A comparison of HDR brachytherapy and IMRT techniques for dose escalation in prostate cancer: a radiobiological modeling study.

A course of one to three large fractions of high dose rate (HDR) interstitial brachytherapy is an attractive alternative to intensity modulated radiation therapy (IMRT) for delivering boost doses to the prostate in combination with additional external beam irradiation for intermediate risk disease. The purpose of this work is to quantitatively compare single-fraction HDR boosts to biologically equivalent fractionated IMRT boosts, assuming idealized image guided delivery (igIMRT) and conventional delivery (cIMRT). For nine prostate patients, both seven-field IMRT and HDR boosts were planned. The linear-quadratic model was used to compute biologically equivalent dose prescriptions. The cIMRT plan was evaluated as a static plan and with simulated random and setup errors. The authors conclude that HDR delivery produces a therapeutic ratio which is significantly better than the conventional IMRT and comparable to or better than the igIMRT delivery. For the HDR, the rectal gBEUD analysis is strongly influenced by high dose DVH tails. A saturation BED, beyond which no further injury can occur, must be assumed. Modeling of organ motion uncertainties yields mean outcomes similar to static plan outcomes.

Anniversary paper: fifty years of AAPM involvement in radiation dosimetry.

This article reviews the involvement of the AAPM in various aspects of radiation dosimetry over its 50 year history, emphasizing the especially important role that external beam dosimetry played in the early formation of the organization. Topics covered include the AAPM's involvement with external beam and x-ray dosimetry protocols, brachytherapy dosimetry, primary standards laboratories, accredited dosimetry chains, and audits for machine calibrations through the Radiological Physics Center.

Respiratory regularity gated 4D CT acquisition: concepts and proof of principle.

Four-dimensional CT images are generally sorted through a post-acquisition procedure correlating images with a time-synchronized external respiration signal. The patient's ability to maintain reproducible respiration is the limiting factor during 4D CT, where artifacts occur in approximately 85% of scans with current technology. To reduce these artifacts and their subsequent effects during radiotherapy planning, a method for improved 4D CT image acquisition that relies on gating 4D CT acquisition based on the real time monitoring of the respiration signal has been proposed. The respiration signal and CT data acquisition are linked, such that data from irregular breathing cycles, which cause artifacts, are not acquired by gating CT acquisition by the respiratory signal. A proof-of-principle application of the respiratory regularity gated 4D CT method using patient respiratory signals demonstrates the potential of this method to reduce artifacts currently found in 4D CT scans. Numerical simulations indicate a potential reduction in motion within a respiratory phase bin by 20-40% depending on tolerances chosen. Additional advantages of the proposed method are dose reduction by eliminating unnecessary oversampling and obviating the need for post-processing to create the 4D CT data set.

Indels and imperfect duplication have driven the evolution of human Complement Receptor 1 (CR1) and CR1-like from their precursor CR1 alpha: importance of functional sets.

This study examines the effects of duplication and insertions-deletions (indels) by comparing human complement receptor 1 (CR1) and human CR1-like (CR1L) with syntenic genes from four other vertebrates (chimpanzee, baboon, rat, and mouse). By phylogenetic analysis, the domains of these genes can be classified into 10 distinct subfamilies (a, b, c, d, e, f, g(-like), h, j, and k), which have been largely conserved throughout vertebrate and invertebrate evolution. In spite of many complex and diverse duplications and indels, the subfamily order of domains (a, j, e, f, b, k, d, g(-like)) has been maintained. The number of domain sets has increased progressively, thereby expanding the functional repertoire.

Dependence of radiochromic film response kinetics on fractionated doses.

Because the rate and magnitude of the post-exposure growth of the MD-55 radiochromic film (RCF) dosimeter response depends significantly on total dose, we have investigated the influence of fractionation and protracted dose delivery on optical density (OD) as a function of total dose and the exposure-to-densitometry time interval for a 633-nm scanning laser densitometer. Both measurements and models demonstrate that fractionation induces transient OD over responses, which can be as large as 20%, but rapidly dissipate within 24 h. However, the superposition model predicts 2-5% over responses that persist as long as 700 h.

Dose calculations about shielded gynecological colpostats.

Although shielded gynecological colpostats have been shown experimentally to reduce doses to bladder and rectal tissue by as much as 50%, nearly all previously described dose computation algorithms ignore applicator heterogeneities. We describe the use of realistic Monte Carlo calculations to study the dosimetric effects of applicator structure. Use of sophisticated solid modeling techniques allows the complex internal structure of two commercially-available Fletcher-Suit colpostats, as well as that of 226Ra or 137Cs tubes, to be accurately simulated. Our results show significant differences among these source-applicator combinations. In addition, a novel dose computation algorithm for efficiently estimating absorbed dose near shielded applicators is described. Our approach is based upon empirical separation of primary- and scatter-dose components. The algorithm requires a small base of Monte Carlo-generated data, reproduces the Monte Carlo dose estimates within 3%, and is faster than Monte Carlo by a factor of 15,000. The scatter-separation method has the potential to make accurate dose estimates to bladder, rectum, tumor, and vagina available for clinical treatment planning and for extraction of more meaningful dose-response curves from clinical data.

The accuracy of the line and point source approximations in Ir-192 dosimetry.

The dosimetric approximations used in computer-aided treatment planning of Ir-192 seed implants generally ignore individual seed dimensions and internal structure. Most commonly, each seed is approximated by an isotropic point source. Alternately, each ribbon assembly consisting of uniformly spaced seeds is replaced by an unfiltered line source. Using filtration corrections applicable to platinum- and steel-encapsulated seeds calculated by the Monte Carlo method, the dosimetric errors introduced by these models into two-and-three-dimensional dose distributions arising from multiple plane implants are analyzed. Our results demonstrate that when anisotropy correction factors of 0.96 and 0.99 are used for platinum- and steel-filtered seeds, respectively, the point source model is accurate within 2%. The accuracy of the line source approximation depends significantly upon the details of its implementation. If the linear density of the line is set equal to individual seed strength for an inter-seed spacing of 1 cm, and filtration correction factors of 0.94 and 0.97 are used for platinum- and steel-clad seeds, respectively, the accuracy of the line source model is 1.5% near the implant center. The method of dose-volume histograms is used to compare the predictions of the different models.

Monte Carlo and analytic calculation of absorbed dose near 137Cs intracavitary sources.

This paper presents dose-rate tables and treatment planning data needed to accurately implement the Sievert line-source integral, found on most commercial computer-aided treatment planning systems, for two recently introduced 137Cs intracavitary sources. One source uses a high-density active core designed to reproduce the non-elliptical isodose curves characteristic of the traditional radium tube. The other source consists of two or three discrete 137Cs seeds encapsulated in stainless steel. Using Monte Carlo dose-rate calculations as the standard of accuracy, we show that the Sievert model, using conventionally defined filtration corrections, overestimates kerma-rate in free space by as much as 20%. In addition, tissue attenuation and scatter build-up factors, derived from an isotropic point source, do not accurately characterize the distribution of scatter dose about heavily filtered sources. By varying the input parameters of the Sievert line-source integral so as to optimize its agreement with the more rigorous Monte Carlo data, accuracy of 3% can be achieved.

The Sievert integral revisited: evaluation and extension to 125I, 169Yb, and 192Ir brachytherapy sources.

PURPOSE: The goal of this study is to assess the accuracy of the Sievert integral dose calculation model for medium and low-energy brachytherapy sources (photon energy: 25-500 keV). A simple modification of the basic model, the isotropic scattering correction, is proposed that significantly improves its accuracy. METHODS AND MATERIALS: Both the classical model and revised Sievert algorithms were tested against 2D dose distributions derived from Monte Carlo photon transport (MCPT) calculations for the following sources: a 169Yb interstitial source, pulsed and high dose rate 192Ir sources and the model 6702 125I source. The Sievert model was implemented as a 3D numerical integral over the radioactivity distribution and included photon attenuation and scattering by the surrounding medium. The Sievert filtration coefficients were approximated by linear energy absorption coefficients, parameters of best fit, and curve fits to simulated open-air transmission measurements. The revised model consists of using the Sievert integral only to calculate the primary dose distribution using contact absorber filtration coefficients. The dose component due to photon scattering in the medium is assumed to be isotropically distributed and is modeled by point-source scatter-to-primary dose ratios. RESULTS: The classical Sievert integral produces maximum and RMS average dose-calculation errors ranging from -53 to -20% and 3 to 19%, respectively. In contrast, the revised model reproduces the MCPT dose distribution with maximum and RMS mean errors ranging from 5 to 13% and 1 to 6%, respectively. CONCLUSIONS: The classical Sievert model fails to accurately describe brachytherapy dose distributions around heavily filtered sources emitting photons with average energies of 28 to 400 keV. The revised Sievert model accurately models single-source dose distributions for a wide range of sources, using well-defined filtration coefficients and scatter ratios that can be measured or calculated without knowledge of the final dose distribution. The model is potentially useful as a single-source dose-array generator for clinical treatment planning in the low energy domain.

Monte Carlo-based dose-rate tables for the Amersham CDCS.J and 3M model 6500 137Cs tubes.

PURPOSE: (1) To present reference-quality dose-rate distributions for the Amersham CDCS.J-type 137Cs intracavitary source (hitherto unavailable in the literature) and updated tables for the 3M model 6500/6D6C source. (2) To assess the accuracy of the widely used 1D pathlength (Sievert integral) algorithm for lightly filtered 137Cs tube sources. METHODS AND MATERIALS: A Monte Carlo photon-transport code is used to calculate the dose-rate distributions about the 3M source and the CDCS.J source based on radiographic examination of the sources and the vendors' specifications. Dose-rate distributions are provided in the form of Cartesian "away-and-along" lookup tables. Using a general form of the Sievert integral, calculated dose-rate distributions were compared to the Monte Carlo benchmark calculations treating the filtration coefficients as best-fit parameters as well as approximating them by linear energy absorption coefficients. In addition, the errors introduced by approximating the active source core by uniform cylinders or line sources was evaluated. RESULTS: The Model CDCS.J dose distribution differs from that of the 3M model 6500 source by -5.9% to +14.4% (root-mean-square [RMS] average: 2.6%). The RMS accuracy of the Sievert algorithm is 2.4% to 2.8% (error range of -1.4% to 7.6%) when filtration coefficients for steel and ceramic media are approximated by linear energy absorption coefficients. If the filtration coefficients are treated as parameters of best fit, selected to minimize the discrepancies between 1D pathlength and Monte Carlo calculations, the RMS error is reduced to 0.8% (error range of -1.8% to 4.1%). The optimal values of stainless steel and low-density ceramic or glass filtration coefficients are approximately independent of the source geometry. CONCLUSIONS: The widely used Sievert integral algorithm accurately characterizes the dose distribution around stainless-steel clad low-density matrix 137Cs sources, particularly if design-independent best-fit values of the filtration coefficients are used. Although both families of source designs studied produce similar dose distributions, source-design specific dose distributions should be used for clinical treatment planning and dose-algorithm validation.

Dosimetric analysis of stereo and orthogonal reconstruction of interstitial implants.

We have evaluated the performance of stereo and orthogonal computerized three-dimensional implant reconstruction algorithms. Sets of orthogonal and stereo radiographs of a simulated interstitial seed implant were taken. Computer reconstructed coordinates of each source were compared with their measured positions in order to assess the geometric accuracy of the two methods. We find that the effect of random measurement errors on reconstructed coordinates from stereo films is 3-5 times that of the orthogonal method. Although relative individual seed positions may be in error by as much as 1 cm, these errors do not significantly change the shape or area of isodose curves surrounding the implant periphery.

Anatomic variation of gynecologic brachytherapy prescription points.

PURPOSE: The purpose of this report is to evaluate the geometric movement (relative to the bony pelvis) of fixed brachytherapy reference points during the time interval of the first and second gynecologic intracavitary implant. METHODS AND MATERIALS: The radiation therapy records of 40 consecutive patients with all stages of carcinoma of the cervix treated at the Radiation Oncology Center, Mallinckrodt Institute of Radiology, from January 1991 through December 1991 were reviewed. All patients received external beam irradiation and two intracavitary implants with Fletcher-Suit tandem and ovoids. Prescription points, per ICRU #38, were used: point A, B, and P, bladder, and rectum. Comparison of the location of points for the first and second implants (2 weeks time interval) was performed using the bony pelvis as reference anatomy and calculating the 3-dimensional spherical coordinates from a common origin for both implants. RESULTS: Analysis of the movement of the second implant relative to the first revealed that the x, y, and z coordinates for the various points shifted from 0 to 6 mm in a single plain. Vector analysis showed that the magnitude and direction of the average shift was on the order of 1.0 to 1.5 cm with displacement posteriorly and inferiorly. The effect of this movement on dose rates to the various points can result in dose rate differences up to 35%. A regression analysis was performed to identify factors affecting this movement. The time interval from the first to the second implant was the factor which correlated best with the movement. CONCLUSION: There is movement of the absolute position of ICRU #38 reference points between the first and second intracavitary implants. This movement results in significant differences in absolute dose rates to these reference points in the two implants. The clinical significance of this movement relative to dose to the tumor is unknown.

Source configuration and dose rates for the Selectron afterloading equipment for gynecologic applicators.

This paper describes source train configurations of the Selectron LDR-6 that can be used to duplicate commonly used manually afterloadable source configurations for gynecological intracavitary brachytherapy. We demonstrate that the recommended source configuration using 2.5 mm 137Cs spheres is dosimetrically equivalent to its manually afterloaded counterparts using 3M 137Cs sources (20 mm overall length, 14 mm active length). Isodose distributions are in agreement to within 5% for the Fletcher-Suit-Delclos family of tandems, colpostats, and vaginal cylinders. Our data also demonstrate that the isotropic point source model used by most computerized treatment planning systems is a good approximation to source trains of filtered, spherical sources interspersed with solid steel spacers. We conclude that the use of 2.5 mm 137Cs sources in the Selectron remote afterloading system can accurately reproduce the isodose distribution achieved with linear 137Cs manually afterloaded sources.

Pre-installation empirical testing of room shielding for high dose rate remote afterloaders.

PURPOSE: Many facilities are acquiring high dose rate remote afterloading units. It is economical that these units be placed in existing shielded teletherapy rooms. Scatter-radiation barriers marginally protect uncontrolled areas from a high dose rate source especially in a room that houses a non-dynamic Cobalt-60 unit. In addition the exact thickness and material composition of the barriers are unknown and therefore, a calculation technique may give misleading results. Also, it would be impossible to evaluate an entire wall barrier by taking isolated core samples in order to assist in the calculations. A quick and inexpensive measurement of dose equivalent using a rented high activity 192Ir source evaluates the barriers and locates shielding deficiencies. METHODS AND MATERIALS: We performed transmission calculations for primary and scattered radiation based on National Council on Radiation Protection and Measurements Reports 49 and 51, respectively. We then rented a high activity 21.7 Ci (8.03 x 10(11) Bq) Ir-192 source to assess our existing teletherapy room shielding for adequacy and voids. This source was placed at the proposed location for clinical high dose rate treatment and measurements were performed. RESULTS: No deficiencies were found in controlled areas surrounding the room, but large differences were found between the calculated and measured values. Our survey located a region in the uncontrolled area above the room requiring augmented shielding which was not predicted by the calculations. A canopy shield was designed to potentially augment the shielding in the ceiling direction. CONCLUSION: Pre-installation testing by measurement is an invaluable method for locating shielding deficiencies and avoiding unnecessary enhancement of shielding particularly when there is lack of information of the inherent shielding.

Estimation of tissue volume irradiated by intracavitary implants.

PURPOSE: The volume of space enclosed by a specified isodose surface arising from an intracavitary implant may correlate with clinical outcome. Several investigators have proposed using the product of the three maximum orthogonal dimensions of the isodose surface as a measure of this volume. We have examined the accuracy of this proposal and compared it to a simpler model for estimating volume which requires only knowledge of the mgRaEq-hrs (total reference air kerma) and the dose level. METHODS AND MATERIALS: Orthogonal films from 204 intracavitary implants of 128 patients with carcinoma of the cervix were used to reconstruct the 137Cs-source coordinates. The source location, strength and duration data were used to calculate dose-volume histograms, yielding the volume enclosed by each dose level as well as its orthogonal dimensions: thickness, width, and height. Using bony landmarks to align films for different insertions in the same patient, similar calculations were repeated for composite implant source coordinates. RESULTS: Curve-fitting techniques revealed that the volume encompassed by each isodose level could be predicted by a modified power-law function of the mgRaEq-hr/dose ratio: predicted volume = [104.8 - 8.103.(M/D) + 0.437.(M/D)2].(M/D)1.635 where M/D = mgRaEq-hr/cGy. The volume predicted by this simple model is accurate within +/- 10% in 95% of the implants when mgRaEq-hr/cGy = 0.8. Accuracy increases with increasing mgRaEq-hr/cGy. In contrast, the ratio, product of orthogonal dimensions/actual volume, varies widely from implant-to-implant, as well as differing systematically from one implant type to another. Investigation of the individual orthogonal dimensions demonstrated that width and height, but not thickness, were moderately well correlated with corresponding maximum implant dimensions. However, in all cases the dimensions were more sensitive to changes in mgRaEq-hr/cGy than to changes in implant geometry. CONCLUSIONS: The product of the orthogonal dimensions is an unsatisfactory estimator of the actual irradiated volume encompassed by an isodose surface. Isodose surface volumes can be accurately estimated knowing only mgRaEq-hr. Prescribing intracavitary brachytherapy by mgRaEq-hr, or its derivative, total reference air kerma, is equivalent to requiring that an isodose surface encompass a specified volume which does not depend on the implant geometry. Constraining the mgRaEq-hr delivered therefore serves to limit the volume of tissue irradiated to high doses.

High dose-rate induced temperature artifacts: thermometry considerations for simultaneous interstitial thermoradiotherapy.

PURPOSE: The goal of the present study was to investigate the effect of high dose-rate radiation on a flouroptic thermometry system commonly used during microwave hyperthermia. METHODS AND MATERIALS: Measurements were performed by placing the flouroptic thermometry sensors at distances of < or = 1.5, 5, 10, and 15 mm from a remote afterloading high dose-rate 192Ir source in a water bath (at two different temperatures) and in a tissue equivalent radiation bolus medium. A simulated volumetric clinical setup using a radiation bolus medium was performed with thermometry sensors placed at 1.5, 7.5, 8.4, and 10.6 mm from a scanning high dose-rate source. RESULTS: It was found that high dose-rate radiation caused thermometry artifacts greater than 1.5 degrees C within 2 min for flouroptic thermometers placed 1.5 mm from a 5 Ci activity high dose-rate source. Simple calculations showed that artifacts of this magnitude could not be due to any heating caused by the energy deposited by the high dose-rate source. The artifact decayed, but was still evident 24 h after the exposure. The effect strongly depended on distance with a 0.7 degrees C artifactual increase in temperature seen for the probe 5 mm from the high dose-rate source. Moreover, experiments performed under conditions that represented a clinical setup with a 7 Ci high dose-rate source showed that for exposure times of 10 s, at distances of 1.5 mm, significant artifacts (> 0.5 degrees C) are produced. CONCLUSIONS: These findings indicate that high dose-rate-induced temperature artifacts should be taken into account in the quality assurance procedures for the treatment of patients with simultaneous interstitial thermoradiotherapy.

Dosimetry and dose specification for a new gynecological brachytherapy applicator.

A new afterloadable gynecological intracavitary applicator has been designed and is now in use for the treatment of a wide range of vaginal, cervical, and endometrial cancers in a single application, with a dose distribution that more closely matches the prescribed treatment than previous methods. The plexiglass applicator consists of a bulbous section that is inserted up to the vaginal apex and loaded with right and left ovoid sources. The distal portion is cylindrical with a central channel for tandem sources. A straight tandem is used when the target volume extends to the vaginal apex, but an intrauterine tandem can also be used. Extensive dosimetric and radiographic evaluations were performed to guide design refinements and to validate the surface dose predictions of the brachytherapy treatment planning system. The final applicator design delivers 110-120 cGy/hr to the vaginal apex surface and 95-100 cGy/hr to the distal vaginal surfaces when loaded with a 144.6 U (20 mgRaEq) cesium tube in each ovoid channel and 72.3, 72.3, and 144.6 U (10, 10, and 20 mgRaEq) cesium tubes in the vaginal cylinder channel. A system for treatment dose specification has been established that includes dose tables for manually calculating surface doses to reference points. A dose distribution comparison with a sequential colpostat-vaginal cylinder treatment demonstrates that dose delivery is more precise and uniform with this new applicator.

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.

Uterosacral space involvement in locally advanced carcinoma of the uterine cervix.

PURPOSE: Radiation therapy is the standard management for locally advanced cervical cancer, but it has not yielded fully satisfactory results; a relatively high incidence of local failure remains. Standard radiation therapy techniques combine external beam radiation and brachytherapy generating a homogeneously composite dose distribution covering the lateral parametria but may not be adequate in the uterosacral and perirectal areas due to the spatial arrangement of intracavitary system and the constraints of rectal tolerance. We hypothesize that these dosimetric characteristics might lead to a higher incidence of central/marginal failures when the uterosacral space is involved by locally advanced carcinoma of uterine cervix. METHODS AND MATERIALS: Between January 1970 and December 1989, 343 patients with clinical Stage IIIB cervical cancer were treated at the Mallinckrodt Institute of Radiology with radiation therapy alone. We identified 83 patients with clinical evidence of tumor in the uterosacral region; the remaining 260 patients either did not have uterosacral involvement or were unspecified. The dose of external beam irradiation ranged from 18.02 to 33.20 Gy to the central pelvis and 48.22 to 59.40 Gy to the lateral parametrium. The average total dose, including brachytherapy contribution, to point A and the lateral pelvis was 80.30 to 86.46 Gy and 60.50 to 73.40 Gy, respectively. External beam dose to the lateral parametria was, on average, 10 Gy higher in patients with uterosacral involvement. RESULTS: We categorized the patterns of pelvic failure into central/marginal (including medial parametrium) and lateral parametria. The cumulative incidence of central/marginal failure at 5 years was significantly higher in the group of patients with uterosacral involvement (36% compared with 21% for patients without uterosacral involvement or unspecified) (p = 0.002). Lateral parametrial failure was similar for patients with and without uterosacral involvement (39% and 38% at 5 years, respectively) (p = 0.42). The actuarial incidence of distant metastasis was identical in the two groups: 46% at 5 years. Multivariate analysis confirmed that uterosacral space involvement increased the risk of pelvic recurrence (p = 0.044) and was the most significant factor that influenced the central/marginal pelvic failure (p = 0.002). CONCLUSIONS: Uterosacral involvement by locally advanced carcinoma of the uterine cervix significantly increased overall pelvic failure and was the most significant prognosticator of central/marginal pelvic failure. This is the result of the spatial constraints of the standard intracavitary geometry that deliver inadequate dose posteriorly to encompass the uterosacral space. Plausible ways to compensate the underdose in the uterosacral space include increasing whole pelvis dose without compromising the intracavitary brachytherapy dose, using a supplemental interstitial implant or adding a posterior oblique external beam boost.

Dosimetric characteristics, air-kerma strength calibration and verification of Monte Carlo simulation for a new Ytterbium-169 brachytherapy source.

PURPOSE: Ytterbium-169 (169Yb) is a promising new isotope for brachytherapy with a half life of 32 days and an average photon energy of 93 KeV. It has an Ir-192-equivalent dose distribution in water but a much smaller half-value layer in lead (0.2 mm), affording improved radiation protection and customized shielding of dose-limiting anatomic structures. The goals of this study are to: (a) experimentally validate Monte Carlo photon transport dose-rate calculations for this energy range, (b) to develop a secondary air-kerma strength standard for 169Yb, and (c) to present essential treatment planning data including the transverse-axis dose-rate distribution and dose correction factors for a number of local shielding materials. METHODS AND MATERIALS: Several interstitial 169Yb sources (type 6) and an experimental high dose-rate source were made available for this study. Monte-Carlo photon-transport (MCPT) simulations, based upon validated geometric models of source structure, were used to calculate dose rates in water. To verify MCPT predictions, the transverse-axis dose distribution in homogeneous water medium was measured using a silicon-diode detector. For use in designing shielded applicators, heterogeneity correction factors (HCF) arising from small cylindrical heterogeneities of lead, aluminum, titanium, steel and air were measured in a water medium. Finally, to provide a sound experimental basis for comparing experimental and theoretical dose-rate distributions, the air-kerma strength of the sources was measured using a calibrated ion chamber. To eliminate the influence of measurement artifacts on the comparison of theory and measurement, simulated detector readings were compared directly to measured diode readings. The final data are presented in the format endorsed by the Interstitial Collaborative Working Group. RESULTS: The in-air calibration revealed that the air-kerma strength per unit activity (mCi), as quoted by the vendor, varied from 1.30 to 1.57 cGy.cm2/mCi.h depending on seed design. The maximum difference between measured and MCPT-simulated absolute diode readings on the transverse axis was less than 2%, indicating that MCPT accurately predicts dose rate in medium for brachytherapy sources in this energy range. Comparison of measured and simulated HCFs for each of the 16 different cylindrical heterogeneities demonstrated 1-3% agreement. The HCFs vary by as much as 200% with respect to distance and by as much as 48% as a function of disk diameter, showing that HCF is strongly dependent on heterogeneity location and lateral dimensions as well as thickness. The dose-rate constant for water medium was found to be 1.225 cGy/h per kerma unit air-strength and 1.962 cGy/h per unit mCi as measured by the vendor. CONCLUSION: Monte Carlo simulation is an accurate and powerful tool for dosimetric characterization of brachytherapy sources in this energy range. Thin lead foils produce shielding factors comparable to standard shielded applicators for 137Cs. Meaningful theoretical absolute dose calculations in brachytherapy require accurately implemented air-kerma strength standards.