Fletcher-Suit-Delclos gynecologic applicator: evaluation of a new instrument

Commissioning a CT-compatible LDR tandem and ovoid applicator using Monte Carlo calculation and 3D dosimetry

PURPOSE

To determine the geometric and dose attenuation characteristics of a new commercially available CT-compatible LDR tandem and ovoid (T&O) applicator using Monte Carlo calculation and 3D dosimetry.

METHODS

For geometric characterization, we quantified physical dimensions and investigated a systematic difference found to exist between nominal ovoid angle and the angle at which the afterloading buckets fall within the ovoid. For dosimetric characterization, we determined source attenuation through asymmetric gold shielding in the buckets using Monte Carlo simulations and 3D dosimetry. Monte Carlo code MCNP5 was used to simulate 1.5 × 10(9) photon histories from a (137)Cs source placed in the bucket to achieve statistical uncertainty of 1% at a 6 cm distance. For 3D dosimetry, the distribution about an unshielded source was first measured to evaluate the system for (137)Cs, after which the distribution was measured about sources placed in each bucket. Cylindrical PRESAGE(®) dosimeters (9.5 cm diameter, 9.2 cm height) with a central channel bored for source placement were supplied by Heuris Inc. The dosimeters were scanned with the Duke Large field of view Optical CT-Scanner before and after delivering a nominal dose at 1 cm of 5-8 Gy. During irradiation the dosimeter was placed in a water phantom to provide backscatter. Optical CT scan time lasted 15 min during which 720 projections were acquired at 0.5° increments, and a 3D distribution was reconstructed with a (0.05 cm)(3) isotropic voxel size. The distributions about the buckets were used to calculate a 3D distribution of transmission rate through the bucket, which was applied to a clinical CT-based T&O implant plan.

RESULTS

The systematic difference in bucket angle relative to the nominal ovoid angle (105°) was 3.1°-4.7°. A systematic difference in bucket angle of 1°, 5°, and 10° caused a 1% ± 0.1%, 1.7% ± 0.4%, and 2.6% ± 0.7% increase in rectal dose, respectively, with smaller effect to dose to Point A, bladder, sigmoid, and bowel. For 3D dosimetry, 90.6% of voxels had a 3D γ-index (criteria = 0.1 cm, 3% local signal) below 1.0 when comparing measured and expected dose about the unshielded source. Dose transmission through the gold shielding at a radial distance of 1 cm was 85.9% ± 0.2%, 83.4% ± 0.7%, and 82.5% ± 2.2% for Monte Carlo, and measurement for left and right buckets, respectively. Dose transmission was lowest at oblique angles from the bucket with a minimum of 56.7% ± 0.8%, 65.6% ± 1.7%, and 57.5% ± 1.6%, respectively. For a clinical T&O plan, attenuation from the buckets leads to a decrease in average Point A dose of ∼3.2% and decrease in D(2cc) to bladder, rectum, bowel, and sigmoid of 5%, 18%, 6%, and 12%, respectively.

CONCLUSIONS

Differences between dummy and afterloading bucket position in the ovoids is minor compared to effects from asymmetric ovoid shielding, for which rectal dose is most affected. 3D dosimetry can fulfill a novel role in verifying Monte Carlo calculations of complex dose distributions as are common about brachytherapy sources and applicators.

On the validity of the superposition principle in dose calculations for intracavitary implants with shielded vaginal colpostats

Intracavitary vaginal applicators typically incorporate internal shielding to reduce dose to the bladder and rectum. While dose distributions about a single colpostat have been extensively measured and calculated, these studies neglect dosimetric perturbations arising from the contralateral colpostat or the intrauterine tandem. Dosimetric effects of inhomogeneities in brachytherapy is essential for both dose-based implant optimization as well as for a comparison with alternate modalities, such as intensity modulated radiation therapy. We have used Monte Carlo calculations to model dose distributions about both a Fletcher-Suit-Delclos (FSD) low dose-rate system and the microSelectron high dose-rate remote afterloading system. We have evaluated errors, relative to a Monte Carlo simulation based upon a complete applicator system, in superposition calculations based upon both precalculated single shielded applicator dose distributions as well as single unshielded source dose distributions. Errors were largely dominated by the primary photon attenuation, and were largest behind the shields and tandem. For the FSD applicators, applicator superposition showed differences ranging from a mean of 2.6% at high doses (>Manchester Point A dose) to 4.3% at low doses (<Manchester Point A dose) compared to the full geometry simulation. Source-only superposition yielded errors higher than 10% throughout the dose range. For the HDR applicator system, applicator superposition-induced errors ranging from 3.6%-6.3% at high and low doses, respectively. Source superposition caused errors of 5%-11%. These results indicate that precalculated applicator-based dose distributions can provide an excellent approximation of a full geometry Monte Carlo dose calculation for gynecological implants.

Monte Carlo dose calculations for a new ovoid shield system for carcinoma of the uterine cervix

The dose distribution for an ovoid with a new tungsten shielding design was determined using Monte Carlo simulation. Standard Cesium-137 tube sources, tungsten shielding, and aluminum ovoid applicator were each modeled as a collection of solid objects. Dose was calculated in planes above, below, in front of, and on the sides of the colpostat. The Monte Carlo results were compared with the results from a parametrized calculation algorithm and good agreement was obtained. The dose distribution matrix derived from the parametrized algorithm can be used for clinical treatment planning.

Three-dimensional applicator system for carcinoma of the uterine cervix

PURPOSE

Intracavitary dose prescription for cancer of the uterine cervix has been based on the use of plane orthogonal films. Computed tomography (CT) and magnetic resonance imaging can provide three-dimensional (3D) anatomic information with which more sophisticated treatment planning can be carried out. This work describes a new tandem and ovoids design that permits modern 3D dosimetry and has the same placement flexibility for the physician as the applicators currently being used.

METHODS AND MATERIALS

The external shape of the Fletcher-Suit-Delclos (FSD) minicolpostat tandem and ovoids system has been used as a model to build a prototype of a new applicator. The prototype colpostats are constructed out of aluminum and steel. The tandems are made of aluminum. The Fletcher shields are eliminated. A new method of using tungsten for dose attenuation and shielding has been designed. Longitudinal alignment of the tungsten shields makes the new system possible. This applicator is CT-compatible.

RESULTS

Dose calculations for the new design are compared to a commercial version of the FSD applicator. Both the aluminum prototype and a simple extension of the prototype to a plastic applicator system are considered. It is shown that the principal difference in dose is that the dose is reduced in the region inferior to the center of the ovoids. All configurations (plastic caps on or off) are equivalently shielded for the new device. In addition, an intermediate mini-ovoid configuration can be used clinically via the introduction of a D-shaped cap. The latter reduces the high dose to the vaginal mucosal surfaces.

CONCLUSION

For a single ovoid, a comparison of dose with the FSD shows differences; however, the difference in dose is insignificant when the complete applicator, tandem, and ovoids are compared. With this new applicator, it is now possible to accumulate very accurate and detailed 3D dose-distribution data for the critical structures and other points of interest in the vicinity of the applicator. These data will permit future analysis of the correlation of dose and outcome for carcinoma of the cervix.

Reduction of the rectal dose in gynecological brachytherapy: modification to the Fletcher-Suit applicator

The dose to the anterior rectal wall is a known limiting factor for the delivery of radical doses of radiation to the uterine cervix with brachytherapy. We developed a modification to the Fletcher-Suit afterloading applicator, consisting of two small inflatable balloons attached to the posterior end of each colpostat. The balloons are connected to catheters that emerge from the vagina attached to the colpostat's handles. The balloons were affixed to the colpostats with a plastic adaptor and are inserted empty. After an anterior radiograph is taken, the balloons are filled with radiological contrast material and a lateral orthogonal film is made. This lateral film taken with the balloons filled with contrast typically shows a significant posterior displacement of the anterior rectal wall away from the vaginal sources. The International Commission on Radiation Units (ICRU) rectal point is then determined 5 mm beyond the posterior boundary of the opacified balloons. We have performed 90 applications using this device, including brachytherapy applications for cervical cancer, as well as vaginal applications for endometrial carcinoma following TAH-BSO. On average, the ICRU rectal point was displaced 14 mm away from the colpostats, thus reducing the dose rate by 60% and resulting in an average dose sparing of about 1000 cGy to the anterior rectal wall.

Development of a shielded 241Am applicator for continuous low dose rate irradiation of rat rectum

This paper describes the development of sources, applicators, and techniques that can be used to irradiate rat rectums with continuous irradiation at dose rates of interest in brachytherapy, either with the full circumference of the rectum irradiated, or with half of the circumference shielded from the radiation. The system uses encapsulated 241Am sources, to irradiate rat rectum with 60 keV photons continuously at a dose rate of up to 50 cGy/hr. Details of the design and fabrication of the 241Am sources, the rectal applicator, the dosimetry of the system, and the protocols for preparing and irradiating the rats, and for detecting early rectal injury using histological examination of irradiated rectum are presented. Highly effective shielding (attenuation factors as low as 0.04) of half of the circumference of the rat rectum was achievable. Unidirectional 241Am irradiators for intracavitary brachytherapy offer a unique tool for examining the effects of shielding a portion of the circumference of the rat rectum, on the radiation tolerance of the rectum.

Design of a plastic minicolpostat applicator with shields

A plastic intracavitary applicator system for the treatment of cancer of the uterine cervix is described. This applicator has a minicolpostat and a mechanism for affixing the tandem to the colpostats. Traditional afterloading refers only to the radioactive source. Both the source and the ovoid shield are afterloaded together in this applicator in contrast to traditional afterloading systems which afterload the source alone. A potential advantage of our applicator system is that it allows high quality CT localization because the sources and shields can be removed and the applicator is made of plastic. The advantages and disadvantages of this variation to the Fletcher system as well as other aspects of applicator design are discussed. An experimentally verified dose calculation method for shielded sources is applied to the design problems associated with this applicator. The dose distribution calculated for a source-shield configuration of the plastic applicator is compared to that obtained with a commercial Fletcher-Suit-Delclos (FSD) applicator. Significant shielding improvements can be achieved for the smallest diameter ovoid, that is, in the minicolpostat. The plastic minicolpostat dose distributions are similar to those produced by the conventional larger diameter colpostats. In particular, the colpostat shielding for rectum and bladder, which is reduced in the metal applicator's minicolpostat configuration, is maintained for the plastic minicolpostat. Further, it is shown that, if desired, relative to the FSD minicolpostat, the mucosa dose can be reduced by a suitable change of the minicolpostat source position.

Incidence of complications with mini vaginal culpostats in carcinoma of the uterine cervix

Between 1980 and 1987, 298 patients with carcinoma of the uterine cervix were treated at the University of Louisville Department of Radiation Oncology. Of these, 197 (66.1%) were treated for cure by radiotherapy alone: 36 by external beam alone and 161 by external beam and tandem and ovoid applications. The F.I.G.O. staging of the 161 patients was 82 (50.1%) Stage IB, 9 (5.6%) Stage IIA, 40 (24.9%) Stage IIB, and 30 (18.6%) Stage III. The usual treatment was whole pelvis irradiation followed by two intracavitary applications using the Fletcher Suit Applicators of tandem and ovoids in 79/161 patients (49%), a 3-M Mini Applicator (Fletcher Suit Delcos Applicator) in 52/161 patients (32.3%), and a 3-M Mini Applicator with Caps in 30/161 patients (18.6%). The incidence of grade 3-4 gastrointestinal or genitourinary complications as defined by the RTOG was 19.3% (31/161). Various treatment parameters were analyzed to define possible contributing factors. Grade 3-4 complications were seen in 7.6% (6/79) of patients treated with the standard ovoid Fletcher system, 26.9% (14/52) treated with the mini-ovoid system, and 36.6% (11/30) treated with the mini-ovoid system with caps (p = .0006). Although trends were noted, neither the vaginal surface dose (VSD) from the ovoids nor the addition of the external beam dose to the VSD (total vaginal surface dose = TVSD) were significant independent variables (p = 0.19 and = 0.133, respectively). The TVSD was significant when comparisons were made between different ovoid systems (p = 0.05 for less than 12,000 cGy and p = 0.004 for greater than 12,000 cGy). In this study, the 3-M mini applicator was associated with a significant increase in grade 3-4 complications as compared to the Standard Fletcher Suit Applicator.

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.

A CT-compatible version of the Fletcher system intracavitary applicator: clinical application and 3-dimensional treatment planning

A new acrylic version of the familiar Fletcher intracavitary applicator, the Ann Arbor (AA) applicator, has been developed. This new device eliminates the problem of "streak" artifacts on CT images, but unlike other plastic applicators the ability to shield portions of the bladder and rectum is retained through the use of tungsten alloy shields which are afterloaded with the radioactive sources. To minimize changes in placement geometry and to take advantage of the wide clinical experience with the Fletcher system, the new applicator nearly duplicates the physical dimensions of the Fletcher applicator. With the Ann Arbor applicator in place, dummy sources are easier to locate on standard radiographic simulations. CT scans are free of artifact and provide clear, detailed visualizations of cross-sectional anatomy. The new applicator thus allows CT images to be used to their potential in evaluating crucial anatomic relationships and in performing 3-D dosimetry with dose volume analysis. Using a treatment planning system with 3-D capabilities, solid surface graphic display of applicator, cervix, rectum, bladder, and treatment isodose volume has been performed. In addition, dose volume histograms can be generated to obtain precise measurements of the volume of cervix, rectum, or bladder receiving specified doses.

Dosimetric comparison of the Fletcher family of gynecologic colpostats 1950-1980

The Fletcher gynecologic applicator was developed for irradiation of carcinoma of the uterine cervix in the early 1950's. Since that time, numerous modifications and changes have been made in the colpostat construction and in the location of the shields that provide a reduced dose to the bladder trigone anteriorly and to the rectal wall posteriorly. The original applicators include the preload radium double colpostat and the preload radium single colpostat. In the 1960's, afterloading colpostats were manufactured as the Fletcher-Suit and the Fletcher-Green devices. With the introduction of the Delclos mini-colpostat, a new generation of applicators followed in the 1970's. The Fletcher-Suit-Delclos colpostat recently manufactured by two companies can be used as a mini-colpostat. By adding a shield-containing cap, these applicators function as the original Fletcher colpostat. With the development of new applicators over the past 30 years, numerous changes in the position of the shields and, therefore, the dose transmitted to the surrounding tissues have been made. This paper describes dosimetric evaluation of all of these applicators and the various changes that have occurred through the generations of Fletcher colpostats in an attempt to provide information for radiation therapists and gynecologists who are using these instruments in their clinical practice.

Screens in ovoids of a Selectron cervix applicator

The addition of screens in the vaginal source holders of a cervix applicator for intracavitary brachytherapy reduces the dose to rectum and bladder and therefore diminishes the number of rectal and vesical complications. Shielding properties of tungsten rectal and bladder screens of a Selectron cervix applicator, loaded with spherical cesium sources, were determined for verification of dose calculations. Transmission characteristics of half-disk shaped tungsten screen segments in a single ovoid were measured in a water phantom. The minimum transmission ratios are 60, 70 and 80% for segment thickness of 5.0, 3.5 and 2.0 mm, respectively. The accuracy of the new screen correction algorithm of the Selectron Planning System was assessed by comparing measured and calculated dose rates and was found to be better than +/- 4%. The correction algorithm provides a method to analyse the efficacy of screens in the ovoids for various segment geometries and orientations without extensive phantom measurements. Isotransmission and isodose calculations were made for a typical clinical applicator set-up and source distribution. The dose reduction to rectum and bladder, near the bottom and top of the ovoids was analysed in detail. A 3.5 mm thick rectum and bladder screen in each ovoid reduces the dose approximately by 20% to the rectum and by 15% to the bladder. A distance enlargement of about 5 mm between ovoid and rectum or bladder, e.g. by packing, results in a comparable dose reduction. Shielding properties of a Selectron cervix applicator, provided with screens, were compared with those of some Fletcher-type applicators. Significant differences between the transmission ratios and shielded areas of the screens of both systems near rectum and bladder were observed.