Dosimetry of CT-guided volumetric IR-192 brain implant

Over the past 2 years, an afterloading technique has been developed and refined to implant radioactive Ir-192 sources into brain tumors. The implantation procedure integrates a stereotaxic system with computerized tomography (CT), which provides tumor position, volume, and guides the placement of catheters. A radiolucent ring-frame immobilizes the head as holes are made at 1 cm intervals with the aid of a template. Catheters containing dummy sources 1 cm apart are then inserted to the desired depth, and their position verified in three dimensions to insure complete coverage of visible tumor volume as defined by contrast enhancement. Once catheters are secured, the anesthetized patient is moved to the intensive care unit where the dummy sources are replaced by ribbons of Ir-192 seeds (specific activity 0.6-1.0 mg Ra eq). CT scans with the dummy sources in place are used to designate spatial coordinates of the active sources. A computer program converts position data and source strength into isodose contours in any plane. The implant duration (70-100 hours) for the desired dose to the tumor periphery (60-120 Gy) is then calculated. Dose rate contours are superimposed on preimplant CT scans. Maximum and minimum doses are determined in each of the various planes. Verification dosimetry has been carried out with thermoluminescent dosimeters placed in a catheter located in a plane along the tumor periphery. In vivo isodose values compared to idealized plans agree within +/-5%-10%.     

Use of three-dimensional radiation therapy planning tools and intraoperative ultrasound to evaluate high dose rate prostate brachytherapy implants

PURPOSE: We performed a pilot study to evaluate the quality of high dose rate (HDR) prostate implants using a new technique combining intraoperative real-time ultrasound images with a commercially available 3-dimensional radiation therapy planning (3D RTP) system. METHODS AND MATERIALS: Twenty HDR prostate implants performed by four different physicians on a phase I/II protocol were evaluated retrospectively. Radiation therapy (RT) consisted of pelvic external beam RT (EBRT) to a dose of 46 Gy in 2-Gy fractions over 5 weeks and 2 HDR implants (prescribed dose of 950 cGy per implant). Our in-house real-time geometric optimization technique was used in all patients. Each HDR treatment was delivered without moving the patient. Ultrasound image sets were acquired immediately after needle placement and just prior to HDR treatment. The ultrasound image sets, needle and source positions and dwell times were imported into a commercial computerized tomography (CT) based 3D RTP system. Prostate contours were outlined manually caudad to cephalad. Dose-volume histograms (DVHs) of the prostate were evaluated for each implant. RESULTS: Four patients with stage T2a carcinoma, 4 with stage T2b, and 3 with stage T1c were studied. The median number of needles used per implant was 16 (range 14-18). The median treated volume of the implant (volume of tissue covered by the 100% isodose surface) was 82.6 cc (range 52.6-96.3 cc). The median target volume based on the contours entered in the 3D RTP system was 44.83 cc (range 28.5-67.45 cc). The calculated minimum dose to the target volume was 70% of the prescribed dose (range 45-97%). On average 92% of the target volume received the prescribed dose (range 75-99 %). The mean homogeneity index (fraction of the target volume receiving between 1.0 to 1.5 times the prescribed dose) was 80% or 0.8 (range 0.55-0.9). These results compare favorably to recent studies of permanent implants which report a minimum target volume dose of 43% (range 29-50%) and an average of 85% of the target volume (range 76-92%) receiving the prescribed dose. CONCLUSIONS: The feasibility of evaluating HDR prostate implants using ultrasound images (acquired immediately prior to treatment) with a commercially available 3D RTP system was established. The dosimetric characteristics of these HDR implants appear to be substantially different compared to permanent implants. These developments allow quantitative evaluation of the dosimetric quality of HDR prostate treatments. Future studies will examine any correlation between the dosimetric quality of the implant and clinical/biochemical outcomes.     

A 192Ir nomogram system for single plane implants

Nomograms for square planar arrays spanning the range from 3 X 3 cm to 10 X 10 cm were developed. The nomograms are intended to be used for pretreatment planning of implant geometry, so that the therapist may enter the operating room with a plan for the optimal implant in mind. We show that clinically useful implants are those in which the reference isodoses are fully coupled. Decoupling occurs when ribbon spacing exceeds approximately 1.2 cm and leads to undesirable "cold spots" within the treatment volume. Ribbon spacing of 1.0 cm is recommended. This represents a trade-off between adequate ribbon coupling and minimum tissue damage from trocar placement. For clinically useful arrays, the area enclosed by the reference isodose contour (85% of the maximum dose rate) is approximately 50% of the array area. Reference isodose contour and its enclosed area are independent of seed strength for a given array, as long as all seeds within the array are of equal strength.     

The guide gutter or loop techniques of interstitial implantation and the Paris system of dosimetry

The predictive dosimetry system for implants known as the Paris system can be used with either loops or hairpins. When using the guide gutter technique, implant geometry is predetermined by the inherent spacing and parallelism of the branches of the hairpins. When using loops, their branches should not be spaced too widely apart and should be parallel over an adequate distance to obtain a fairly regular dose distribution between them. The basic principles of implantation are the same as for rectilinear sources. Branches must be rectilinear, parallel, arranged so that their centers are located in the same plane (central plane). Adjacent branches must be equidistant from each other and the reference linear kerma rate (or the linear activity) must be uniform and identical for all sources. When these conditions are met, the dimensions of the treated volume (volume encompassed by the reference isodose surface with a value equal to 85% of the basal dose rate) can be estimated at the time of the implantation procedure. In practice, only a few relationships presented in this paper, with examples of application, must be known. Although, the Paris system permits forecasting the final dosimetry, the geometry of the implant must be verified and the dose calculated according to the implantation as actually achieved. The best method of checking the exact position of radioactive sources in an implant and determining the dose rate at any desired point is a reconstruction by computer program although alternative methods are occasionally appropriate.(ABSTRACT TRUNCATED AT 250 WORDS)     

A spreadsheet program for brachytherapy planning

A computer program (brachy-spread) which allows spreadsheet-like interactive adjustment of the loading of any brachytherapy application has been implemented. Sources are collected into objects, each assigned an activity and duration of implant. Activities and times may be adjusted by moving a cursor to the datum to be edited and entering a new value from the keyboard. Alternatively, the desired total dose to a given calculation point may be edited resulting in a recalculation of the time for all objects. For each of a set of calculation points, dose rates, total doses, and the percent contribution of each object to the point are displayed and instantly updated as the times and activities are adjusted. The program design includes rapidly updated display of isodose curves in previously selected arbitrary planes. A strategy for providing rapid dose display involving precalculation of fractional dose tables is used. The program has significantly reduced the time required to determine the appropriate loading of GYN applications and of implants which involve a combination of line sources and seeds.     

Some treatment planning considerations for 103Pd and 125I permanent interstitial implants.

Sealed sources of palladium-103 (103Pd), which decay with a half life of 17 days and emit on average 21 keV photons, are now in clinical use for permanent implants. For seed implantation of prostatic cancer, 103Pd implants are usually planned to deliver 115 Gy to full decay at an initial dose rate of 19.7 cGy/hr whereas 125I implants are usually planned to deliver 160 Gy at an initial dose rate of 7.72 cGy/hr. Because of the lower energy of photons emitted by 103Pd compared to the 125I sources (27 keV average energy), the tissue attenuation is more severe for 103Pd sources. The radial dose function drops more steeply with distance from the 103Pd sources compared to the 125I sources, raising a concern about the possibility of cold spots in the tumors implanted with 103Pd sources. To investigate this issue, a detailed analysis of the dependence of dose uniformity as a function of seed spacing for 125I and 103Pd sources in various cubic and spherical configurations was carried out. Using the measured single source dosimetry data as input, dose distributions for a variety of cubic and spherical implants were generated on a computerized treatment planning system. This study indicates that relative dose distributions for 125I and 103Pd implants with the same geometric configuration and number of seeds are very similar inside the implanted volume for implants. Dose uniformity within a target volume implanted with 103Pd seeds is also very similar to that for 125I. To expedite clinical implementation of 103Pd, an atlas of dose distributions for 103Pd implants has been produced for various seed configurations, seed spacings, and target volumes. Using 125I implants as a guideline, clinical procedures for planning of 103Pd implants have been developed. It was found that the total source strength implanted divided by the dimension of the implant can be expressed as an exponential function of implant size, resulting in a simple method for estimating the strength of seeds necessary in an implant. Also, the air kerma strength of 103Pd seeds is about 3.3 times that of 125I sources in an implant with the same geometric configuration and number of seeds, provided treatment doses of 115 Gy and 160 Gy are chosen for 103Pd and 125I implants, respectively.     

A method to improve the dose distribution of interstitial breast implants using geometrically optimized stepping source techniques and dose normalization.

BACKGROUND AND PURPOSE: The standard linear source breast implant of our institution was compared with alternative linear source implant geometries and a stepping source implant, to evaluate the possibility of minimizing the treated volume. Normalization to a higher isodose than the conventional 85% of the mean central dose (MCD) was investigated for the stepping source implant to reduce the thickness of the treated volume and to increase dose uniformity. The purpose of this study was to develop an implant geometry yielding a high conformity and a more uniform dose distribution over the target volume. MATERIALS AND METHODS: The dose distributions of four implant geometries were compared for a planning target volume (PTV) of 48 cm(3). Implants #1 (standard) and #2 had linear sources arranged in a triangular pattern of equal lengths and lengths adapted to the shape of the PTV. Implants #3 and #4 were squared pattern arranged implants with linear sources and a stepping source with geometric optimized dwell times. The active lengths were adapted to the shape of the PTV. Using implant #4 for PTVs of different volumes, the reference dose (RD) was normalized to 85 and 91% of the MCD. RESULTS: Comparing implants #2, #3, and #4 with #1, the treated volume (V(100)) encompassed by the reference isodose was reduced by 22, 35, and 37%, respectively. The volumes receiving a dose of at least 125% (V(125)) of the reference dose was reduced by 16, 30, and 30%, respectively. The conformation number increased being 0.30, 0.39, 0.47, and 0.48 for implants #1, #2, #3, and #4, respectively. The average reduction of V(125) when the dose was normalized to 91% compared with 85% of the MCD was 18%. CONCLUSIONS: A conformal treatment to a PTV could be best achieved with a geometrically optimized stepping source plan with needles arranged in a squared pattern. Reduction of high dose volumes within the implant was obtained by normalizing the RD to 91% instead of 85% of the MCD.     

A comprehensive three-dimensional radiation treatment planning system

A comprehensive software system has been developed to allow 3-dimensional planning of radiation therapy treatments using the extensive anatomical information made available by imaging modalities such as CT and MR. Biological structures of interest and tumor volumes are defined by outlines drawn on a sequence of CT slices. Beam set-ups may then be determined in three dimensions by displaying the structure contours in a beam's eye view, or in two dimensions using a single CT cut. Each beam defined may be shaped by the specification of block aperture contours, and its intensity may be modified with the use of planar compensators. 3D dose calculation algorithms are discussed. To evaluate the calculation results, dose volume histograms are provided, as well as various types of displays in two and three dimensions, including dose on arbitrarily oriented planes, dose on the surface of anatomical objects, and isodose surfaces. Computer generated beam films are also available as an aid in patient set-up verification. These tools, and others, provide the basis for a comprehensive 3D system that can be used throughout the treatment planning process.     

Method of radiotherapy planning for head and neck tumors using simulated CT images and radiographic data, developed at the Gustave Roussy Institute]

The paper deals with the recent improvements introduced in the most usual method applied in the Institut Gustave Roussy radiotherapy department for obtaining the anatomical data of patients treated for head and neck tumors. For each of these patients, five to seven transverses slices and a lateral radiographic film are taken from a Mecaserto simulator-CT. The anatomical representation of the patient sagittal plane is carried out from the digitalisation of the radiographic film on a Vidar Vxr-12 Plus film scanner and integrated into the Dosigray dose calculation programme in order to be used as a support for the laying out of the dose distribution in reference to the treatment. The sagittal anatomical representation obtained from the radiographic film digitalisation is compared with the one resulting from the interpolation between a limited number of irregularly-spaced transverse slices taken on the simulator-CT. The method using the simulator-scanner transverse slices and the radiographic film digitalisation represents an interesting alternative for obtaining an anatomy simulation representative of the patient in hospitals where a scanner is not available full-time for the needs of the radiotherapy process.     

Dosimetric considerations of stereotactic brain implants

Dose distributions of stereotactic brain implants performed by four institutions were analyzed. In these implants 192Ir or 125I sources were used. The analyses involved an evaluation of the isodose distributions in two orthogonal planes, the dose gradient outside, and the dose homogeneity within the target volume. Quantitative evaluation of the dose homogeneity was performed using three volumetric irradiation indices. The dose homogeneity was observed to improve as the number of catheters increased. However, the number of catheters used is influenced by neurosurgical considerations. Thus, it is necessary to make a compromise between dose homogeneity and the maximum number of catheters to be used. The dose gradient, a centimeter outside the target volume, was found to depend on the geometry of the implant and at distances beyond, it was found to depend on the type of radioisotopes used.     

The effect of finite patient dimensions and tissue inhomogeneities on dosimetry planning of Ir HDR breast brachytherapy: A Monte Carlo dose verification study

PURPOSE: To evaluate the accuracy of clinical dosimetry planning using commercially available treatment planning systems in (192)Ir high-dose-rate (HDR) breast brachytherapy, with emphasis on skin dose, in view of potential uncertainties owing to the patient finite dimensions and the presence of the lung. METHODS AND MATERIALS: A patient-equivalent mathematical phantom was constructed on the basis of the patient computed tomography scan used in the clinical treatment planning procedure. The actual treatment plan delivered to the patient, involving an implant of six plastic catheters and 26 programmed source dwell positions, was simulated by means of the Monte Carlo method. Results are compared with corresponding dose calculations of a commercially available treatment planning system in the form of prescribed dose percentage isodose contours and cumulative dose-volume histograms. RESULTS: The comparison of Monte Carlo results and treatment planning system calculations revealed that all percentage isodose contours greater than 60% of the prescribed dose are not affected by the finite breast dimensions or the presence of the lung. Treatment planning system calculations overestimate dose in the lung as well as lower isodose contours at points lying both close to the breast or lung surface and relatively away from the implant. In particular, skin dose is overestimated by 5% in the central breast region and within 10% at all other points. CONCLUSIONS: Dose-volume histogram and all other relevant planning quality indices for the planning target volume calculated by the treatment planning system are credible. Skin and lung dose calculations by the treatment planning system can be thought of as a conservative approach in view of the reported dose overestimation.     

Uniform analysis of dose distribution in interstitial brachytherapy dosimetry systems

In brachytherapy, articles are published with dose and homogeneity specifications using different systems which are hard to compare. For the same "stated dose", the dose delivered, volume treated and activity chosen are not the same in different systems, due to the fact that the source placement rules are different. In this article, the authors have circumvented this problem by the use of an approach not applied hitherto, viz. by determining the value of the Uniformity index (UI) of an implant using different systems. This value takes into account the integral dose within the treatment volume and is compared with an idealized implant, where the dose is uniform and the target and treatment volumes are the same. This method of evaluation has been applied for single plane, multiple plane and cylindrical volume implants using the Manchester, Quimby and Paris systems. Although the results obtained are different, the degree of closeness of these values are striking, with some minor exceptions. Thus, it is possible to combine all the brachytherapy parameters, such as: dose, homogeneity, treatment volume within a single value to determine the quality of an implant.     

Interstitial volume implants with I-125 seeds

Interstitial implants with I-125 seeds are generally carried out using a dosimetric system that is based on implanting a specified number of mCi, depending on the average dimension of the gland. On the other hand, traditional interstitial radium implants are carried out using either the Patterson Parker or the Quimby systems. An absorbed dose of 6000 rad delivered over a 7 day period is considered optimal for most sites using either of these systems. In the case of permanent interstitial implants using I-125 seeds, the allowance that must be made for gradually declining dose rate changes the dose to be delivered by several orders of magnitude. Furthermore, it is not clear at the present time whether the dose delivered should vary with the volume of the implant and if so in what manner. In this paper, we describe a protocol in which the mCi and the number of seeds to be implanted are selected on the basis of delivering a prescribed dose in rad. Two specific doses within an implant have been identified. One is the minimum peripheral dose (MPD), defined as the lowest dose found at the intersection of the periphery of each seed array and a plane halfway between the planes carrying the seeds. Another one is the maximum central dose, defined as the maximum dose found in a plane halfway between the planes carrying the seeds. A simple and unique way of accurately assessing the glandular volume at surgery is also described.     

Adaptive brachytherapy treatment planning for cervical cancer using FDG-PET

PURPOSE: A dosimetric study was conducted to compare intracavitary brachytherapy using both a conventional and a custom loading intended to cover a positron emission tomography (PET)-defined tumor volume in patients with cervix cancer. METHODS AND MATERIALS: Eleven patients who underwent an [(18)F]-fluoro-deoxy-D-glucose (FDG)-PET in conjunction with their first, middle, or last brachytherapy treatment were included in this prospective study. A standard plan that delivers 6.5 Gy to point A under ideal conditions was compared with an optimized plan designed to conform the 6.5-Gy isodose surface to the PET defined volume. RESULTS: A total of 31 intracavitary brachytherapy treatments in conjunction with an FDG-PET were performed. The percent coverage of the target isodose surface for the first implant with and without optimization was 73% and 68% (p = 0.21). The percent coverage of the target isodose surface for the mid/final implant was 83% and 70% (p = 0.02), respectively. The dose to point A was higher with the optimized plans for both the first implant (p = 0.02) and the mid/last implants (p = 0.008). The dose to 2 cm(3) and 5 cm(3) of both the bladder and rectum were not significantly different. CONCLUSIONS: FDG-PET based treatment planning allowed for improved dose coverage of the tumor without significantly increasing the dose to the bladder and rectum.     

Interstitial IR-192 implants of the oral cavity: the planning and construction of volume implants

The success of radioactive implant therapy for head and neck carcinomas depends critically on careful planning and execution of the implant procedure. In this paper we discuss our experience with oral tongue and floor of mouth implants, and some innovations introduced to facilitate these procedures. Implants were carried out using standard angiocatheters modified with magnetic caps at the open end and terminated with Teflon spacers and lead shot at the closed end. The importance of accurate source placement and careful determination of the target dose rate is discussed with numerical examples. Differential hot-loading of sources is clearly indicated in cases of extension of the lesion to the dorsal or lateral tongue surface. For dorsal surface extension the use of Teflon spacers at the closed ends of the catheters helps to ensure adequate coverage of the target volume with the higher dose region enclosing the demonstrable tumor. All 10 patients implanted with this technique are controlled without recurrence at a median follow-up of 38 months. The two complications observed appeared to be associated with excessive hot-loading of edge plane sources.     

Permanent planar iodine-125 implants: the dosimetric effect of geometric parameters for idealized source configurations

PURPOSE: To provide dosimetric information about permanent planar (125)I implants in a manner that is useful to the brachytherapist in the operative setting. METHODS AND MATERIALS: Reference planar permanent implants were simulated for a variety of areas with sources placed uniformly on a 1-cm grid. Implants having variable source spacing and curvature were simulated and compared with the reference implants. Dosimetric measures were calculated at 0.5 and 1.0 cm from the implant plane. RESULTS: A method for calculating dosimetric statistics for permanent implants ranging from 5 x 5 cm to 13 x 13 cm is presented. A formula to predict the reference source strength needed to achieve a desired dosimetric quantity is also presented. The effect of adjusting strand spacing to compensate for source activity is presented and is shown to be an effective means to adjust implants to use source strengths other than the reference strength. The effect of implant curvature compared with flat implants on dosimetric statistics is presented as a function of radius of curvature. CONCLUSIONS: The results presented in this work may be used to provide information about dose delivered from planar permanent implants.