Correction factors for Farmer-type chambers for absorbed dose determination in 60Co and 192Ir brachytherapy dosimetry

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

Dose calibrator response to brachytherapy sources: A Monte Carlo and analytic evaluation

It is well known that the dose calibrator response/unit exposure rate depends significantly upon source energy. However, investigation of 137Cs, 192Ir, and 226Ra brachytherapy sources by empirical, analytical, and Monte Carlo techniques shows that source filtration significantly affects the calibrator reading to exposure rate conversion factor. The results demonstrate that for each clinically used filtration thickness an exposure calibrated standard source is required to establish the response of the well chamber. An interesting consequence of this analysis is that the Sievert point dose algorithm for clinical sources overestimates the dose on the order of 3% at distances of approximately 3.5 cm from the source.     

Analytical approach to heterogeneity correction factor calculation for brachytherapy

In brachytherapy treatment planning, the effects of tissue and applicator heterogeneities are commonly neglected due to lack of accurate, general, and fast three-dimensional (3D) dose-computational algorithms. A novel approach, based on analytical calculation of scattered photon fluxes inside and around a disk-shaped heterogeneity, has been developed for use in the three-dimensional scatter-subtraction algorithm. Specifically, our model predicts the central-ray dose distribution for a collimated photon isotropic source or brachytherapy "minibeam" in the presence of a slab of heterogeneous material. The model accounts for the lateral dimensions, location, composition, density, and thickness of the heterogeneity using precalculated scatter-to-primary ratios (SPRs) for the corresponding homogeneous problem. The model is applicable to the entire brachytherapy energy range (25 to 662 keV) and to a broad range of materials having atomic numbers of 13 to 82, densities of 2.7 g.cm-3 (Al) to 21.45 g.cm-3 (Pt) and thicknesses up to 1 mean free path. For this range of heterogeneous materials, the heterogeneity correction factors (HCFs) vary from 0.09 to 0.75. The model underestimates HCF when multiple scattering prevails and overestimates HCF when absorption dominates. However, the analytic model agrees with Monte Carlo photon transport (MCPT) benchmark calculations within 1.8% to 10% for 125I, 169Yb, 192Ir, and 137Cs for a wide variety of materials, with the exception of Ag. For 125I shielded by Ag, where the mean discrepancy can exceed 25%, the error is due to K-edge characteristic x rays originating within the heterogeneity. The proposed approach provides reductions in CPU time required of 5 x 10(4)-10(5) and 100 in comparison with direct MCPT simulation and 1D numerical integration, respectively. The limitations of model applicability, as determined by the physical properties of heterogeneity material and accuracy required, are also discussed.     

Two-dimensional scatter integration method for brachytherapy dose calculations in 3D geometry

In brachytherapy clinical practice, applicator shielding and tissue heterogeneities are usually not explicitly taken into account. None of the existing dose computational methods are able to reconcile accurate dose calculation in complex three-dimensional (3D) geometries with high efficiency and simplicity. We propose a new model that performs two-dimensional integration of the scattered dose component. The model calculates the effective primary dose at the point of interest and estimates the scatter dose as a superposition of the scatter contributions from pyramid-shaped minibeams. The approach generalizes a previous scatter subtraction model designed to calculate the dose for axial points in simple cylindrically symmetric geometry by dividing the scattering volume into spatial regions coaxial with the source-to-measurement point direction. To allow for azimuthal variation of the primary dose, these minibeams were divided into equally spaced azimuthally distributed pyramidal volumes. The model uses precalculated scatter-to-primary ratios (SPRs) for collimated isotropic sources. Effective primary dose, which includes the radiation scattered in the source capsule, is used to achieve independence from the source structure. For realistic models of the 192Ir HDR and PDR sources, the algorithm agrees with Monte Carlo within 2.5% and for the 125I type 6702 seed within 6%. The 2D scatter integration (2DSI) model has the potential to estimate the dose behind high-density heterogeneities both accurately and efficiently. The algorithm is much faster than Monte Carlo methods and predicts the dose around sources with different gamma-ray energies and differently shaped capsules with high accuracy.     

A calculation of the relative biological effectiveness of 125I and 103Pd brachytherapy sources using the concept of proximity function

The clinical application of encapsulated radioactive sources in brachytherapy plays an important role in the treatment of malignancy. 125I and 103Pd sources have been widely used in the permanent implant of prostate cancer. An important consideration for the choice of brachytherapy sources is their relative biological effectiveness (RBE). Previous calculations of this quantity have used the dose-averaged lineal energy, yD, as a measure of biological effectiveness. In this approach, however, the selection of a relevant site size remains an open question. Here we avoid this problem by using the generalized theory of dual radiation action to calculate the initial slope, alpha, of the dose-effect curves using the proximity function, t(x), and the biological response function, gamma(x). At low doses and/or low dose rates (e.g., prostate implants) the parameter alpha determines the RBE. Proximity function, t(x), is the probability distribution function of distances between pairs of sublesions; and the biological function, gamma(x), is the probability that two sublesions at a distance x apart results in a lesion. Functions t(x) have been calculated for each source using the Monte Carlo transport codes PHOEL and PROTON5. The function gamma(x) has been taken from a published analysis. The RBE values thus obtained are: 1.5 for 125I and 1.6 for 103Pd. The question of whether an "effective" site size exists where yD approximates best the variation of alpha with radiation quality is also addressed.     

Calculated dosimetric parameters of the IoGold 125I source model 3631-A

Basic dosimetric parameters as recommended by the AAPM Task Group No. 43 (TG-43) have been determined for recently available IoGold 125I brachytherapy seeds. Monte Carlo methods (MCNP) were used in the calculation of these parameters in water, and results compared with soon to be published experimental parameters also for 125I IoGold seeds as well with parameters for model 6702 and 6711 125I seeds. These parameters were the radial dose function, anisotropy factor and constant, and the dose rate constant. Using MCNP, values for the radial dose function at 0.5, 2.0, and 5.0 cm were 1.053, 0.877, and 0.443, respectively. The anisotropy factor was 0.975, 0.946, 0.945, and 0.952 at 0.5, 1.0, 2.0, and 5.0 cm, respectively, with an anisotropy constant of 0.95. The IoGold dose rate constant was determined by excluding the low energy titanium characteristic x rays produced in the IoGold titanium capsule. Using this post TG-43 revised NIST air kerma methodology, the IoGold dose rate constant was 0.96 cGy h-1 U-1. These calculatively determined parameters for IoGold seeds were compared with those determined experimentally for IoGold seeds, and also compared with parameters determined for model 6702 and 6711 seeds as presented in TG-43.     

In-water calibration of PDR 192Ir brachytherapy sources with an NE2571 ionization chamber

An ionometric calibration procedure for 192Ir PDR brachytherapy sources in terms of dose rate to water is presented. The calibration of the source is performed directly in a water phantom at short distances (1.0, 2.5 and 5.0 cm) using an NE2571 Farmer type ion chamber. To convert the measured air-kerma rate in water to dose rate to water a conversion factor (CF) was calculated by adapting the medium-energy x-ray dosimetry protocol for a point source geometry (diverging beam). The obtained CF was verified using two different methods. Firstly, the CF was calculated by Monte Carlo simulations, where the source-ionization chamber geometry was modelled accurately. In a second method, a combination of Monte Carlo simulations and measurements of the air-kerma rate in water (at 1.0, 2.5 and 5.0 cm distance) and in air (1 m distance) was used to determine the CF. The obtained CFs were also compared with conversion factors calculated with the adapted dosimetry protocol for high-energy photons introduced by T?lli. All calculations were done for a Gammamed PDR 192Ir source-NE2571 chamber geometry. The conversion factors obtained with the four different methods agree to within 1% at the three distances of interest. We obtained the following values (medium-energy x-ray protocol): CF(1 cm) = 1.458; CF(2.5 cm) = 1.162; CF(5.0 cm) = 1.112 (1 sigma = 0.7% for the three distances of interest). The obtained results were checked with TLD measurements. The values of the specific dose rate constant and the radial dose function calculated in this work are in accordance with the literature data.     

Experimental determination of dosimetry functions of Ir-192 sources

Experimental data related to the dosimetric characteristics of Ir-192 brachytherapy sources are limited. The aim of this work was to obtain the dosimetry functions required by the American Association of Physicists in Medicine Task Group 43 for both a low and a high dose-rate iridium-192 brachytherapy source through dose measurements in a water-equivalent phantom. Dose measurements have been performed using lithium fluoride thermoluminescent detectors positioned in a polystyrene phantom at distances from the source that vary from 1 to 10 cm, with 1 cm intervals, and at angles that vary from 0 degree to 170 degrees with 10 degrees intervals. The anisotropy functions, radial dose functions, and dose rate constants were determined for both brachytherapy sources. The precision of results obtained on those relatively fine intervals of angles and distances provides clinics with the possibility to use and interpolate the complete data sets for treatment planning.     

A survey of physics and dosimetry practice of permanent prostate brachytherapy in the United States

PURPOSE: To obtain data with regard to current physics and dosimetry practice in transperineal interstitial permanent prostate brachytherapy (TIPPB) in the U.S. by conducting a survey of institutions performing this procedure with the greatest frequency. METHODS AND MATERIALS: Seventy brachytherapists with the greatest volume of TIPPB cases in 1995 in the U.S. were surveyed. The four-page comprehensive questionnaire included questions on both clinical and physics and dosimetry practice. Individuals not responding initially were sent additional mailings and telephoned. Physics and dosimetry practice summary statistics are reported. Clinical practice data is reported separately. RESULTS: Thirty-five (50%) surveys were returned. Participants included 29 (83%) from the private sector and 6 (17%) from academic programs. Among responding clinicians, 125I (89%) is used with greater frequency than 103Pd (83%). Many use both (71%). Most brachytherapists perform preplans (86%), predominately employing ultrasound imaging (85%). Commercial treatment planning systems are used more frequently (75%) than in-house systems (25%). Preplans take 2.5 h (avg.) to perform and are most commonly performed by a physicist (69%). A wide range of apparent activities (mCi) is used for both 125I (0.16-1.00, avg. 0.41) and 103Pd (0.50-1.90, avg. 1.32). Of those assaying sources (71%), the range in number assayed (1 to all) and maximum accepted difference from vendor stated activity (2-20%) varies greatly. Most respondents feel that the manufacturers criteria for source activity are sufficiently stringent (88%); however, some report that vendors do not always meet their criteria (44%). Most postimplant dosimetry imaging occurs on day 1 (41%) and consists of conventional x-rays (83%), CT (63%), or both (46%). Postimplant dosimetry is usually performed by a physicist (72%), taking 2 h (avg.) to complete. Calculational formalisms and parameters vary substantially. At the time of the survey, few institutions have adopted AAPM TG-43 recommendations (21%). Only half (50%) of those not using TG-43 indicated an intent to do so in the future. Calculated doses at 1 cm from a single 1 mCi apparent activity source permanently implanted varied significantly. For 125I, doses calculated ranged from 13.08-40.00 Gy and for 103Pd, from 3.10 to 8.70 Gy. CONCLUSION: While several areas of current physics and dosimetry practice are consistent among institutions, treatment planning and dose calculation techniques vary considerably. These data demonstrate a relative lack of consensus with regard to these practices. Furthermore, the wide variety of calculational techniques and benchmark data lead to calculated doses which vary by clinically significant amounts. It is apparent that the lack of standardization with regard to treatment planning and dose calculation practice in TIPPB must be addressed prior to performing any meaningful comparison of clinical results between institutions.     

Chronic lymphocytic leukemia presenting as pyogenic arthritis of the proximal interphalangeal joint

PURPOSE: To evaluate the results of 125I brachytherapy in colorectal cancers recurrent in the pelvis and paraortics. METHODS AND MATERIALS: From September 1989 to January 1997, 29 patients with colorectal adenocarcinoma recurrent in the pelvis or the paraortic nodes were treated intraoperatively with permanent 125iodine seed implantation at the James Cancer Center of The Ohio State University (OSU). All patients had undergone prior surgery; 72% had prior EBRT. The implanted residual tumor volume was microscopic in 38% and gross in 62%. The implanted area (median 25 cc) received a median minimal peripheral dose of 140 Gy to total decay. An omental pedicle was used to minimize irradiation of the bowel. Five patients received additional postimplant EBRT (20-50 Gy; median 30 Gy). RESULTS: The 1-, 2-, and 4-year actuarial local-regional control rates were 38%, 17%, and 17%, respectively, with a median time to local failure of 11 months (95% CI 10-12 months). The first manifestation of disease progression in 52 % of the patients was local-regional. In addition, 22 patients (75%) developed distant metastases. The 1-, 2-, and 4-year actuarial overall survival rates were 70%, 35%, and 21%, (median = 18 months; 95% CI: 14-22 months). Overall survival was better for patients smaller volume implants (p = 0.007), with a lower total activity implanted (p = 0.0003), with a smaller number of implanted sites (p = 0.004), and with microscopic residual disease (p = 0.01). Patients receiving additional EBRT also had a better prognosis (p = 0.005). Local tumor progression was the cause of death in 34% of the patients who have died at the time of this report and 56% died of distant metastases. Of the patients, 13 (45%) experienced 15 toxic events, including 3 patients (10%) with enteric fistula. Neuropathy was not observed. CONCLUSIONS: 125I brachytherapy can be successfully used for salvage in patients with recurrent colorectal cancer. Patients with isolated, microscopic, or minimal gross residual disease requiring small-volume implants and those receiving additional EBRT have a better prognosis. Postimplant EBRT is now routinely added, even for previously irradiated patients, in an attempt to improve local control. Compared to IOERT and IOHDR, 125I brachytherapy is not associated with clinical neuropathy, probably due to the continuous low dose rate irradiation delivered by the 125I seeds.     

Identification of patients at increased risk for prolonged urinary retention following radioactive seed implantation of the prostate

PURPOSE: Urinary retention is a frequently reported complication following radioactive seed implantation of the prostate. If retention is refractory, a post-implant transurethral prostatic resection may ultimately be required to relieve obstruction, leading to an increased risk of urinary incontinence. In this series the incidence of prolonged urinary retention was determined, and the effect of pretreatment and treatment related factors was analyzed to identify high risk patients. MATERIALS AND METHODS: A total of 251 patients with organ confined prostate carcinoma underwent transperineal prostate seed implantation. Of the patients 114 were implanted with 103palladium (103Pd) and 137 with 125iodine seeds. Of the patients who were implanted with 103Pd 90 received 3 months of neoadjuvant hormonal therapy. All patients had International Prostate Symptom Scores (I-PSS) recorded before implantation to assess the degree of urinary symptoms. In the patients receiving neoadjuvant hormones prostate volumes and I-PSS were recorded before initiation of hormone treatment and 3 months later at the time of implant. RESULTS: Urinary retention developed in 14 patients requiring catheterization for more than 48 hours. Median time to onset was 1 day after implant. Of these patients 6 ultimately required transurethral prostatic resection to relieve urinary obstruction. No patient had urinary incontinence following implantation or transurethral prostatic resection. Multivariate analysis revealed that pretreatment I-PSS, and combined treatment with hormonal therapy and 103Pd predicted for the development of retention. Patients with I-PSS 20 or greater had a 29% risk, I-PSS 10 to 19, 11% risk and I-PSS less than 10, 2% risk of retention. Neither patient age, clinical stage, prostate specific antigen, Gleason score, use of 125I nor prostate volume was significant. A subgroup analysis of patients receiving hormonal therapy and 103Pd revealed that those with persistent urinary symptoms (I-PSS 10 or greater) following 3 months of hormonal therapy had the greatest risk of prolonged retention (37%). CONCLUSIONS: The overall risk of prolonged urinary retention following prostate implantation was low in our series. Using the I-PSS questionnaire, high risk patients can be identified before treatment. Patients with significant pretreatment urinary symptoms or persistent urinary symptoms following 3 months of hormonal therapy and then implantation with 103Pd have the greatest risk.     

Validation of Monte Carlo dose calculations near 125I sources in the presence of bounded heterogeneities

PURPOSE: Dose distributions around low energy (< 60 keV) brachytherapy sources, such as 125I, are known to be very sensitive to changes in tissue composition. Available 125I dosimetry data describe the effects of replacing the entire water medium by heterogeneous material. This work extends our knowledge of tissue heterogeneity effects to the domain of bounded tissue heterogeneities, simulating clinical situations. Our goals are three-fold: (a) to experimentally characterize the variation of dose rate as a function of location and dimensions of the heterogeneity, (b) to confirm the accuracy of Monte Carlo dose calculation methods in the presence of bounded tissue heterogeneities, and (c) to use the Monte Carlo method to characterize the dependence of heterogeneity correction factors (HCF) on the irradiation geometry. METHODS AND MATERIALS: Thermoluminescent dosimeters (TLD) were used to measure the deviations from the homogeneous dose distribution of an 125I seed due to cylindrical tissue heterogeneities. A solid water phantom was machined accurately to accommodate the long axis of the heterogeneous cylinder in the transverse plane of a 125I source. Profiles were obtained perpendicular to and along the cylinder axis, in the region downstream of the heterogeneity. Measurements were repeated at the corresponding points in homogeneous solid water. The measured heterogeneity correction factor (HCF) was defined as the ratio of the detector reading in the heterogeneous medium to that in the homogeneous medium at that point. The same ratio was simulated by a Monte Carlo photon transport (MCPT) code, using accurate modeling of the source, phantom, and detector geometry. In addition, Monte Carlo-based parametric studies were performed to identify the dependence of HCF on heterogeneity dimensions and distance from the source. RESULTS: Measured and calculated HCFs reveal excellent agreement (< or = 5% average) over a wide range of materials and geometries. HCFs downstream of 20 mm diameter by 10 mm thick hard bone cylinders vary from 0.12 to 0.30 with respect to distance, while for an inner bone cylinder of the same dimension, it varies from 0.72 to 0.83. For 6 mm diameter by 10 mm thick hard bone and inner bone cylinders, HCF varies 0.27-0.58 and 0.77-0.88, respectively. For lucite, fat, and air, the dependence of HCF on the 3D irradiation geometry was much less pronounced. CONCLUSION: Monte Carlo simulation is a powerful, convenient, and accurate tool for investigating the long neglected area of tissue composition heterogeneity corrections. Simple one dimensional dose calculation models that depend only on the heterogeneity thickness cannot accurately characterize 125I dose distributions in the presence of bone-like heterogeneities.     

The Baylor College of Medicine experience with gold seed implantation

Advances in imaging technology and implant technique have led to the resurgent interest and practice of brachytherapy for the treatment of prostate cancer. Brachytherapy is a form of radiation treatment in which radioactive sources are placed directly into the tumor; it offers the advantage of maximizing the radiation dose delivered to the tumor while sparing the adjacent normal tissue. Permanent implants have become an important component of radiation delivery. Interstitial gold radioisotope (Au-198) implants for prostate cancer were introduced at Baylor College of Medicine in 1965. The rationale for using Au-198, instead of the two most commonly used radioisotopes, Palladium-103 (Pd-103) and Iodine-125 (I-125), is discussed, and the Baylor implant technique is compared to that used in other centers. Retrospective review divides the patient population into pre-ultrasound versus post-ultrasound eras. Dosimetric calculation and disease control with the Au-198 seed implant for prostatic cancer are reviewed for the two different eras; toxicity is evaluated in the post-ultrasound era only. In the pre-ultrasound era, 510 patients were treated with pelvic lymph node sampling and gold seed insertion of the prostate followed by external beam radiation. In the post-ultrasound era, 54 patients were treated definitively with ultrasound-guided transperineal Au-198 implant followed by external beam irradiation. A small group of 30 patients in the post-ultrasound era were evaluated for the efficacy of Au-198 re-implantation for locally recurrent disease.     

Clinical evaluation of an interstitial remote afterloading device for multichannel intracavitary irradiation

PURPOSE: The purpose of this report is to describe the clinical implementation and evaluation of an interstitial remote afterloading device for multichannel intracavitary brachytherapy. METHODS AND MATERIALS: Two 15-channel low dose rate devices were adapted for use with Fletcher-Suit tandem and ovoids and Simon-Heyman capsules. The technical records for 103 intracavitary brachytherapy procedures performed from February 1989 through February 1991 were reviewed. RESULTS: Isodose distributions from fixed source trains for Microselectron low dose rate gynecologic applicators closely approximate standard manual afterloading sources and applicators. Device malfunctions occurred in 51% (53 out of 103) of the procedures. Malfunctions by applicator type were 70% (51 out of 73) for tandem, ovoids, and capsules, 12% (2 out of 17) for tandem and ovoids, and none (0 out of 13) for ovoids only. The most common malfunction occurred during source transfer. Total implant time was prolonged 0 to 4% by malfunction and 10% by patient care interruptions, depending on applicator type. CONCLUSION: The adaptation of the Microselectron device for multichannel gynecologic intracavitary brachytherapy results in similar dose distributions as standard manual after loading sources and a decreased radiation exposure to nursing personnel. The system has a high rate of malfunctions but a low overall prolongation of implant time due to malfunction.     

Optimization of permanent 125I prostate implants using fast simulated annealing

PURPOSE: Treatment planning of ultrasound-guided transperineal 125I permanent prostatic implants is a time-consuming task, due to the large number of seeds used and the very large number of possible source arrangements within the target volume. The goal of this work is to develop an algorithm based on fast simulated annealing allowing consistent and automatic dose distribution optimization in permanent 125I prostatic implants. METHODS AND MATERIALS: Fast simulated annealing is used to optimize the dose distribution by finding the best seed distribution through the minimization of a cost function. The cost function includes constraints on the dose at the periphery of the planned target volume and on the dose uniformity within this volume. Adjustment between peripheral dose and the dose uniformity can be achieved by varying the weight factor in the cost function. RESULTS: Fast simulated annealing algorithm finds very good seed distributions within 20,000 iterations. The computer time needed for the optimization of a typical permanent implant involving 60 seeds and 14 needles is approximately 15 min. An additional 5 min are necessary for isodose distribution computations and miscellaneous outputs. CONCLUSION: The use of fast simulated annealing allows for an efficient and rapid optimization of dose distribution. This algorithm is now routinely used at our institution in the clinical planning of 125I permanent transperineal prostate implants for early stage prostatic carcinoma.     

Factors affecting mandibular complications in low dose rate brachytherapy for oral tongue carcinoma with special reference to spacer

PURPOSE: To evaluate the efficacy of a spacer in the prevention of mandibular complications in low dose rate (LDR) brachytherapy (BRT) for oral tongue carcinoma. METHODS AND MATERIALS: A retrospective analysis was conducted using 103 patients with T1 or T2 tongue carcinoma treated by a single plane implantation of iridium (192Ir) pins between 1979-1994. Of these patients, 60 were treated by BRT alone, and the rest were combined with external irradiation (Ext) and/or chemotherapy (CHT). Forty-eight and 55 patients were given BRT with and without a spacer, respectively. Spacers were individually made of acrylic resin according to a prosthetic technique so as to obtain the thickness of 7-10 mm at the lingual part of the implanted side. Variables, including a spacer, which may be associated with the development of osteoradionecrosis (ORN) of the mandible, were analyzed by the Cox proportional hazards regression analysis. RESULTS: Our spacer reduced about 50% of the absorbed dose at the lingual side surface of the lower gingiva (LSG) to that in the absence of a spacer. Absolute incidence of ORN was 2.1% (1 of 48) and 40.0% (22 of 55), with and without a spacer, respectively, and the difference was statistically significant by univariate analysis (p = 0.0004). It was revealed by the Cox analysis that the spacer (p = 0.0247), combined CHT (p = 0.0295), and combined Ext (p = 0.0279) were significant independent factors associated with the development of ORN. The spacer was shown to be a significant factor by univariate analysis (p = 0.0037), but not by multivariate analysis when analysis was restricted to the patients who did not receive CHT. The absorbed dose, dose rate, and biological effective dose (BED) reflecting early or late response were estimated at the LSG, and prognosticators associated with the incidence of ORN were also determined by the Cox analysis. Particularly, BED for late response by BRT, the total absorbed dose, and any BED by Ext plus BRT were highly significant factors in the whole population. Essentially similar results were obtained in the patients without receiving CHT. CONCLUSIONS: It was clarified that our spacer effectively prevents mandibular complications in LDR BRT by 192Ir for oral tongue carcinoma. Furthermore, introduction of a spacer provided novel information concerning the development of ORN, where BED particularly for late response given by BRT, the total absorbed dose, and any BED by Ext plus BRT could be good prognostic factors only when estimated at the LSG.     

Computer-assisted CT-guided stereotactic biopsy and brachytherapy of brain tumors

From March 1991 to September 1993, 26 patients (aged 4-78 years) with brain tumors (4 glioblastoma multiforme, 10 nonglioblastoma multiforme, 1 mixed oligoastrocytoma, 2 carniopharyngiomas, 2 meningiomas and 7 metastases) were treated with stereotactic techniques at the Centro Internacional de Restauración Neurológica, La Habana, Cuba. A total of 28 stereotactic surgical procedures were performed with no operative mortality; they included biopsies in all cases, 1 stereotactic microsurgical resection and 12 permanent implants of 192Ir, followed by external beam fractionated radiation therapy (40-60 Gy). The present paper shows that the combined use of a stereotactic approach, a comprehensive and reliable stereotactic dosimetric planning system, stereotactic brachytherapy with 192Ir and complementary percutaneous radiation treatment constitutes a promising strategy for brain tumor management.     

Influence of timing on the dosimetric analysis of transperineal ultrasound-guided, prostatic conformal brachytherapy

Postoperative computed tomography (CT)-based dosimetric analysis of transperineal ultrasound-guided conformal prostate brachytherapy provides detailed information regarding the coverage and uniformity of the implant. However, there is no generally accepted standard for the optimal timing of the postoperative dosimetry. This report details dosimetric analysis and the effect of timing based upon CT and orthogonal film evaluation for ten unselected patients implanted with either iodine-125 (125I) or palladium-103 (103Pd). Within 2 hours after implantation, patients underwent a CT scan and the first of four sequential sets of orthogonal films. Subsequent orthogonal films were obtained on days 3, 14, and 28 postimplant. CT-based dosimetry revealed coverage of the prostate to the prescribed minimal peripheral dose (mPD) at 93.1 +/- 3.6% of the volume, the prostate volume receiving 150% of mPD was 38.2 +/- 8.7%, and the urethral and rectal doses were 114 +/- 12% and 78 +/- 19% of mPD, respectively. The implanted seeds seen on orthogonal films acted as markers for temporal changes in prostate dimensions, and the standard deviation of each dimension was used as input in an ellipsoidal volume calculation. Seed coordinates were self normalized to the center of gravity of each two-dimensional view and were measured relative to the linear regression line in the superior-inferior direction. The reproducibility of the anteroposterior (AP) film setup in terms of temporal variation in the angle of the regression line was markedly better than that of the lateral films, 1.8 degrees +/- 1.2 degrees vs. 4.3 degrees +/- 2.6 degrees, respectively. Dimensional contraction from day 0 to day 28 averaged 11.3% in the superior-inferior direction, 8.5% in the AP/PA (posteroanterior) direction, and 2.5% in the right-left lateral direction. This translated into a volume change of 20.9% (ranged 11.6-31.6%), which was determined by using the ellipsoid method. The half-life for edema resolution was 10.6 +/- 1.8 days (range 8.6-14.3 days). However, because of variability in the degree and extent of edema and its rate of resolution, we believe that it may be futile to define a single point in time as the most accurate indicator of the postoperative dose distribution. Rather, it may be preferable to accept universal standardization of timing and methodology for CT-based postoperative dosimetry, which would facilitate comparison of results between centers and maximize the information content of that single measurement. We conclude that day 0 represents the optimal time, because dosimetric evaluation at that time minimizes patient discomfort and inconvenience (a catheter is already in place), provides information about edema when it is near its maximum extent, and provides prompt closure of the learning loop and, as such, hopefully will result in improved implantation techniques and results.