Permanent prostate seed implant brachytherapy: report of the American Association of Physicists in Medicine Task Group No. 64

Clinical practice and quality assurance challenges in modern brachytherapy sources and dosimetry

Modern brachytherapy has led to effective treatments through the establishment of broadly applicable dosimetric thresholds for maximizing survival with minimal morbidity. Proper implementation of recent dosimetric consensus statements and quality assurance procedures is necessary to maintain the established level of safety and efficacy. This review classifies issues as either "systematic" or "stochastic" in terms of their impact on large groups or individual patients, respectively. Systematic changes affecting large numbers of patients occur infrequently and include changes in source dosimetric parameters, prescribing practice, dose calculation formalism, and improvements in calculation algorithms. The physicist must be aware of how incipient changes accord with previous experience. Stochastic issues involve procedures that are applied to each patient individually. Although ample guidance for quality assurance of brachytherapy sources exists, some ambiguities remain. The latest American Association of Physicists in Medicine guidance clarifies what is meant by independent assay, changes source sampling recommendations, particularly for sources in sterile strands and sterile preassembled needles, and modifies action level thresholds. The changing environment of brachytherapy has not changed the fact that the prime responsibility for quality assurance in brachytherapy lies with the institutional medical physicist.

On the use of Kodak CR film for quality assurance of needle loading in I-125 seed prostate brachytherapy

Low dose rate brachytherapy using implanted I-125 seeds as a monotherapy for prostate cancer is now in use in many hospitals. In contrast to fractionated brachytherapy treatments, where the effect of incorrect positioning of the source in one treatment fraction can be diminished by correcting the position in subsequent fractions, the I-125 seed implant is permanent, making correct positioning of the seeds in the prostate essential. The seeds are inserted into the prostate using needles. Correct configuration of seeds in the needles is essential in order to deliver the planned treatment. A comparison of an autoradiograph obtained by exposing film to the seed-loaded needles with the patient treatment plan is a valuable quality assurance tool. However, the time required to sufficiently expose Kodak XOMAT V film, currently used in this department is significant. This technical note presents the use of Kodak CR film for acquisition of the radiograph. The digital radiograph can be acquired significantly faster, has superior signal-to-noise ratio and contrast and has the usual benefits of digital film, e.g. a processing time which is shorter than that required for non-digital film, the possibility of image manipulation, possibility of paper printing and electronic storage.

Observations on rotating needle insertions using a brachytherapy robot

A robot designed for prostate brachytherapy implantations has the potential to greatly improve treatment success. Much of the research in robotic surgery focuses on measuring accuracy. However, there exist many factors that must be optimized before an analysis of needle placement accuracy can be determined. Some of these parameters include choice of the needle type, insertion velocity, usefulness of the rotating needle and rotation speed. These parameters may affect the force at which the needle interacts with the tissue. A reduction in force has been shown to decrease the compression of the prostate and potentially increase the accuracy of seed position. Rotating the needle as it is inserted may reduce frictional forces while increasing accuracy. However, needle rotations are considered to increase tissue damage due to the drilling nature of the insertion. We explore many of the factors involved in optimizing a brachytherapy robot, and the potential effects each parameter may have on the procedure. We also investigate the interaction of rotating needles in gel and suggest the rotate-cannula-only method of conical needle insertion to minimize any tissue damage while still maintaining the benefits of reduced force and increased accuracy.

Brachytherapy scatter dose calculation in heterogeneous media: I. A microbeam ray-tracing method for the single-scatter contribution

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.

Brachytherapy seed localization using geometric and linear programming techniques

We propose an optimization algorithm to solve the brachytherapy seed localization problem in prostate brachytherapy. Our algorithm is based on novel geometric approaches to exploit the special structure of the problem and relies on a number of key observations which help us formulate the optimization problem as a minimization integer program (IP). Our IP model precisely defines the feasibility polyhedron for this problem using a polynomial number of half-spaces; the solution to its corresponding linear program is rounded to yield an integral solution to our task of determining correspondences between seeds in multiple projection images. The algorithm is efficient in theory as well as in practice and performs well on simulation data (approximately 98% accuracy) and real X-ray images (approximately 95% accuracy). We present in detail the underlying ideas and an extensive set of performance evaluations based on our implementation.

Postimplant Dosimetry Using a Monte Carlo Dose Calculation Engine: A New Clinical Standard

PURPOSE

To use the Monte Carlo (MC) method as a dose calculation engine for postimplant dosimetry. To compare the results with clinically approved data for a sample of 28 patients. Two effects not taken into account by the clinical calculation, interseed attenuation and tissue composition, are being specifically investigated.

METHODS AND MATERIALS

An automated MC program was developed. The dose distributions were calculated for the target volume and organs at risk (OAR) for 28 patients. Additional MC techniques were developed to focus specifically on the interseed attenuation and tissue effects.

RESULTS

For the clinical target volume (CTV) D(90) parameter, the mean difference between the clinical technique and the complete MC method is 10.7 Gy, with cases reaching up to 17 Gy. For all cases, the clinical technique overestimates the deposited dose in the CTV. This overestimation is mainly from a combination of two effects: the interseed attenuation (average, 6.8 Gy) and tissue composition (average, 4.1 Gy). The deposited dose in the OARs is also overestimated in the clinical calculation.

CONCLUSIONS

The clinical technique systematically overestimates the deposited dose in the prostate and in the OARs. To reduce this systematic inaccuracy, the MC method should be considered in establishing a new standard for clinical postimplant dosimetry and dose-outcome studies in a near future.

Tumour and target volumes in permanent prostate brachytherapy: A supplement to the ESTRO/EAU/EORTC recommendations on prostate brachytherapy

The aim of this paper is to supplement the GEC/ESTRO/EAU recommendations for permanent seed implantations in prostate cancer to develop consistency in target and volume definition for permanent seed prostate brachytherapy. Recommendations on target and organ at risk (OAR) definitions and dosimetry parameters to be reported on post implant planning are given.

A greedy heuristic using adjoint functions for the optimization of seed and needle configurations in prostate seed implant

We continue our work on the development of an efficient treatment-planning algorithm for prostate seed implants by incorporation of an automated seed and needle configuration routine. The treatment-planning algorithm is based on region of interest (ROI) adjoint functions and a greedy heuristic. As defined in this work, the adjoint function of an ROI is the sensitivity of the average dose in the ROI to a unit-strength brachytherapy source at any seed position. The greedy heuristic uses a ratio of target and critical structure adjoint functions to rank seed positions according to their ability to irradiate the target ROI while sparing critical structure ROIs. Because seed positions are ranked in advance and because the greedy heuristic does not modify previously selected seed positions, the greedy heuristic constructs a complete seed configuration quickly. Isodose surface constraints determine the search space and the needle constraint limits the number of needles. This study additionally includes a methodology that scans possible combinations of these constraint values automatically. This automated selection scheme saves the user the effort of manually searching constraint values. With this method, clinically acceptable treatment plans are obtained in less than 2 min. For comparison, the branch-and-bound method used to solve a mixed integer-programming model took close to 2.5 h to arrive at a feasible solution. Both methods achieved good treatment plans, but the speedup provided by the greedy heuristic was a factor of approximately 100. This attribute makes this algorithm suitable for intra-operative real-time treatment planning.

Multi-species prostate implant treatment plans incorporating Ir192 and I125 using a Greedy Heuristic based 3D optimization algorithm

The goals of interstitial implant brachytherapy include delivery of the target dose in a uniform manner while sparing sensitive structures, and minimizing the number of needles and sources. We investigated the use of a multi-species source arrangement (192Ir with 125I) for treatment in interstitial prostate brachytherapy. The algorithm utilizes an "adjoint ratio," which provides a means of ranking source positions and is the criterion for the Greedy Heuristic optimization. Three cases were compared, each using 0.4 mCi 125I seeds: case I is the base case using 125I alone, case II uses 0.12 mCi 192Ir seeds mixed with 125I, and case III uses 0.25 mCi 192Ir mixed with 125I. Both multi-species cases result in lower exposure of the urethra and central prostate region. Compared with the base case, the exposure to the rectum and normal tissue increases by a significant amount for case III as compared with the increase in case II, signifying the effect of slower dose falloff rate of higher energy gammas of 192Ir in the tissue. The number of seeds and needles decreases in both multi-species cases, with case III requiring fewer seeds and needles than case II. Further, the effect of 192Ir on uniformity was investigated using the 0.12 mCi 192Ir seeds in multi-species implants. An increase in uniformity was observed with an increase in the number of 0.12 mCi 1921r seeds implanted. The effects of prostate size on the evaluation parameters for multi-species implants were investigated using 0.12 mCi 192Ir and 0.4 mCi 125I, and an acceptable treatment plan with increased uniformity was obtained.

Présent et avenir de la radiothérapie guidée par l'image (IGRT) et ses applications possibles dans le traitement des cancers bronchiques

These last years, the new irradiation techniques as the conformal 3D radiotherapy and the IMRT are strongly correlated with the technological developments in radiotherapy. The rigorous definition of the target volume and the organs at risk required by these irradiation techniques, imposed the development of various image guided patient positioning and target tracking techniques. The availability of these imaging systems inside the treatment room has lead to the exploration of performing real-time adaptive radiation therapy. In this paper we present the different image guided radiotherapy (IGRT) techniques and the adaptive radiotherapy (ART) approaches. IGRT developments are focused in the following areas: 1) biological imaging for better definition of tumor volume; 2) 4D imaging for modeling the intra-fraction organ motion; 3) on-board imaging system or imaging devices registered to the treatment machines for inter-fraction patient localization; and 4) treatment planning and delivery schemes incorporating the information derived from the new imaging techniques. As this paper is included in the "Cancer-Radiotherapie" special volume dedicated to the lung cancers, in the description of the different IGRT techniques we try to present the lung tumors applications when this is possible.

Dosimetric consideration of individual 125I source strength measurement and a large-scale comparison of that measured with a nominal value in permanent prostate implant brachytherapy

PURPOSE

We investigated the difference between measured and manufacturer's nominal source strength in a large sample of a single model of (125)I seeds. Physical characteristics of single seed measurement by the well-type ionization chamber were also investigated to provide dosimetric data.

MATERIALS AND METHODS

A well-type ionization chamber with a single seed holder was used to measure source strength of all 1935 (125)I seeds implanted in the initial 28 patients in our hospital. Physical characteristics including linearity of readings for different integral time intervals, reproducibility, isotropy, and axial positional sensitivity were assessed. To calculate the source strength, the integral charge during 30 s was measured and converted to air kerma strength. The nominal activity stated by the manufacturer was compared with the measured value.

RESULTS

Linearity, reproducibility, and isotropy of the well-type ionization chamber were within 0.2%. Measured source strength was on average 2.1% (range -7.6% to +7.2%), lower than the nominal value. Standard deviation of all measured seeds was 2.0%. The maximum difference between the measured and the manufacturer's nominal source strength in each patient was -3.7%. The standard deviation averaged 1.6%.

CONCLUSION

The nominal source strength of the (125)I seeds agreed well with the measured value. Our study can be helpful as guidance for individual (125)I seed source strength measurement.

Overview of image-guided radiation therapy

Radiation therapy has gone through a series of revolutions in the last few decades and it is now possible to produce highly conformal radiation dose distribution by using techniques such as intensity-modulated radiation therapy (IMRT). The improved dose conformity and steep dose gradients have necessitated enhanced patient localization and beam targeting techniques for radiotherapy treatments. Components affecting the reproducibility of target position during and between subsequent fractions of radiation therapy include the displacement of internal organs between fractions and internal organ motion within a fraction. Image-guided radiation therapy (IGRT) uses advanced imaging technology to better define the tumor target and is the key to reducing and ultimately eliminating the uncertainties. The purpose of this article is to summarize recent advancements in IGRT and discussed various practical issues related to the implementation of the new imaging techniques available to radiation oncology community. We introduce various new IGRT concepts and approaches, and hope to provide the reader with a comprehensive understanding of the emerging clinical IGRT technologies. Some important research topics will also be addressed.

Innovative Technologies in Radiation Therapy: Brachytherapy

Brachytherapy has changed dramatically in the last decade, with the widespread introduction of high-dose rate afterloading systems having greater flexibility of source loading patterns and smaller sources enabling new anatomic sites to be considered. This has been harnessed to the major developments in cross-sectional imaging and dosimetry planning systems to enable highly conformal radiotherapy to be delivered accurately and reliably by brachytherapy. There has been a major change in the distribution of sites treated, the majority being prostate, gynecologic, and breast treatments, whereas an emerging role as a simple but effective palliative treatment in bronchus, esophagus, and rectal cancers has also been recognized.

Calibration of multiple LDR brachytherapy sources

A trend is underway toward the use of prepackaged low dose rate brachytherapy sources, which come in the form of strands, coiled line sources, preloaded needles, and sterile cartridge packs. Since the medical physicist is responsible for verification of source strength prior to patient treatment, development of prepackaged source strength verification methods is needed. Existing guidelines are reviewed to establish the situation that medical physicists find with respect to prepackaged sources. This investigation presents an experimental evaluation of the effect of some of these multiseed geometries on source strength measurements. Multiseed strands and coils, whether 125I, 103Pd, or 192Ir can be measured in a chamber with a long, sensitive axial length with a uniform response. Sterile seed cartridge packs can also be measured but require a correction factor to be applied. Sources in needles, however, cannot be measured in the needle since there is too great a variation in needle composition and needle tolerance thickness. Removing these seeds from the needle into a sterile measurement insert, which maintains sterility is a practical source strength verification method, similar to those done for multiple seed configurations in a well chamber with adequate axial uniformity. Values are compared with individual air kerma strength calibrations, and correction factors, are presented' where needed. In each case, care must be taken to maintain sterility as multiple seeds are measured in well chamber inserts.

Impact of interseed attenuation and tissue composition for permanent prostate implants

The purpose is to evaluate the impact of interseed attenuation and prostate composition for prostate treatment plans with 125I permanent seed implants using the Monte Carlo (MC) method. The effect of seed density (number of seeds per prostate unit volume) is specifically investigated. The study focuses on treatment plans that were generated for clinical cases. For each plan, four different dose calculation techniques are compared: TG-43 based calculation, superposition MC, full MC with water prostate, and full MC with realistic prostate tissue. The prostate tissue description is from the ICRP report 23 (W. S. Snyer, M. J. Cook, E. S. Nasset, L. R. Karkhausen, G. P. Howells, and I. H. Tipton, "Report of the task group on reference man," Technical Report 23, International Commission on Radiological Protection, 1974). According to the comparisons, the seed density has an influence on interseed attenuation. A plan with a typical low seed density (42 0.6 mCi seeds in a 26 cm3 prostate) suffers a 1.2% drop in the CTV D90 value due to interseed attenuation. A drop of 3.0% is calculated for a higher seed density (75 0.3 mCi seeds, same prostate). The influence of the prostate composition is similar for all seed densities and prostate sizes. The difference between MC simulations in water and MC simulations in prostate tissue is between 4.4% and 4.8% for the D90 parameter. Overall, the effect on D90 is ranging from 5.8% to 12.8% when comparing clinically approved TG-43 and MC simulations in prostate tissue. The impact varies from one patient to the other and depends on the prostate size and the number of seeds. This effect can reach a significant level when reporting correlations between clinical effect and deposited dose.

Feasibility study of using MOSFET detectors for in vivo dosimetry during permanent low-dose-rate prostate implants

PURPOSE

To investigate the feasibility of using new micro-MOSFET detectors for QA and in vivo dosimetry of the urethra during transperineal interstitial permanent prostate implants (TIPPB).

METHODS AND MATERIALS

This study involves measurements for several patients who have undergone the implant procedure with iodine-125 seeds. A new micro-MOSFET detector is used as a tool for in vivo measurement of the initial dose rate within the urethra. MOSFETs are calibrated using a single special order calibration seed. The angular response is investigated in a 100 kVp X-ray beam.

RESULTS

micro-MOSFETs are found to have a calibration factor of 0.03 cGy/mV for low energy X-rays and a high isotropic response (within 2.5%). Prostate volume and shape changes during TIPPB due to edema caused by the trauma of needle insertion, making it difficult to achieve the planned implant geometry and hence the desired dose distribution. MOSFET measurements help us to evaluate the overall quality of the implant, by analyzing the maximum dose received by urethra, the prostate base coverage, the length of the prostatic urethra that is irradiated, and the apex coverage.

CONCLUSIONS

We demonstrate that ease of use, quick calibration and the instantaneous reading of accumulated dose make micro-MOSFETs feasible for in vivo dosimetry during TIPPB.

Image-Guided Brachytherapy for Prostate Cancer

Prostate brachytherapy offers and effective treatment for organ-confined prostate carcinoma. It is rapidly delivered compared with external beam radiation therapy or surgery and well tolerated by patients. Volumetric imaging and image guidance play critical roles in patient selection, treatment planning, treatment delivery, and postimplant assessment. Costs, availability and ease of use often dictate the local and regional differences in imaging approach, whether ultrasound, CT, or MR. Future volumetric image developments may permit multimodality image fusion to integrate tumor-specific imaging such as MR spectroscopy or positron emission tomography/CT into real-time ultrasound, CT, or MR.

Dosimetry of a thyroid uptake detected in seed migration survey following a patient's iodine-125 prostate implant and in vitro measurements of intentional seed leakages

As a quality control procedure, a post-implant seed migration survey has been accomplished on 340 prostate cancer patients since November 2001. Pulmonary seed embolization and intracardiac seed embolization have been detected. A case of thyroid uptake due to leaking iodine-125 (I-125) sources was also seized. In order to determine the dose to the thyroid, a dosimetry method was developed to link in vivo measurements and the cumulated dose to the thyroid. The calculated source leakage half-life in the case was approximately 15 days based on the measurements and the estimated cumulated dose to thyroid was 204 cGy. It is concluded that one seed was leaking. In order to verify the in vivo measurements, intentional in vitro seed leakage tests were performed. A seed was cut open and placed in a sealed glass container filled with a given volume of saline. The I-125 concentration in the saline was subsequently measured over a period of six months. Consistent in vivo and in vitro results were obtained. Recent incidents of seed leaks reported from other centers have drawn practitioners' attention to this problem. In order to make the measurements more useful, the seed leakage tests were expanded to include I-125 seeds from six other vendors. The results show that the leakage half-lives of those seeds varied from nine days to a half-year. Two seed models demonstrated least leakage. Since the measurements lasted for six months, the escape of iodine resulted from oxidation of iodide in the saline was a concern for the measurement accuracy. As a reference, another set of leakage tests were performed by adding sodium thiosulfate salt (Na2S2O3 x 5 H2O) to the saline. Sodium thiosulfate is a reducing agent that prevents the conversion of iodide to iodate so as to minimize I-125 evaporation. As a result, significantly shortened leakage half-lives were observed in this group. Seed agitation was also performed and no significant deviations of the leakage rates were observed. Considering the body fluid is more complex than saline, the in vivo leakage half-life, in case a source leak is encountered, may vary significantly from what is presented in this paper due to chemical reactions. In vivo measurements thus may produce a more accurate estimation of leakage half-life and thyroid uptake dose.

Class solution for inversely planned permanent prostate implants to mimic an experienced dosimetrist

The purpose of this paper is to present a method for the selection of inverse planning parameters and to establish a set of inverse planning parameters (class solution) for the inverse planning included in a commercial permanent prostate implant treatment planning system. The manual planning of more than 750 patients since 1996 led to the establishment of general treatment planning rules. A class solution is tuned to fulfill the treatment planning rules and generate equivalent implants. For ten patients, the inverse planning is compared with manual planning performed by our experienced physicist. The prostate volumes ranged from 17 to 51 cc and are implanted with low activity 1-125 seeds. Dosimetric indices are calculated for comparison. The inverse planning needed about 15 s for each optimization (400 000 iterations on a 2.5 GHz PC). In comparison, the physicist needed about 20 min to perform each manual plan. A class solution is found that consistently produces dosimetric indices equivalent or better than the manual planning. Moreover, even with strict seed placement rules, the inverse planning can produce adequate prostate dose coverage and organ at risk protection. The inverse planning avoids implant with seeds outside of the prostate and too close to the urethra. It also avoids needles with only one seed and needles with three consecutive seeds. This reduces the risk of complication due to seed misplacement and edema. The inverse planning also uses a smaller number of needles, reducing the cause of trauma. The quality of the treatment plans is independent of the gland size and shape. A class solution is established that consistently and rapidly produces equivalent dosimetric indices as manual planning while respecting severe seed placement rules. The class solution can be used as a starting point for every patient, dramatically reducing the time needed to plan individual patient treatments. The class solution works with inverse preplanning, intraoperative inverse preplanning, and intraoperative real-time planning. This technology is not intended to replace the physicist but to accelerate the planning process, making intraoperative treatment planning more effective.

Robotically assisted prostate brachytherapy with transrectal ultrasound guidance--Phantom experiments

PURPOSE

To report the preliminary experimental results obtained with a robot-assisted transrectal ultrasound (TRUS)-guided prostate brachytherapy system.

METHODS AND MATERIALS

The system consists of a TRUS unit, a spatially coregistered needle insertion robot, and an FDA-approved treatment planning and image-registered implant system. The robot receives each entry/target coordinate pair of the implant plan, inserts a preloaded needle, and then the seeds are deposited. The needles/sources are tracked in TRUS, thus allowing the plan to be updated as the procedure progresses.

RESULTS

The first insertion attempt was recorded for each needle, without adjustment. All clinically relevant locations were reached in a prostate phantom. Nonparallel and parallel needle trajectories were demonstrated. Based on TRUS, the average transverse placement error was 2 mm (worst case 2.5 mm, 80% less than 2 mm), and the average sagittal error was 2.5 mm (worst case 5.0 mm, 70% less than 2.5 mm).

CONCLUSIONS

The concept and technical viability of robot-assisted brachytherapy were demonstrated in phantoms. The kinematically decoupled robotic assistant device is inherently safe. Overall performance was promising, but further optimization is necessary to prove the possibility of improved dosimetry.

A study of a pretreatment method to predict the number of I-125 seeds required for prostate brachytherapy

PURPOSE

Prediction of the number of iodine seeds (I-125) required for prostate implantation is an important tool to reduce the number of unused seeds for brachytherapy. This study was designed to investigate the relationship between the number of seeds implanted vs. prostate volume. This can produce a tool to accurately estimate the number of seeds required for a given target volume. In addition, total cost of treatment, personal radiation risks during storage and handling, and errors in accounting for seeds can be reduced.

METHODS AND MATERIALS

Data from two groups of patients who had I-125 seed prostate implants (Oncura/Amersham RAPIDStrand model 6711 I-125) have been separately analyzed: (A) The relationship between prostate volume vs. number of seeds implanted was based on 401 patients treated between 1999 and 2002 who were implanted with seeds of air kerma strength (AKS) of 0.459 microGy h(-1) @ 1 m per seed. (B) The relationship between prostate volume vs. total seed AKS was analyzed. This was based on 628 patients treated between 1999 and 2002 who were implanted with a range of seed strengths from 0.381 to 0.521 U. Both patient groups were subdivided into integer prostate volume bins. For each bin, the mean and 95% confidence intervals (CI) for the implanted number of seeds or total AKS implanted were calculated. The upper 95% CI was used to investigate the relationship between the number of seeds implanted and total AKS implanted vs. prostate volume.

RESULTS

The new method of predicting the number of seeds shows valid and accurate results. The required number of seeds can be predicted, which helps to reduce the number of leftover seeds to 3% of the total number of seeds ordered.

CONCLUSION

The number of I-125 seeds or the total activity that is required to deliver the prescribed dose for the target volume can be predicted. This could reduce the overall treatment cost by accurate seed ordering before implantation.

Accuracy of seed reconstruction in prostate postplanning studied with a CT- and MRI-compatible phantom

BACKGROUND AND PURPOSE

Postimplant dosimetry of prostate seed implants is usually performed by seed localisation on transversal CT or MR images. In order to obtain reliable dosimetric evaluation data, it is important that seeds are reconstructed accurately. Currently, there is no comparative data available on seed localisation accuracy of CT-and MRI-based reconstructions, mainly due to the lack of a suitable QA tool. In this study, we developed a CT-and MRI compatible prostate phantom to investigate the intrinsic accuracy of seed detection for both imaging modalities.

PATIENTS AND METHODS

A 60 seed geometry was created according to a clinically meaningful plan, including rotated and shifted seeds. After implantation of the seeds in the phantom, CT and MRI scans with 3, 4 and 5mm slice thickness were performed. The seed locations were reconstructed in the treatment planning system and compared with the known reference positions.

RESULTS

Due to the comparable density and relaxation times of the phantom material to prostate tissue, the seeds are visualised similarly as on real patient images. The observed mean reconstruction uncertainties were in general smaller for CT (0.9+/-0.6, 0.9+/-0.6, 2.1+/-0.8 mm on 3, 4 and 5mm scans, respectively), than for MRI (Philips 1.5 T: 2.1+/-1.4, 1.6+/-1.2, 1.9+/-0.9 mm on 3, 4 and 5 mm scans, respectively, and Siemens 1.5 T: 2.3+/-0.8, 2.0+/-1.6, 1.6+/-0.8 mm on 3, 4 and 5mm scans, respectively).

CONCLUSIONS

For our clinical sequences of both CT and MRI, the mean deviation of the reconstructed seed positions were all within acceptable limits for clinical use (<2.3 mm). The phantom was found to be a suitable quality assurance tool to assess the reliability and accuracy of the seed reconstruction procedure. Moreover, as the phantom material has the same imaging characteristics as real prostate tissue, it is a useful device to define proper MRI sequences.

Prostate segmentation by feature enhancement using domain knowledge and adaptive region based operations

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.

Interstitial brachytherapy dosimetry update

In March 2004, the American Association of Physicists in Medicine (AAPM) published an update to the AAPM Task Group No. 43 Report (TG-43) which was initially published in 1995. This update was pursued primarily due to the marked increase in permanent implantation of low-energy photon-emitting brachytherapy sources in the United States over the past decade, and clinical rationale for the need of accurate dosimetry in the implementation of interstitial brachytherapy. Additionally, there were substantial improvements in the brachytherapy dosimetry formalism, accuracy of related parameters and methods for determining these parameters. With salient background, these improvements are discussed in the context of radiation dosimetry. As an example, the impact of this update on the administered dose is assessed for the model 200 (103)Pd brachytherapy source.

AAPM Task Group 103 report on peer review in clinical radiation oncology physics

This report provides guidelines for a peer review process between two clinical radiation oncology physicists. While the Task Group's work was primarily focused on ensuring timely and productive independent reviews for physicists in solo practice, these guidelines may also be appropriate for physicists in a group setting, particularly when dispersed over multiple separate clinic locations. To ensure that such reviews enable a collegial exchange of professional ideas and productive critique of the entire clinical physics program, the reviews should not be used as an employee evaluation instrument by the employer. Such use is neither intended nor supported by this Task Group. Detailed guidelines are presented on the minimum content of such reviews, as well as a recommended format for reporting the findings of a review. In consideration of the full schedules faced by most clinical physicists, the process outlined herein was designed to be completed in one working day.

MCPI©: A sub-minute Monte Carlo dose calculation engine for prostate implants

An accelerated Monte Carlo code [Monte Carlo dose calculation for prostate implant (MCPI)] is developed for dose calculation in prostate brachytherapy. MCPI physically simulates a set of radioactive seeds with arbitrary positions and orientations, merged in a three-dimensional (3D) heterogeneous phantom representing the prostate and surrounding tissue. MCPI uses a phase space data source-model to account for seed self-absorption and seed anisotropy. A "hybrid geometry" model (full 3D seed geometry merged in 3D mesh of voxels) is used for rigorous treatment of the interseed attenuation and tissue heterogeneity effects. MCPI is benchmarked against the MCNP5 code for idealized and real implants, for 103Pd and 125I seeds. MCPI calculates the dose distribution (2-mm voxel mesh) of a 103Pd implant (83 seeds) with 2% average statistical uncertainty in 59 s using a single Pentium 4 PC (2.4 GHz). MCPI is more than 10(3) and 10(4) times faster than MCNP5 for prostate dose calculations using 2- and 1-mm voxels, respectively. To illustrate its usefulness, MCPI is used to quantify the dosimetric effects of interseed attenuation, tissue composition, and tissue calcifications. Ignoring the interseed attenuation effect or slightly varying the prostate tissue composition may lead to 6% decreases of D100, the dose delivered to 100% of the prostate. The presence of calcifications, covering 1%-5% of the prostate volume, decreases D80, D90, and D100 by up to 32%, 37%, and 58%, respectively. In conclusion, sub-minute dose calculations, taking into account all dosimetric effects, are now possible for more accurate dose planning and dose assessment in prostate brachytherapy.

A three-dimensional computed tomography–assisted Monte Carlo evaluation of ovoid shielding on the dose to the bladder and rectum in intracavitary radiotherapy for cervical cancer

PURPOSE

To determine the effects of Fletcher Suit Delclos ovoid shielding on dose to the bladder and rectum during intracavitary radiotherapy for cervical cancer.

METHODS AND MATERIALS

The Monte Carlo method was used to calculate the dose in 12 patients receiving low-dose-rate intracavitary radiotherapy with both shielded and unshielded ovoids. Cumulative dose-difference surface histograms were computed for the bladder and rectum. Doses to the 2-cm(3) and 5-cm(3) volumes of highest dose were computed for the bladder and rectum with and without shielding.

RESULTS

Shielding affected dose to the 2-cm(3) and 5-cm(3) volumes of highest dose for the rectum (10.1% and 11.1% differences, respectively). Shielding did not have a major impact on the dose to the 2-cm(3) and 5-cm(3) volumes of highest dose for the bladder. The average dose reduction to 5% of the surface area of the bladder was 53 cGy. Reductions as large as 150 cGy were observed to 5% of the surface area of the bladder. The average dose reduction to 5% of the surface area of the rectum was 195 cGy. Reductions as large as 405 cGy were observed to 5% of the surface area of the rectum.

CONCLUSIONS

Our data suggest that the ovoid shields can greatly reduce the radiation dose delivered to the rectum. We did not find the same degree of effect on the dose to the bladder. To calculate the dose accurately, however, the ovoid shields must be included in the dose model.

Treatment planning consideration for prostate implants with the new linear RadioCoil 103Pd brachytherapy source

Recently, various linear source models, for example, 103Pd RadioCoil, have been introduced to overcome the shortcomings of traditional "seed" type interstitial prostate brachytherapy implants, such as migration and clumping of the seeds. However, the existing prostate treatment-planning systems have not been updated to perform dose calculation for implants with linear sources greater than 1.0 cm in length. In these investigations, two new models are developed for 3D dose calculation for a prostate implant with linear brachytherapy sources using the commercially available treatment-planning systems. The proposed models are referred to as the linear-segmented source (LSS) model and the point-segmented source (PSS) model. The calculated dose distributions obtained by these models for a single linear source have been validated by their comparison with the Monte Carlo-simulated data. Moreover, these models were used to calculate the dose distributions for a multilinear source prostate implant, and the results were compared to "seed" type implants. The results of these investigations show that the LSS model better approximates the linear sources than the PSS model. Moreover, these models have shown a better approximation of the dose distribution from a linear source for 0.5 cm source segments as compared to 1.0 cm source segments. However, for the points close to the longitudinal axis of the source located outside the region bounded by the active length, both models show differences of approximately +/-15%. These deficiencies are attributed to the limitations of the TG43 formalism for elongated sources.

Localization of linked 125I seeds in postimplant TRUS images for prostate brachytherapy dosimetry

PURPOSE

To demonstrate that (125)I seeds can be localized in transrectal ultrasound (TRUS) images obtained with a high-resolution probe when the implant is performed with linked seeds and spacers. Adequate seed localization is essential to the implementation of TRUS-based intraoperative dosimetry for prostate brachytherapy.

METHODS AND MATERIALS

Thirteen preplanned peripherally loaded prostate implants were performed using (125)I seeds and spacers linked together in linear arrays that prevent seed migration and maintain precise seed spacing. A set of two-dimensional transverse images spaced at 0.50-cm intervals were obtained with a high-resolution TRUS probe at the conclusion of the procedure with the patient still under anesthesia. The image set extended from 1.0 cm superior to the base to 1.0 cm inferior to the apex. The visible echoes along each needle track were first localized and then compared with the known construction of the implanted array. The first step was to define the distal and proximal ends of each array. The visible echoes were then identified as seeds or spacers from the known sequence of the array. The locations of the seeds that did not produce a visible echo were interpolated from their known position in the array. A CT scan was obtained after implantation for comparison with the TRUS images.

RESULTS

On average, 93% (range, 86-99%) of the seeds were visible in the TRUS images. However, it was possible to localize 100% of the seeds in each case, because the locations of the missing seeds could be determined from the known construction of the arrays. Two factors complicated the interpretation of the TRUS images. One was that the spacers also produced echoes. Although weak and diffuse, these echoes could be mistaken for seeds. The other was that the number of echoes along a needle track sometimes exceeded the number of seeds and spacers implanted. This was attributed to the overall length of the array, which was approximately 0.5 cm longer than the center-to-center distance between the first and last seed owing to the finite length of the seeds at the ends of the array. When this occurred, it was necessary to disregard either the most distal or most proximal echo, which produced a 0.5-cm uncertainty in the location of the array in the axial direction. For these reasons, simply localizing the visible echoes in the TRUS images did not guarantee the reliable identification of the seeds.

CONCLUSION

Our results have demonstrated that a high percentage (>85%) of the implanted (125)I seeds can be directly visualized in postimplant TRUS images when the seeds and spacers are linked to preclude seed migration and rotation and when the images are obtained with a high-resolution TRUS probe. Moreover, it is possible to localize 100% of the seeds with the mechanism of linked seeds because the locations of the missing seeds can be determined from the known construction of the arrays.

Dosimetric effects of seed anisotropy and interseed attenuation for Pd103 and I125 prostate implants

A Monte Carlo study is carried out to quantify the effects of seed anisotropy and interseed attenuation for 103Pd and 125I prostate implants. Two idealized and two real prostate implants are considered. Full Monte Carlo simulation (FMCS) of implants (seeds are physically and simultaneously simulated) is compared with isotropic point-source dose-kernel superposition (PSKS) and line-source dose-kernel superposition (LSKS) methods. For clinical pre- and post-procedure implants, the dose to the different structures (prostate, rectum wall, and urethra) is calculated. The discretized volumes of these structures are reconstructed using transrectal ultrasound contours. Local dose differences (PSKS versus FMCS and LSKS versus FMCS) are investigated. The dose contributions from primary versus scattered photons are calculated separately. For 103Pd, the average absolute total dose difference between FMCS and PSKS can be as high as 7.4% for the idealized model and 6.1% for the clinical preprocedure implant. Similarly, the total dose difference is lower for the case of 125I: 4.4% for the idealized model and 4.6% for a clinical post-procedure implant. Average absolute dose differences between LSKS and FMCS are less significant for both seed models: 3 to 3.6% for the idealized models and 2.9 to 3.2% for the clinical plans. Dose differences between PSKS and FMCS are due to the absence of both seed anisotropy and interseed attenuation modeling in the PSKS approach. LSKS accounts for seed anisotropy but not for the interseed effect, leading to systematically overestimated dose values in comparison with the more accurate FMCS method. For both idealized and clinical implants the dose from scattered photons represent less than 1/3 of the total dose. For all studied cases, LSKS prostate DVHs overestimate D90 by 2 to 5% because of the missing interseed attenuation effect. PSKS and LSKS predictions of V150 and V200 are overestimated by up to 9% in comparison with the FMCS results. Finally, effects of seed anisotropy and interseed attenuation must be viewed in the context of other significant sources of dose uncertainty, namely seed orientation, source misplacement, prostate morphological changes and tissue heterogeneity.

Experimental determination of the dosimetric characteristics for the I-Plant model 3600 125I brachytherapy source

The dosimetric characteristics for a new brachytherapy seed source (I-Plant model 3600) were measured using LiF thermoluminescent dosimeters and appropriate phantom materials in conformance with the methodology and guidance provided by the AAPM Task Group 43. The I-Plant model 3600 is the successor to the I-Plant model 3500. The major difference between these sources is that the model 3600 contains a leaded-glass core to provide radio-opacity (while the model 3500 contains a silver core), which does not produce spectral contamination upon neutron activation. The dose rate constant lambda for the model 3600 was determined to be 1.00 Gy h(-1) U(-1) (with a 6% overall relative standard deviation), compared to 1.01 cGy h(-1) U(-1) reported for the model 3500 in previous studies. The remaining dosimetric characteristics also are similar for both sources.

A permanent breast seed implant as partial breast radiation therapy for early-stage patients: a comparison of palladium-103 and iodine-125 isotopes based on radiation safety considerations

PURPOSE

A permanent breast seed implant (PBSI) technique has been developed as a new form of partial adjuvant radiation therapy for early-stage breast cancer. This study compares iodine-125 ((125)I) and palladium-103 ((103)Pd) isotopes by examining the exposure and effective dose (ED) to a patient's partner.

METHODS AND MATERIALS

A low-energy survey meter was used to measure exposure rates as a function of bolus thickness placed over (103)Pd or (125)I seeds. A general mathematical expression for the initial exposure rate at 1 m (x(o,1m)) from the skin surface as a function of the implant size, R, and the distance between the skin surface and the implant, d, was derived. Also, a second general equation is proposed to calculate the ED to the patient's partner.

RESULTS

The initial exposure rate at 1 meter and the ED are calculated as follows: x(o,1m) = 3alpha2R(3) ; ;beta(3) [e(-beta(2R+d))(betaR + 1) + e(-betad)(betaR - 1)], and ED = aR(b) [e(-c(2R+d)) (cR + 1) + e(-cd) (cR - 1)]. For (125)I, the parameters are: alpha = 0.154409, beta = 0.388460, a = 197, b = -0.95, and c = 0.38846. For (103)Pd, they are: alpha = 0.06877, beta = 0.421098, a = 18.6, b = -0.78, and c = 0.421098. For implant diameters varying from 2 to 6 cm and skin-to-implant distances varying from 0.7 to 4 cm, the ED is consistently below 2.6 mSv using the (103)Pd isotope, but more than 5 mSv in many instances and possibly up to 20 mSv using (125)I.

CONCLUSIONS

PBSI using (103)Pd seeds appears safe because the patient's partner ED is consistently below 5 mSv. The(125)I isotope is not recommended for PBSI.

On the question of 3D seed reconstruction in prostate brachytherapy: the determination of x-ray source and film locations

Inaccuracy in seed placement during permanent prostate implants may lead to significant dosimetric deviations from the intended plan. In two recent publications (Todor et al 2002 Phys. Med. Biol. 47 2031-48, Todor et al 2003 Phys. Med. Biol. 48 1153-71), methodology was described for identifying intraoperatively the positions of seeds already implanted, thus allowing reoptimization of the treatment plan and correcting for such seed misplacement. Seed reconstruction is performed using fluoroscopic images and an important (and non-trivial) component of this approach is the ability to accurately determine the position of the gantry relative to the treatment volume. We describe the methodology for acquiring this information, based on the known geometry of six markers attached to the ultrasound probe. This method does not require the C-arm unit to be isocentric and films can be taken with the gantry set at any arbitrary position. This is significant because the patient positioning on the operating table (in the lithotomy position) restricts the range of angles at which films can be taken to a quite narrow (typically +/- 10 degrees) interval and, as a general rule, the closer the angles the larger the uncertainty in the seed location reconstruction along the direction from the x-ray source to the film.

[Retrospective study of 130 patients with organconfined prostate cancer treated with brachytherapy]

INTRODUCTION

The prostate brachytherapy with I 125 seeds has an indication in patients with organconfined prostate cancer. Our objective is to describe the population treated in our institution with permanent I125 seeds implants, the dosimetric characteristics of the technique and the preliminary results of our group-study in terms of evolution and toxicity.

MATERIAL AND METHODS

Between May 2000 and March 2003, 130 patients with permanent implants of I125 seeds were treated. Beforehand we did prostate volumetric with transrectum prostate echography in order to assess the configuration of the implant, number of seeds and their place in the prostate with the objective to get a fine coverage of PTV (planet target volume). Stage distribution: 75.72% T1c; 24.28% T2a; Gleason<6, 94%. The PSA pretreatment average was 6.38 ng/ml. The average prostate volume was 30 cc. The 16.67% of the patients included had hormonal treatment previously to get the implants. The average age was 64 years. The characteristic techniques of the implants were: the average width of the needle as 24 (14-35) and the average of the seeds 76 (46-111). Finally the average activity was 0.39 mCi/seed, which means average total implant activity of 80 mCi.

RESULTS

We analyzed 130 patients with average follow up 6 months. A 1 to 2 year surveillance was carried out on 98.9% and the global free disease surveillance (biochemic relapse) of 98.9% at the year and of the 87.8% at the end of the 2 years. The relapse in the low risk patients was 98.8% after the first year and 88.7% at the end of 2 year. On the contrary in the middle risk was of 100% and 83% respectively, although the amount of patients in significantly less. As a relevant acute secondary effects we found slight rectitys or GI (RTOG scale) in 1.4 and that needs synthomatic medication or GII (RTOG scale) in 0.8%. We found slide hematuria or GI (RTOG scale) in the 53% and other measures or GII (RTOG scale) in the 2.64% was needed. Finally we had to set a urinary prove for acute retention in 4.3%.

CONCLUSION

The prostate brachyterapy is a complex procedure that needs a multidisciplinary team participation in order to be able to carry out. It avoids a long term hospitalization and allows for the patient to have daily activity within a short period of time. Despite the fact of the brief follow-up, the results over biochemical relapse and toxicity were similar to the ones in the literature. Tolerance to the implant was good. It would necessary a longer follow-up in order to be able to come to long term conclusions.

Brachytherapy needle deflection evaluation and correction

In prostate brachytherapy, an 18-gauge needle is used to implant radioactive seeds. This thin needle can be deflected from the preplanned trajectory in the prostate, potentially resulting in a suboptimum dose pattern and at times requiring repeated needle insertion to achieve optimal dosimetry. In this paper, we report on the evaluation of brachytherapy needle deflection and bending in test phantoms and two approaches to overcome the problem. First we tested the relationship between needle deflection and insertion depth as well as whether needle bending occurred. Targeting accuracy was tested by inserting a brachytherapy needle to target 16 points in chicken tissue phantoms. By implanting dummy seeds into chicken tissue phantoms under 3D ultrasound guidance, the overall accuracy of seed implantation was determined. We evaluated methods to overcome brachytherapy needle deflection with three different insertion methods: constant orientation, constant rotation, and orientation reversal at half of the insertion depth. Our results showed that needle deflection is linear with needle insertion depth, and that no noticeable bending occurs with needle insertion into the tissue and agar phantoms. A 3D principal component analysis was performed to obtain the population distribution of needle tip and seed position relative to the target positions. Our results showed that with the constant orientation insertion method, the mean needle targeting error was 2.8 mm and the mean seed implantation error was 2.9 mm. Using the constant rotation and orientation reversal at half insertion depth methods, the deflection error was reduced. The mean needle targeting errors were 0.8 and 1.2 mm for the constant rotation and orientation reversal methods, respectively, and the seed implantation errors were 0.9 and 1.5 mm for constant rotation insertion and orientation reversal methods, respectively.

A technical evaluation of the Nucletron FIRST system: conformance of a remote afterloading brachytherapy seed implantation system to manufacturer specifications and AAPM Task Group report recommendations

The Fully Integrated Real‐time Seed Treatment (FIRST™) system by Nucletron has been available in Europe since November 2001 and is being used more and more in Canada and the United States. Like the conventional transrectal ultrasound implant procedure, the FIRST system utilizes an ultrasound probe, needles, and brachytherapy seeds. However, this system is unique in that it (1) utilizes a low‐dose‐rate brachytherapy seed remote afterloader (the seedSelectron), (2) utilizes 3D image reconstruction acquired from electromechanically controlled, nonstepping rotation of the ultrasound probe, (3) integrates the control of a remote afterloader with electromechanical control of the ultrasound probe for integrating the clinical procedure into a single system, and (4) automates the transfer of planning information and seed delivery to improve quality assurance and radiation safety. This automated delivery system is specifically intended to address reproducibility and accuracy of seed positioning during implantation. The FIRST computer system includes two software environments: SPOT PRO™ and seedSelectron™; both are used to facilitate treatment planning and brachytherapy seed implantation from beginning to completion of the entire procedure. In addition to these features, the system is reported to meet certain product specifications for seed delivery positioning accuracy and reproducibility, seed calibration accuracy and reliability, and brachytherapy dosimetry calculations. Consequently, a technical evaluation of the FIRST system was performed to determine adherence to manufacturer specifications and to the American Association of Physicists in Medicine (AAPM) Task Group Reports 43, 53, 56, 59, and 64 and recommendations of the American Brachytherapy Society (ABS). The United States Nuclear Regulatory Commission (NRC) has recently added Licensing Guidance for the seedSelectron system under 10 CFR 35.1000. Adherence to licensing guidance is made by referencing applicable AAPM Task Group recommendations. In general, results of this evaluation indicated that the system met its claimed specifications as well as the applicable recommendations outlined in the AAPM and ABS reports.

PACS number(s): 87.53.Xd, 87.53.Jw

Impact of selection of post-implant technique on dosimetry parameters for permanent prostate implants

PURPOSE

To investigate the variability of prostate implant quality indices between three different methods of calculating the post-implant dose distribution.

METHODS AND MATERIALS

In a study of 9 permanent prostate implant patients, post-implant dosimetry was carried out using three methods of identifying seed positions within the prostate volume: (1) prostate volumes defined by transrectal ultrasound (TRUS) immediately following implant were registered with shift-film defined seed positions, (2) seeds were identified directly from the post-implant TRUS images, and (3) CT was used to define seed positions and prostate volumes from images acquired at 41-65 days post-implant. For each method, the volume of prostate receiving 90%, 100%, and 150% of the prescribed dose (V90, V100, V150) and the dose delivered to 90% of the prostate volume (D90) were calculated.

RESULTS

Post-implant TRUS volumes were within 15% of the preimplant TRUS volumes in 8 of the 9 patients investigated. The post-implant CT volume was within 15% of the preimplant (TRUS) volume in only 3 of the 9 cases. The value of the dosimetry parameters was dependent on the method used and varied by 5-25% for V90, 5-30% for V100, 42-134% for V150, and 9-60% for D90. No simple relationship was found between change in volume and the resultant change in dosimetry parameter. Differences in dosimetry parameters due to source localization uncertainties was found to be small (< or = 10% for V100) when comparing methods (1) and (2).

CONCLUSIONS

There are many uncertainties in the calculation of parameters that are commonly used to describe the quality of a permanent prostate implant. Differences in the parameters calculated were most likely a result of a combination of factors including uncertainties in delineating the prostate with different imaging modalities, differences in source identification techniques, and intraobserver variability.

MR and CT image fusion for postimplant analysis in permanent prostate seed implants

PURPOSE

To compare the outcome of two different image-based postimplant dosimetry methods in permanent seed implantation.

METHODS AND MATERIALS

Between October 1999 and October 2002, 150 patients with low-risk prostate carcinoma were treated with (125)I and (103)Pd in our institution. A CT-MRI image fusion protocol was used in 21 consecutive patients treated with exclusive brachytherapy. The accuracy and reproducibility of the method was calculated, and then the CT-based dosimetry was compared with the CT-MRI-based dosimetry using the dose-volume histogram (DVH) related parameters recommended by the American Brachytherapy Society and the American Association of Physicists in Medicine.

RESULTS

Our method for CT-MRI image fusion was accurate and reproducible (median shift <1 mm). Differences in prostate volume were found, depending on the image modality used. Quality assurance DVH-related parameters strongly depended on the image modality (CT vs. CT-MRI): V(100) = 82% vs. 88%, p < 0.05. D(90) = 96% vs. 115%, p < 0.05. Those results depend on the institutional implant technique and reflect the importance of lowering inter- and intraobserver discrepancies when outlining prostate and organs at risk for postimplant dosimetry.

CONCLUSIONS

Computed tomography-MRI fused images allow accurate determination of prostate size, significantly improving the dosimetric evaluation based on DVH analysis. This provides a consistent method to judge a prostate seed implant's quality.

Distortions induced by radioactive seeds into interstitial brachytherapy dose distributions

In a previous article, we presented development and verification of an integral transport equation-based deterministic algorithm for computing three-dimensional brachytherapy dose distributions. Recently, we have included fluorescence radiation physics and parallel computation to the standing algorithms so that we can compute dose distributions for a large set of seeds without resorting to the superposition methods. The introduction of parallel computing capability provided a means to compute the dose distribution for multiple seeds in a simultaneous manner. This provided a way to study strong heterogeneity and shadow effects induced by the presence of multiple seeds in an interstitial brachytherapy implant. This article presents the algorithm for computing fluorescence radiation, algorithm for parallel computing, and display results for an 81-seed implant that has a perfect and imperfect lattice. The dosimetry data for a single model 6711 seeds is presented for verification and heterogeneity factor computations using simultaneous and superposition techniques are presented.

Dosimetric characteristics of the new RadioCoil™ Pd103 wire line source for use in permanent brachytherapy implants

Recently, a novel linear brachytherapy source in the form of a coiled wire has become available for use in interstitial implants of various treatment sites such as prostate gland. This source type employs a design completely different from that of most "seed" sources currently on the market, one which improves upon or eliminates several common problems with such sources. Dosimetric characteristics of these sources with active lengths 0.5 cm to 5.0 cm were determined for clinical application. For 0.5 cm and 1.0 cm active length sources, the dose rate constant, radial dose function, and two-dimensional (2D) anisotropy function were experimentally and theoretically determined following the updated AAPM Task Group 43 (TG-43U1) recommendations. Radial dose functions and/or "along-away" matrix functions were also obtained for sources with active lengths 2.0 cm to 5.0 cm. Measurements were performed with LiF thermoluminescent dosimeters in Solid Water phantoms. Measured data was compared to Monte Carlo simulated data in Solid Water utilizing the PTRAN code, version 7.43. After finding the data to be in agreement, Monte Carlo calculations were performed in liquid water to obtain clinically applicable dosimetric data as per TG-43U1 recommendations. The results indicated the dose rate constant of the 0.5 cm long RadioCoil 103Pd source in Solid Water to be 0.641 cGy h(-1) U(-1) when measured, and 0.636 cGy h(-1) U(-1) when simulated by Monte Carlo. The calculated dose rate constant in liquid water was found to be 0.650 cGy h(-1) U(-1). These values are comparable to other commercially available sources. Complete dosimetric data and simulation results are described in this paper. Per TG-43U1, clinical treatment planning systems should utilize the values reported for liquid water.

Update of AAPM Task Group No. 43 Report: A revised AAPM protocol for brachytherapy dose calculations

Since publication of the American Association of Physicists in Medicine (AAPM) Task Group No. 43 Report in 1995 (TG-43), both the utilization of permanent source implantation and the number of low-energy interstitial brachytherapy source models commercially available have dramatically increased. In addition, the National Institute of Standards and Technology has introduced a new primary standard of air-kerma strength, and the brachytherapy dosimetry literature has grown substantially, documenting both improved dosimetry methodologies and dosimetric characterization of particular source models. In response to these advances, the AAPM Low-energy Interstitial Brachytherapy Dosimetry subcommittee (LIBD) herein presents an update of the TG-43 protocol for calculation of dose-rate distributions around photon-emitting brachytherapy sources. The updated protocol (TG-43U1) includes (a) a revised definition of air-kerma strength; (b) elimination of apparent activity for specification of source strength; (c) elimination of the anisotropy constant in favor of the distance-dependent one-dimensional anisotropy function; (d) guidance on extrapolating tabulated TG-43 parameters to longer and shorter distances; and (e) correction for minor inconsistencies and omissions in the original protocol and its implementation. Among the corrections are consistent guidelines for use of point- and line-source geometry functions. In addition, this report recommends a unified approach to comparing reference dose distributions derived from different investigators to develop a single critically evaluated consensus dataset as well as guidelines for performing and describing future theoretical and experimental single-source dosimetry studies. Finally, the report includes consensus datasets, in the form of dose-rate constants, radial dose functions, and one-dimensional (1D) and two-dimensional (2D) anisotropy functions, for all low-energy brachytherapy source models that met the AAPM dosimetric prerequisites [Med. Phys. 25, 2269 (1998)] as of July 15, 2001. These include the following 125I sources: Amersham Health models 6702 and 6711, Best Medical model 2301, North American Scientific Inc. (NASI) model MED3631-A/M, Bebig/Theragenics model I25.S06, and the Imagyn Medical Technologies Inc. isostar model IS-12501. The 103Pd sources included are the Theragenics Corporation model 200 and NASI model MED3633. The AAPM recommends that the revised dose-calculation protocol and revised source-specific dose-rate distributions be adopted by all end users for clinical treatment planning of low energy brachytherapy interstitial sources. Depending upon the dose-calculation protocol and parameters currently used by individual physicists, adoption of this protocol may result in changes to patient dose calculations. These changes should be carefully evaluated and reviewed with the radiation oncologist preceding implementation of the current protocol.

The impact of technological advances on the evolution of 3D conformal brachytherapy for early prostate cancer

Permanent implantation of I-125 and Pd-103 seeds is one of the widely used treatment options for the early stage prostate cancer with minimum normal tissue complications and long-term local control of the tumor. This is possible because of several technological advances made in the past decade to better understand the procedural aspects of implantations with the desired clinical outcome and with acceptable morbidities. In addition, with the widespread use of PSA testing, more widely disseminated information about prostate cancer and increased patient awareness, over 70% of patients are diagnosed early with localized disease and therefore are candidates for definitive local therapy. Delineation of soft tissue structures including the prostate, rectum, urethra and bladder has become more accurate with the use of imaging modalities including Ultrasound and MRI, with or without the CT. A re-evaluation of the dosimetric parameters of the radioactive sources has lead to a more precise estimate of the dose delivered to the prostate and the associated critical normal structures. Technological improvements in the post implant dosimetry have helped to understand the factors, which makes an implant a "good implant" or a "poor implant". Intraoperative treatment planning with on line dosimetry is emerging as one of the best approaches for prostate brachytherapy. In addition, better software is now available producing dose-volume histograms with 3D target and normal tissue reconstruction. The combination of seed implant followed by IMRT would provide scope for differentially boosting the regions under-dosed because of uncontrollable and unexpected reasons during the implant and unsuspected micro extensions of the tumor.

What is the optimal dose for 125I prostate implants? A dose-response analysis of biochemical control, posttreatment prostate biopsies, and long-term urinary symptoms

PURPOSE

To define the optimal dose for 125I prostate implants by correlating post implant CT dosimetry findings with urinary symptoms, biochemical failure, and posttreatment biopsies.

METHODS AND MATERIALS

Patients with T1-T2, Gleason score 2-6 prostate cancer treated with I-125 brachytherapy were analyzed. Group 1 (276 patients) was observed from 18 to 108 months (median, 34 months) and had urinary symptoms prospectively assessed using the International Prostate Symptom Score (IPSS) system. Group 2 (181 patients) observed from 24 to 108 months (median, 44 months) and did not receive hormonal therapy. Implant dose was defined as the D90 (dose delivered to 90% of the prostate on a dose-volume histogram). Patients were analyzed by dose categories: <140 Gy, 140 to <160 Gy, 160 to <180 Gy, and > or =180 Gy. In Group 1, the mean pre- to postimplant IPSS scores were compared in different dose categories by using a matched paired t test. In Group 2, the effect of dose on biochemical control was tested with actuarial methods by using the American Society for Therapeutic Radiology and Oncology definition and on local control with posttreatment biopsies (113 patients).

RESULTS

A comparison of pre- with postimplant IPSS revealed no significant changes in scores in the dose groups <180 Gy except for small changes in urgency and bladder emptying in the dose group <140 Gy. In dose group >180 Gy, mean scores changed from 0.5 to 1.0 (p=0.002) for emptying, 0.76 to 1.29 (p=0.004) for weak stream, 0.24 to 0.51 (p=0.009) for straining, 1.55 to 1.82 (p=0.05) for nocturia, and 6.3 to 8.45 (p=0.0009) for the total score. Freedom from biochemical failure (FFBF) at 5 years was 68% for doses <140 Gy, 97% for 140 to <160 Gy, 98% for 160 to <180 Gy, and 95% for > or =180 Gy (p=0.0025). Overall, patients with doses <140 Gy (median follow-up, 66 months) had an FFBF of 68%, compared with 96% for patients with doses > or =140 Gy (median follow-up, 35 months; p=0.0002). Multivariate analysis found dose to be the most significant factor affecting FFBF. Positive biopsies were found in 23% for doses <140 Gy, 21% for 140 to <160 Gy, 10% for 160 to <180 Gy, and 8% for > or =180 Gy. Overall, biopsies were positive in 22% for doses <160 Gy vs. 9% for > or =160 Gy (p=0.05).

CONCLUSIONS

Optimal 125I prostate implants should deliver a D90 of 140-180 Gy, on the basis of postimplant dosimetry. Doses of <140 Gy are associated with increased biochemical failure, and doses >180 Gy with a small increase in long-term urinary symptoms.