interactive multiobjective optimization (IMOO). In brachytherapy, the goals are to irradiate a tumor without causing damage to healthy tissue. These goals are often conflicting, i.e. when one target is optimized the other will suffer, and the solution is a compromise between them. IMOO is capable of handling multiple and strongly conflicting objectives in a convenient way. With the IMOO approach, a treatment planner's knowledge is used to direct the optimization process. Thus, the weaknesses of widely used optimization techniques (e.g. defining weights, computational burden and trial-and-error planning) can be avoided, planning times can be shortened and the number of solutions to be calculated is small. Further, plan quality can be improved by finding advantageous trade-offs between the solutions. In addition, our approach offers an easy way to navigate among the obtained Pareto optimal solutions (i.e. different treatment plans). When considering a simulation model of clinical 3D HDR brachytherapy, the number of variables is significantly smaller compared to IMRT, for example. Thus, when solving the model, the CPU time is relatively short. This makes it possible to exploit IMOO to solve a 3D HDR brachytherapy optimization problem. To demonstrate the advantages of IMOO, two clinical examples of optimizing a gynecologic cervix cancer treatment plan are presented.

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 (192)Ir brachytherapy sources. In 2004, the National Physical Laboratory (NPL) commissioned a primary standard for the realization of RAKR of HDR (192)Ir brachytherapy sources. This has meant that it is now possible to calibrate ionization chambers directly traceable to an air kerma standard using an (192)Ir 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, Κ(R) (ICRU 1985 ICRU Report No 38 (Washington, DC: ICRU); ICRU 1997 ICRU Report No 58 (Bethesda, MD: ICRU)), or air kerma strength, S(K) (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 (192)Ir 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.

The objective of this study was to determine the magnitude of transit dose contributions to the planned dose in common intracavitary and interstitial brachytherapy treatments delivered using a pulsed dose rate (PDR) remote afterloader. The total transit dose arises from the travel of the radiation source into (entry) and out of (exit) the applicator, and between the dwell positions (inter-dwell). In this paper, we used a well-type ionization chamber to measure the transit dose component for a PDR afterloader and compared the results against measurements for a high dose rate (HDR) afterloader. Our results show that for typical intracavitary and interstitial treatments, the major contribution to transit dose is from the entry+exit source travel, as the inter-dwell component is effectively compensated for (<0.5%) by the afterloader. The transit dose was generally found to be larger for PDR treatments than for HDR treatments, as it is influenced by the source activity, dwell times and number of radiation pulses. The overall increase in the planned dose contributed by the transit dose in a typical intracavitary PDR treatment was estimated to be <2%, but much higher for interstitial treatments. This study shows that the effect of the transit dose on common clinical intracavitary PDR brachytherapy treatments is practically negligible, but requires attention in highly fractionated large volume interstitial treatments.

Plastic scintillator fibre optic dosimeters (FODs) have advantages for both brachytherapy and external beam radiotherapy applications. Convenient real-time monitoring and diagnosis of such dosimeters are desirable because of changes in the optical circuit that may arise in use. In this paper, we propose and demonstrate a real-time method using ultraviolet light emitting diodes (LED) to stimulate the scintillator and to diagnose failures of FODs. Key aspects of the LED FOD dosimetry design are investigated, enabling the design of a stable and accurate real-time monitoring dosimetry system. We demonstrate experimentally that the real-time monitoring FOD system is convenient to be used to monitor FOD dosimeters and to diagnose their failures resulted from different mechanisms.

The presence of heterogeneous media can produce significant perturbations of dose distribution in brachytherapy. In a companion paper, we proposed a dose decomposition approach for dose calculation in a heterogeneous medium, which separately treats dose contributions from primary, once-scattered and multiple-scattered photons. The companion paper also describes and verifies a micro-beam ray-tracing method for evaluating the once-scatter dose. This paper deals with the calculation of the multiple-scatter dose. We present two empirical formulations for evaluating the heterogeneity correction factor for a 27 keV point source in a water sphere containing a disc-shaped heterogeneity. The empirical formulations are based on nonlinear curve fitting of the Monte Carlo multiple-scatter dose estimates calculated for the heterogeneous system. Extensive benchmark comparisons show that these formulations provide results for the multiple-scatter dose that agree within 10% (and mostly within 5%) with corresponding Monte Carlo dose estimates. Combining them with the algorithms for primary and once-scatter dose calculation described in the companion paper yields results for the total dose of equivalent accuracy. The empirical formulations are expressed in simple mathematical forms which involve a separation of the geometry and position variables of the heterogeneous system. Such representation provides a good tool to investigate the heterogeneity-induced perturbation of a multiple-scatter dose at low photon energy.

In this work, we propose a framework for calculating brachytherapy dose distributions in heterogeneous media. The approach taken includes analytical calculation of the primary dose, and separately treats contributions of the once-scatter photons and multiple-scatter photons to the total scatter dose. This paper focuses on the evaluation of the once-scatter dose, which is based on a micro-beam ray-tracing model developed by the authors that incorporates an accurate description of the physical scattering of photons (Compton and Rayleigh scattering) with considerable flexibility in accommodating diverse geometries in a heterogeneous medium. The accuracy of the ray-tracing model has been verified by comparing model-calculated once-scatter doses with corresponding Monte Carlo results. For a 22 keV, 27 keV and 300 keV point source in water containing a disc-shaped heterogeneity of whitlockite, stainless steel or lead, our calculated results for once-scatter doses are in excellent agreement with corresponding Monte Carlo results over a wide range of heterogeneity dimensions and positions. Our investigation also explores the differences between physical scattering and isotropic scattering in evaluating the once-scatter dose, and thus enables the domain of applicability of the latter to be assessed. An appropriate method for evaluating the multiple-scatter dose, which together with the micro-beam method described here provides a means to calculate the total dose, is the subject of a companion paper.

This paper presents a generalization of a previously published methodology which quantified the radiobiological consequences of dose-gradient effects in brachytherapy applications. The methodology uses the linear-quadratic (LQ) formulation to identify an equivalent biologically effective dose (BED(eq)) which, if applied uniformly to a specified tissue volume, would produce the same net cell survival as that achieved by a given non-uniform brachytherapy application. Multiplying factors (MFs), which enable the equivalent BED for an enclosed volume to be estimated from the BED calculated at the dose reference surface, have been calculated and tabulated for both spherical and cylindrical geometries. The main types of brachytherapy (high dose rate (HDR), low dose rate (LDR) and pulsed (PB)) have been examined for a range of radiobiological parameters/dimensions. Equivalent BEDs are consistently higher than the BEDs calculated at the reference surface by an amount which depends on the treatment prescription (magnitude of the prescribed dose) at the reference point. MFs are closely related to the numerical BED values, irrespective of how the original BED was attained (e.g., via HDR, LDR or PB). Thus, an average MF can be used for a given prescribed BED as it will be largely independent of the assumed radiobiological parameters (radiosensitivity and alpha/beta) and standardized look-up tables may be applicable to all types of brachytherapy treatment. This analysis opens the way to more systematic approaches for correlating physical and biological effects in several types of brachytherapy and for the improved quantitative assessment and ranking of clinical treatments which involve a brachytherapy component.

The 50-year tenure of Physics in Medicine and Biology has coincided with some of the most important developments in radiological science, including the introduction of artificial radioactivity, computers and 3D imaging into medicine. These events have profoundly influenced the development of brachytherapy. Although it is not the dominant radiotherapy modality, it continues to play an important role in cancer therapy, more than a century after its introduction. This paper reviews the impact of three broad categories of innovation introduced since 1950 from the North American perspective: the introduction of artificial radioactivity, computer- and image-based treatment planning, and basic single-source dosimetry.

Estimation of prostate location and volume is essential in determining a dose plan for ultrasound-guided brachytherapy, a common prostate cancer treatment. However, manual segmentation is difficult, time consuming and prone to variability. In this paper, we present a semi-automatic discrete dynamic contour (DDC) model based image segmentation algorithm, which effectively combines a multi-resolution model refinement procedure together with the domain knowledge of the image class. The segmentation begins on a low-resolution image by defining a closed DDC model by the user. This contour model is then deformed progressively towards higher resolution images. We use a combination of a domain knowledge based fuzzy inference system (FIS) and a set of adaptive region based operators to enhance the edges of interest and to govern the model refinement using a DDC model. The automatic vertex relocation process, embedded into the algorithm, relocates deviated contour points back onto the actual prostate boundary, eliminating the need of user interaction after initialization. The accuracy of the prostate boundary produced by the proposed algorithm was evaluated by comparing it with a manually outlined contour by an expert observer. We used this algorithm to segment the prostate boundary in 114 2D transrectal ultrasound (TRUS) images of six patients scheduled for brachytherapy. The mean distance between the contours produced by the proposed algorithm and the manual outlines was 2.70 +/- 0.51 pixels (0.54 +/- 0.10 mm). We also showed that the algorithm is insensitive to variations of the initial model and parameter values, thus increasing the accuracy and reproducibility of the resulting boundaries in the presence of noise and artefacts.

Post-implantation dosimetry is an important element of permanent prostate brachytherapy. This process relies on accurate localization of implanted seeds relative to the surrounding organs. Localization is commonly achieved using CT images, which provide suboptimal prostate delineation. On MR images, conversely, prostate visualization is excellent but seed localization is imprecise due to distortion and susceptibility artefacts. This paper presents a method based on fused MR and x-ray images acquired consecutively in a combined x-ray and MRI interventional suite. The method does not rely on any explicit registration step but on a combination of system calibration and tracking. A purpose-built phantom was imaged using MRI and x-rays, and the images were successfully registered. The same protocol was applied to three patients where combining soft tissue information from MRI with stereoscopic seed identification from x-ray imaging facilitated post-implant dosimetry. This technique has the potential to improve on dosimetry using either CT or MR alone.

The need for fast, accurate and high resolution dosimetric quality assurance in radiation therapy has been outpacing the development of new and improved 2D and 3D dosimetry techniques. This paper summarizes the efforts to create a novel and potentially very fast, 3D dosimetry method based on the observation of scintillation light from an irradiated liquid scintillator volume serving simultaneously as a phantom material and as a dose detector medium. The method, named three-dimensional scintillation dosimetry (3DSD), uses visible light images of the liquid scintillator volume at multiple angles and applies a tomographic algorithm to a series of these images to reconstruct the scintillation light emission density in each voxel of the volume. It is based on the hypothesis that with careful design and data processing, one can achieve acceptable proportionality between the local light emission density and the locally absorbed dose. The method is applied to a Ru-106 eye plaque immersed in a 16.4 cm3 liquid scintillator volume and the reconstructed 3D dose map is compared along selected profiles and planes with radiochromic film and diode measurements. The comparison indicates that the 3DSD method agrees, within 25% for most points or within approximately 2 mm distance to agreement, with the relative radiochromic film and diode dose distributions in a small (approximately 4.5 mm high and approximately 12 mm diameter) volume in the unobstructed, high gradient dose region outside the edge of the plaque. For a comparison, the reproducibility of the radiochromic film results for our measurements ranges from 10 to 15% within this volume. At present, the 3DSD method is not accurate close to the edge of the plaque, and further than approximately 10 mm (<10% central axis depth dose) from the plaque surface. Improvement strategies, considered important to provide a more accurate quick check of the dose profiles in 3D for brachytherapy applicators, are discussed.

The use of low-energy photon emitters for brachytherapy applications, as in the treatment of prostate or ocular tumours, has increased significantly over the last few years. Several new seed models utilizing 103Pd and 125I have recently been introduced. Following the TG43U1 recommendations of the AAPM (American Association of Physicists in Medicine) (Rivard et al 2004 Med. Phys. 31 633), dose distributions around these low-energy photon emitters are characterized by the dose rate constant, the radial dose function and the anisotropy function in water. These functions and constants can be measured for each new seed in a solid phantom (i.e. solid water such as WT1) using high spatial resolution detectors such as very small thermoluminescent detectors. These experimental results in solid water must then be converted into liquid water by using Monte Carlo simulations. This paper presents the dosimetric parameters of a new palladium seed, OptiSeed (produced by International Brachytherapy (IBt), Seneffe, Belgium), made with a biocompatible polymeric shell and with a design that differs from the hollow titanium encapsulated seed, InterSource103, produced by the same company. A polymer encapsulation was chosen by the company IBt in order to reduce the quantity of radioactive material needed for a given dose rate, and to improve the symmetry of the radiation field around the seed. The necessary experimental data were obtained by measurements with LiF thermoluminescent dosimeters (1 mm3) in a solid water phantom (WT1) and then converted to values in liquid water using Monte Carlo calculations (MCNP-4C). Comparison of the results with a previous study by Reniers et al (2002 Appl. Radiat. Isot. 57 805) shows very good agreement for the dose rate constant and for the radial dose function. In addition, the results also indicate an improvement in isotropy compared to a conventional titanium encapsulated seed. The relative dose (anisotropy value relative to 90 degrees ) from the seed at a distance of 3 cm is close to 70% at 0 degrees whereas that for the titanium encapsulated InterSource103seed is close to 40%. This paper also presents some new Monte Carlo calculations relating to shadowing produced by the seeds in an array implanted for a prostate cancer treatment. Recently, Mobit and Badragan (2004 Phys. Med. Biol. 49 3171) reported shadowing resulting in a 10% decrease in dose from titanium encapsulated 125I seed. We used Monte Carlo simulations (MCNP-4C) to evaluate shadowing for the InterSource103 titanium encapsulated seed and the OptiSeed polymer encapsulated seed. For a specific geometry specified, dose decreases of 13% and 7% were found for the InterSource103 titanium encapsulated and the OptiSeed polymer encapsulated seed, respectively.

Knowing the location and the volume of the prostate is important for ultrasound-guided prostate brachytherapy, a commonly used prostate cancer treatment method. The prostate boundary must be segmented before a dose plan can be obtained. However, manual segmentation is arduous and time consuming. This paper introduces a semi-automatic segmentation algorithm based on the dyadic wavelet transform (DWT) and the discrete dynamic contour (DDC). A spline interpolation method is used to determine the initial contour based on four user-defined initial points. The DDC model then refines the initial contour based on the approximate coefficients and the wavelet coefficients generated using the DWT. The DDC model is executed under two settings. The coefficients used in these two settings are derived using smoothing functions with different sizes. A selection rule is used to choose the best contour based on the contours produced in these two settings. The accuracy of the final contour produced by the proposed algorithm is evaluated by comparing it with the manual contour outlined by an expert observer. A total of 114 2D TRUS images taken for six different patients scheduled for brachytherapy were segmented using the proposed algorithm. The average difference between the contour segmented using the proposed algorithm and the manually outlined contour is less than 3 pixels.

The dose distribution produced by the high-dose-rate (HDR) 192Ir source is inherently anisotropic due to self-absorption by the high-density source core, oblique filtration by the source capsule and the asymmetric geometry of the source capsule. To account for the dose distribution anisotropy of brachytherapy sources, AAPM Task Group No 43 has included a two-dimensional anisotropy function, F(r, theta), in the recommended dose calculation formalism. Gafchromic HS radiochromic film (RCF) was used to measure anisotropy function for microSelectron HDR 192Ir source (classic/old design). Measurements were carried out in a water phantom using specially fabricated PMMA cylinders at radial distances 1, 2, 3, 4 and 5 cm. The data so generated are comparable to both experimental and Monte Carlo calculated values for this source reported earlier by other authors. The RCF method described in this paper is comparatively high resolution, simple to use and is a general method, which can be applied for other brachytherapy sources as well.

In this paper, we examine the effect of treatment parameters in a model used to evaluate permanent prostate implants. The model considers the prostate to be composed of 12 sub-sections, each sub-section is assigned a cell density based on the probability of finding cancer foci in that sub-section. Wasted dose as a result of the dose rate from the implant falling below a level adequate to counteract repopulation was found to vary by 2-16% over the range of radiosensitivity and repopulation rates considered. Within the model, applied to five dose distributions, the uncertainty in the tumour control probability (TCP) values calculated for each sub-section as a result of differences in the model parameters, was found to be less than 12% in most cases for the good quality implants. The difference in TCP values was much larger for the poor quality implant. Substituting a heterogeneous distribution of alpha for a single mean value resulted in generally lower TCP values though introducing a cutoff value with a Gaussian distribution had a profound effect on the calculated values. Despite uncertainties in the parameters, the model was able to identify sub-sections at risk of local recurrence but as a result of these uncertainties, the TCP values can only be considered in the relative rather than absolute sense.

In order to perform intra-operative or post-implant dosimetry in prostate brachytherapy, the 3D coordinates of the implanted radioactive seeds must be determined. Film or fluoroscopy based seed reconstruction techniques use back projection of x-ray data obtained at two or three x-ray positions. These methods, however, do not perform well when some of the seed images are undetected. To overcome this problem we have developed an alternate technique for 3D seed localization using the principle of Hough transform. The Hough method utilizes the fact that, for each seed coordinate in three dimensions, there exists a unique trajectory in Hough feature space. In this paper we present the Hough transform parametric equations to describe the path of the seed projections from one view to the next and a method to reconstruct the 3D seed coordinates. The results of simulation and phantom studies indicate that the Hough trajectory method can accurately determine the 3D seed positions even from an incomplete dataset.

All international recommendations point out as necessary the calibration or verification of the reference air kerma rate (RAKR) for brachytherapy sources (independent of manufacturer established value) prior to their clinical use. The most common procedure for RAKR measurement in iridium wires is based on the use of well chambers with specific inserts that set the wire in a fixed position; previously, the electrometer plus well chamber with insert (EWI) was calibrated by using a source obtained from an accredited laboratory for which the RKAR was established precisely, called the 'reference' source. The distribution of Ir-192 material in the wire could be not perfectly homogeneous all along its length, and in this case the influence of these inhomogeneities in the calibration process should be studied. This paper focuses on the evaluation of this topic and an analytical and experimental study is presented taking into account the non-homogeneity of Ir-192 material along the wire for both the reference source (of length 14 cm) and a wire of unknown RAKR. This study is based on measurements with a 1 cm iridium wire on a rectilinear insert considering either of the two available reference sources (1 or 14 cm length), and has been experimentally evaluated using two typical well chambers. The main conclusion of the study is that if the non-homogeneity of the wires is lower than 5% the effect of non-homogeneity on RAKR measurements is negligible for rectilinear inserts even for short well chambers.

An automatic localization method of implanted seeds from a series of post-implant computed tomography (CT) images is described in this paper. Post-implant CT studies were obtained for patients who underwent prostate brachytherapy. Bright areas were segmented using binary thresholding in each CT slice, and geometrical information on these areas was collected. Large areas (possibly containing two connected seeds) were split into smaller ones by geometry-based filtering in each slice. The area connectivity along the longitudinal direction was analysed using a geometry-based connection search algorithm executed on every area slice by slice, so that the connected areas were combined into one object. The weighted centroid of each object was taken as the seed position. This method was tested on a seed-containing prostate phantom as well as using CT studies from patients. Statistical analysis demonstrates that it can achieve above 99% detection rate with reliable localization accuracy and high speed. It is reliable and convenient for localizing implanted seeds on CT and can be used to assist post-implant dosimetry for prostate brachytherapy.

This paper describes analytic tools in support of a paradigm shift in brachytherapy treatment planning for prostate cancer--a shift from standard pre-planning to intraoperative planning using dosimetric feedback based on the actual deposited seed positions within the prostate. The method proposed is guided by several desiderata: (a) bringing both planning and evaluation in the operating room (i.e. make post-implant evaluation superfluous) therefore making rectifications--if necessary--still achievable; (b) making planning and implant evaluation consistent by using the same imaging system (ultrasound); and (c) using only equipment commonly found in a hospital operating room. The intraoperative dosimetric evaluation is based on the fusion between ultrasound images and 3D seed coordinates reconstructed from fluoroscopic projections. Automatic seed detection and registration of the fluoroscopic and ultrasound information, two of the three key ingredients needed for the intraoperative dynamic dosimetry optimization (IDDO), are explained in detail. The third one, the reconstruction of 3D coordinates from projections, was reported in a previous article. The algorithms were validated using a custom-designed phantom with non-radioactive (dummy) seeds. Also, fluoroscopic images were taken at the conclusion of an actual permanent prostate implant and compared with data on the same patient obtained from radiographic-based post-implant evaluation. To offset the effect of organ motion the comparison was performed in terms of the proximity function of the two seed distributions. The agreement between the intra- and post-operative seed distributions was excellent.

A new 90Sr/90Y catheter-based system, the Beta-Cath 3.5F system, was recently introduced into the clinic for intravascular brachytherapy. This new system has a smaller diameter of delivery catheter (3.5 French) compared to the original Beta-Cath system (5 French catheter). The new source/seed (3.5F source) has a different design from the original (5F) source. In this paper, we report the EGSnrc Monte Carlo results of two-dimensional dose rate distributions in water as well as the TG-60 dose parameters for the new 3.5F source and the original 5F source. A complete set of the dosimetric data for each individual seed is tabulated. A detailed dosimetric comparison was carried out for the two sources. The comparison shows that differences between the two sources are less than 5% in the region of interest for intravascular brachytherapy. Higher differences (up to 10%) are seen in the near-source region, which is mainly inside the delivery catheter. Based on the present calculation and comparison, we conclude that the new Beta-Cath 3.5F system is dosimetrically equivalent to the original Beta-Cath (5F) system. Therefore, clinical dosimetry and the practice of using both the systems can be treated similarly.

Since the company MDS Nordion Haan GmbH introduced the new afterloading radiation unit GammaMed plus in 1998, new HDR Ir-192 sources with a diameter of 0.9 mm (formerly 1.1 mm) and a length of 4.5 mm (formerly 5.5 mm) have been used. With this equipment it is possible to treat peripheral vessels for prophylaxis of restenosis after percutaneous transluminal angioplasty (PTA). In this case a good knowledge of the dose characteristics in the near area is the prerequisite for safe application. In this study, the dose characteristics for this source type is investigated at close range by means of an ionization chamber. For the description of the dose characteristics, the radial dose distribution and the anisotropy function are used. The measurements were carried out using an ionization chamber, the so-called PinPoint chamber (Type PTW 31006, PTW Freiburg, Germany). The radial dose distribution was examined in the area of 0.26 to 12 cm. The anisotropy function was examined in the range from 0.5 to 5 cm (distance from the effective site of measurement to the source axis). The radial dose function was compared with the correction function k(as,w) (r), published in the German standard DIN 6809 part 2 (1991 Beuth Verlag, Berlin, Germany). In the area up to 4 cm (distance source-axis to measuring point) the deviation maximized up to 2%. The deviation climbs with greater distance from the source with results around 3%. The anisotropy function was compared with that used in the treatment planning system Abacus 3.0 of the same manufacturer. In the close range between 0.5 and 1 cm, there were some major variations (up to 10%) from the anisotropy function used in the planning system. The fault reduces itself with greater distance to under 2%. In view of the results presented in this paper further measurements of the anisotropy function in the close range from 0 to 1.5 cm from the source axis should be made. It can be seen that the function used in the planning system supplies results that are too high in comparison with the results of other authors. This concerns primarily the area of very small and very big angles.

The aim of this work was to investigate the applicability of MR-based polymer gel dosimetry to measure the absorbed dose distribution at short distance from an iridium-192 brachytherapy point source. In this paper, different methodological problems that may result in significant errors in the measured dose distribution are discussed. First of all the extent to which physicochemical mechanisms alter the dose response is discussed. The most important among these are the oxygen permeability of the catheter material and monomer-diffusion-related effects during irradiation. The effect of oxygen on the dose-R2 curve has been determined quantitatively and an oxygen map is performed using a well-defined external irradiation beam. The effect of diffusion of monomer during irradiation has been simulated. Another contribution of errors is related to magnetic susceptibility differences between the catheter and the gel during scanning the irradiated gel. The magnetic field distortion has been mapped by using both an experimental MRI technique and by simulation. Moreover, in constructing a dose-versus-distance curve by polar averaging, the sensitivity to the exact distance between source and point of measurement and to partial volume effects is illustrated. An optimization program is proposed to determine the location of the source on a sub-pixel scale.

In this paper, we present results from three different simulation models that are used to determine the dose distribution around a 188W/188Re coronary brachytherapy source with EGS4 Monte Carlo simulations. The three models are found to give similar results within 10%. Agreement was found with experimental data from measurements in a PMMA phantom. It has been shown that in the therapeutically relevant region the beta line source can be characterized by the radial depth-dose distribution in water.

The paper reports the experimental procedure adopted to determine the absorbed dose rate in water per reference air kerma rate, D(Kr), (d, theta), along the transverse bisector axis of a 137Cs brachytherapy source. The dose rate measurements have been carried out at difference distances, d, from the source using alanine dosemeters in a water phantom. The reference air kerma rate, Kr, was determined adopting a 'direct procedure' that uses a spherical ionization chamber in air. The dose rate constant of the source examined was D(Kr) (1, pi/2) = 0.99 +/- 0.03 cGy h(-1) (microGy h(-1))(-1). The values of the radial dose function along the transverse axis, g(d), determined with an uncertainty of 3.4% (1sigma), were found to be in good agreement with the results reported in the literature. The uncertainty in dose rate value has been estimated as 2.8% (1sigma) for distances from the source up to 7 cm. Kr has been determined with 1.2% (1sigma) uncertainty. So D(r) (d, pi/2) values were determined with 3% (1sigma) uncertainty.

This paper presents experimentally determined correction factors for Farmer-type chambers for absorbed dose determination in 60Co and 192Ir brachytherapy dosimetry. The correction factors were determined from measurements made in a PMMA phantom and calculation of ratios of measured charges. The ratios were corrected for the different volumes of the ionization chambers, determined in external high-energy electron beams. The correction factors for the central electrode effect and the wall material dependency in 60Co brachytherapy dosimetry agree with those used in external 60Co beam dosimetry. In 192Ir dosimetry, the central aluminium electrode increases the response of an NE2571 chamber compared with that of a chamber with a central graphite electrode. The increase is 1.1 and 2.1% at 1.5 and 5.0 g cm(-2) distance, respectively. Similar values are obtained with an NE2577 chamber. The wall correction factor in 192Ir dosimetry for a chamber with an A-150 wall has been determined to be 1.018, independent of the measurement distance. For a graphite walled chamber, the correction factor is 0.996 and 1.001 at 1.5 and 5.0 g cm(-2) distance, respectively. The values of the wall correction factors are evaluated by a theory presented. If the chamber is used according to the 'large cavity' principle, the correction factor to account for the replacement of the phantom material by the ionization chamber was determined to be 0.982 for an NE2571 chamber when used with a Delrin cap, and 0.978 for an NE2581 when used with a polystyrene cap. The correction factors for the 'large cavity' principle are valid at both 60Co and 192Ir qualities.

This paper presents two methods for absorbed dose determination with ionization chambers at short distance from 60Co and 192Ir brachytherapy sources. The methods are modifications of the Bragg-Gray and large cavity principles given in the IAEA code of practice for high- and medium-energy photon beams. A non-uniformity correction factor to account for the non-uniform electron fluence in the air cavity is introduced into the methods. The absorbed dose rates were determined from ionization chamber measurements at distances between 1.5 and 5.0 cm from the brachytherapy sources. The agreement between the two methods is excellent in 60Co brachytherapy dosimetry. For 192Ir dosimetry, the difference is less than 2.5% at all distances. In absorbed dose rate calculations with the 60Co brachytherapy source, the ratios between calculated and experimentally determined absorbed dose rates are 0.987 and 0.994 depending on the method used for absorption and scatter correction. In 192Ir dosimetry, the large cavity principle gives almost identical values to those which can be obtained with the AAPM recommendations. Using the chambers according to the Bragg-Gray principle in 192Ir dosimetry, the agreement with AAPM calculated absorbed dose rates is within 2.5% at all distances. The uncertainty, expressed as one standard deviation, in the experimentally determined absorbed dose is estimated to be between 3 and 4%.

Source anisotropy is a very important factor in the brachytherapy quality assurance of high-dose rate (HDR) 192Ir afterloading stepping sources. If anisotropy is not taken into account then doses received by a brachytherapy patient in certain directions can be in error by a clinically significant amount. Experimental measurements of anisotropy are very labour intensive. We have shown that within acceptable limits of accuracy, Monte Carlo integration (MCI) of a modified Sievert integral (3D generalization) can provide the necessary data within a much shorter time scale than can experiments. Hence MCI can be used for routine quality assurance schedules whenever a new design of HDR or PDR 192Ir is used for brachytherapy afterloading. Our MCI calculation results are compared with published experimental data and Monte Carlo simulation data for microSelectron and VariSource 192Ir sources. We have shown not only that MCI offers advantages over alternative numerical integration methods, but also that treating filtration coefficients as radial distance-dependent functions improves Sievert integral accuracy at low energies. This paper also provides anisotropy data for three new 192Ir sources, one for the microSelectron-HDR and two for the microSelectron-PDR, for which data are currently not available. The information we have obtained in this study can be incorporated into clinical practice.

The Associazione Italianà di Fisica Biomedica (AIFB; Italian Association of Biomedical Physics) has adopted the Italian protocol for the calibration of brachytherapy sources. The AIFB protocol allows measurements of the reference air kerma rate, Kr, within 1.7% (1 sigma). To measure Kr the AIFB protocol has identified a direct and an indirect procedure. The direct procedure is based on the use of spherical or cylindrical ionization chambers as local reference dosimeters positioned along the transverse bisector axis of the source. Once the source is specified by a Kr value, this can be used to calibrate a field instrument, such as a well-type ionization chamber, for further source calibrations by means of an indirect procedure. This paper reports the results obtained by the Physics Laboratory of the Università Cattolica del S Cuore (PL-UCSC), in terms of Kr calibration of five types of source, 169Yb, 192Ir and 137Cs. The role of the Kr determination for a brachytherapy source has been underlined when a new source such as the 169Yb seed model X1267 has been proposed for clinical use. The Kr values for 137Cs spherical sources differed by 5% from the vendor's mean value. The five types of source calibrated in terms of Kr were used to obtain the calibration factor. NKrsource, of an HDR-1000 well-type ionization chamber.

In ionization chamber dosimetry close to brachytherapy sources, the non-uniform photon fluence in the vicinity of the air cavity of the ionization chamber must be corrected for. In this paper, experimentally determined relative non-uniformity correction factors are determined for a PTW23331 chamber and a number of Farmer-type chambers. It is concluded that the 'anisotropic' theory of Bielajew agrees better with the experimental results than the 'isotropic' theory of Kondo and Randolph. The experiments show that neither the material choice nor the radius of the central electrode have any significant effect on the non-uniformity correction factors. Experiments on the wall-material dependence in the non-uniformity correction factor, which is predicted by the anisotropic theory, were inconclusive. Parameters for the Farmer-type chambers for calculation of theoretical non-uniformity correction factors are given. Data to derive anisotropic fluence non-uniformity correction factors are given in tabular form. An error in the Kondo and Randolph equations is reported and the corrected equations presented.

Therapy with intraarterial microspheres is a technique which involves incorporation of radioisotope-labelled microspheres into a capillary bed of tumour and normal tissue. Betaemitters such as 90Y and 166Ho are used for this purpose. This technique provides tumour to normal tissue (TNT) dose ratios in the range of 2-10 and demonstrates significant clinical benefit, which could potentially be increased with more accurate dose predictions and delivery. However, dose calculations in this modality face the difficulties associated with nonuniform and inhomogeneous activity distribution. Most of the dose calculations used clinically do not account for the nonuniformity and assume uniform activity distribution. This paper is devoted to the development of a model which would allow more accurate prediction of dose distributions from microspheres. The model calculates dose assuming that microspheres are aggregated into randomly distributed clusters, and using precomputed dose kernels for the clusters. The dose kernel due to a microsphere cluster was found by numerical integration of a point source dose kernel over the volume of the cluster. It is shown that a random distribution of clusters produces an intercluster distance distribution which agrees well with the one measured by Pillai et al in liver. Dose volume histograms (DVHs) predicted by the model agree closely with the results of Roberson et al for normal tissue and tumour. Dose distributions for different concentrations and types of radioisotope as well as for tumours of different radii, have been calculated to demonstrate the model's possible applications.