Measurements of radiation dose distributions for shielded cervical applicators

Commissioning of a grid-based Boltzmann solver for cervical cancer brachytherapy treatment planning with shielded colpostats

PURPOSE

We sought to commission a gynecologic shielded colpostat analytic model provided from a treatment planning system (TPS) library. We have reported retrospectively the dosimetric impact of this applicator model in a cohort of patients.

METHODS AND MATERIALS

A commercial TPS with a grid-based Boltzmann solver (GBBS) was commissioned for (192)Ir high-dose-rate (HDR) brachytherapy for cervical cancer with stainless steel-shielded colpostats. Verification of the colpostat analytic model was verified using a radiograph and vendor schematics. MCNPX v2.6 Monte Carlo simulations were performed to compare dose distributions around the applicator in water with the TPS GBBS dose predictions. Retrospectively, the dosimetric impact was assessed over 24 cervical cancer patients' HDR plans.

RESULTS

Applicator (TPS ID #AL13122005) shield dimensions were within 0.4 mm of the independent shield dimensions verification. GBBS profiles in planes bisecting the cap around the applicator agreed with Monte Carlo simulations within 2% at most locations; differing screw representations resulted in differences of up to 9%. For the retrospective study, the GBBS doses differed from TG-43 as follows (mean value ± standard deviation [min, max]): International Commission on Radiation units [ICRU]rectum (-8.4 ± 2.5% [-14.1, -4.1%]), ICRUbladder (-7.2 ± 3.6% [-15.7, -2.1%]), D2cc-rectum (-6.2 ± 2.6% [-11.9, -0.8%]), D2cc-sigmoid (-5.6 ± 2.6% [-9.3, -2.0%]), and D2cc-bladder (-3.4 ± 1.9% [-7.2, -1.1%]).

CONCLUSIONS

As brachytherapy TPSs implement advanced model-based dose calculations, the analytic applicator models stored in TPSs should be independently validated before clinical use. For this cohort, clinically meaningful differences (>5%) from TG-43 were observed. Accurate dosimetric modeling of shielded applicators may help to refine organ toxicity studies.

Comparison of dose distributions around the pulsed-dose-rate Fletcher–Williamson and the low-dose-rate Fletcher–Suit–Delclos ovoids: a Monte Carlo study

We performed a Monte Carlo study to compare dose distributions for a Fletcher-Suit-Delclos (FSD) ovoid used with (137)Cs low-dose-rate (LDR) sources with those for a Fletcher-Williamson (FW) ovoid used with an (192)Ir pulsed-dose-rate (PDR) source for intracavitary brachytherapy of cervical cancer. We recently reported on extensive validation of Monte Carlo MCNPX models of these ovoids using radiochromic film measurements. Here, we compared these models assuming identical loading of 10, 15 and 20 mgRaEq (72, 108 and 145 cGy cm(2) h(-1), respectively) in three dose mesh planes: one perpendicular to the ovoid long axis bisecting the ovoid, one parallel to and displaced 2 cm medially from the long axis of the ovoid, and a 'rectal' plane perpendicular to the long axis located 1 cm distal to the distal face of the ovoid cap. The FW ovoid delivered slightly higher doses (within 10%) over all loadings to regions away from the bladder and rectal shields when compared to the FSD ovoid. However, the FW ovoid delivered much higher doses (>50%) in regions near these shields. In the rectal plane, the FW ovoid delivered a slightly higher dose, but within the region directly behind the rectal shield, the FW ovoid delivered a dose ranging from +35% to -35% of the FSD dose distribution. We attribute these differences to intrinsic differences in source characteristics (radial dose function and anisotropy factors) and extrinsic factors such as the solid-angle effect between sources and shields and applicator design.

Dosimetric analysis of a shielded applicator for nasopharyngeal carcinoma intracavitary brachytherapy: Monte Carlo calculation

In nasopharyngeal cancer (NPC) intracavitary brachytherapy, an anatomical dose reference point (in line with that for gynecology work), e.g., at the sphenoid floor, is more precise than the empirical point of 1 cm from the source. However, such increases of the single-source-plan treatment distances may deliver excessive doses inferiorly, to the soft palate. As shielding may help, its efficacy was studied by Monte Carlo simulations in water for 20 and 30 mm diameter spherical NP applicators (representing extremes of sizes for the small NP cavity), with/without lead shielding inferiorly, using a single linear Ir-192, 2 mm steps, equal dwell times for 5 (5DP) and 9 dwell positions (9DP). Dose reductions of the selected points of interest ranged from 1.2% to 40.5% for the 20 mm shielded applicator and a range of 2.9% to 17.9%, for the 30 mm shielded applicator. Dose volume histograms of the "region of interest" (ROI)-a cuboid of 4 x 4 x 0.5 cm3 at the most inferior aspect of the applicator, also differed significantly. The highest doses of the 50% (D50) and 20% (D20) volumes of ROI (for 5DP and 9DP plans) were reduced by 11.9% to 17.9% for the 20 mm applicator and a range of 9.0% to 11.5% for the 30 mm shielded applicator. Doses in unshielded directions were insignificantly changed, for example, with a 20 mm applicator simulated in a 5DP plan, the dose distribution close to the source in the unshielded direction has less than 4% difference at the 50% isodose relative to the dose prescription point. For the 30 mm shielded applicator, despite smaller dose reduction percentages, a more pronounced effective dose reduction was obtained than nominal values when considering radiobiological equivalent doses. Our system was demonstrated to be ready for clinical assessment.

Dosimetric evaluation of the Fletcher–Williamson ovoid for pulsed-dose-rate brachytherapy: a Monte Carlo study

We used radiochromic film dosimetry to validate a Monte Carlo (MC) model of a 192Ir pulsed-dose-rate (PDR) source inside a Fletcher-Williamson ovoid. MD-55-2 radiochromic film was placed in a high-impact polystyrene phantom in a plane parallel to and displaced 2.0 cm medially from the long axis of the ovoid. MC N-particle transport code (MCNPX) version 2.4 was used to model the ovoid and the 192Ir source. Energy deposition was calculated using a track-length estimator modified by an energy-dependent heating function, which is a good approximation of the collision kerma. To convert the estimates of the MC dose per simulated particle to clinically relevant absolute dosimetry, additional MC models of an actual and a virtual 192Ir source in dry air were simulated to determine air kerma strength for the penetrating part of the photon spectrum (>11.3 keV). The absolute dose distributions predicted by MCNPX agreed with the film results and were within +/-9.4% (k = 2) and within +/-2% or within a distance to agreement of 2 mm for 94% of the dose grid. Additional MC models characterized the uncertainty resulting from source positioning inside the ovoid. For a worst-case scenario of 1 mm off centre from the nominal source position in the 3 mm diameter ovoid shaft, the average dose deviation over the film plane was +/-5% (1sigma = +/-4%), with maximum deviation near the sharp dose-gradient provided by the shields of -20% to + 26%. A validated MC model is the first requirement to simulate common LDR clinical loadings (5-20 mgRaEq) and, thus, will aid in the transition from the current 137Cs Selectron LDR ICBT to PDR for treatment of gynecologic cancers.

Comparison of Monte Carlo calculations around a Fletcher Suit Delclos ovoid with radiochromic film and normoxic polymer gel dosimetry

The Fletcher Suit Delclos (FSD) ovoids employed in intracavitary brachytherapy (ICB) for cervical cancer contain shields to reduce dose to the bladder and rectum. Many treatment planning systems (TPS) do not include the shields and other ovoid structures in the dose calculation. Instead, TPSs calculate dose by summing the dose contributions from the individual sources and ignoring ovoid structures such as the shields. The goal of this work was to calculate the dose distribution with Monte Carlo around a Selectron FSD ovoid and compare these calculations with radiochromic film (RCF) and normoxic polymer gel dosimetry. Monte Carlo calculations were performed with MCNPX 2.5.c for a single Selectron FSD ovoid with and without shields. RCF measurements were performed in a plane parallel to and displaced laterally 1.25 cm from the long axis of the ovoid. MAGIC gel measurements were performed in a polymethylmethacrylate phantom. RCF and MAGIC gel were irradiated with four 33 microGy m2 h(-1) Cs-137 pellets for a period of 24 h. Results indicated that MCNPX calculated dose to within +/- 2% or 2 mm for 98% of points compared with RCF measurements and to within +/- 3% or 3 mm for 98% of points compared with MAGIC gel measurements. It is concluded that MCNPX 2.5.c can calculate dose accurately in the presence of the ovoid shields, that RCF and MAGIC gel can demonstrate the effect of ovoid shields on the dose distribution and the ovoid shields reduce the dose by as much as 50%.

Attenuation of intracavitary applicators in 192Ir-HDR brachytherapy

Unlike the penetrating monoenergetic 662 keV gamma rays emitted by 137Cs LDR sources, the spectrum of 192Ir used in HDR brachytherapy contains low-energy components. Since these are selectively absorbed by the high-atomic number materials of which intracavitary applicators are made, the traditional neglect of applicator attenuation can lead to appreciable dose errors. We investigated the attenuation effects of a uterine applicator, and of a set of commonly used vaginal cylinders. The uterine applicator consists of a stainless steel source guide tube with a wall thickness of 0.5 mm and a density of 8.02 g/cm3, whereas the vaginal cylinders consist of the same stainless steel tube plus concentric polysulfone cylinders with a radius of 1 or 2 cm and a density of 1.40 g/cm3. Monte Carlo simulations were performed to compute dose distributions for a bare 192Ir-HDR source, and for the same source located within the applicators. Relative measurements of applicator attenuation using ion-chambers (0.125 cm3) confirmed the Monte Carlo results within 0.5%. We found that the neglect of the applicator attenuation overestimates the dose along the transverse plane by up to 3.5%. At oblique angles, the longer photon path within applicators worsens the error. We defined attenuation-corrected radial dose and anisotropy functions, and applied them to a treatment having multiple dwell positions inside a vaginal cylinder.

Absorbed dose calculations in a brachytherapy pelvic phantom using the Monte Carlo method

Monte Carlo calculations of the absorbed dose at various points of a brachytherapy anthropomorphic phantom are presented. The phantom walls and internal structures are made of polymethylmethacrylate and its external shape was taken from a female Alderson phantom. A complete Fletcher-Green type applicator with the uterine tandem was fixed at the bottom of the phantom reproducing a typical geometrical configuration as that attained in a gynecological brachytherapy treatment. The dose rate produced by an array of five (137)Cs CDC-J type sources placed in the applicator colpostats and the uterine tandem was evaluated by Monte Carlo simulations using the code PENELOPE at three points: point A, the rectum, and the bladder. The influence of the applicator in the dose rate was evaluated by comparing Monte Carlo simulations of the sources alone and the sources inserted in the applicator. Differences up to 56% in the dose may be observed for the two cases in the planes including the rectum and bladder. The results show a reduction of the dose of 15.6%, 14.0%, and 5.6% in the rectum, bladder, and point A respectively, when the applicator wall and shieldings are considered.

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 lookup tables for Henschke applicator cervix treatment by HDR 192IR remote afterloading

PURPOSE

We have generated three-dimensional (3D) lookup tables for dosimetric analysis and optimization of high-dose rate (HDR) gynecological treatments using the Henschke applicator. The new dosimetry data have been compared with two-dimensional (2D) data currently in use. The 3D dosimetry tables have been implemented in an existing cervix treatment-planning system and have been evaluated through analysis of clinical cases.

METHODS AND MATERIALS

A general Monte Carlo N-Particles (MCNP) transport code was used to compute absorbed dose distributions around the intrauterine tandem and tungsten-shielded ovoid separately. The dosimetry data are represented in the x-y coordinate system for the intrauterine tandem table. The 3D table for the ovoid contains a radial dose function and an anisotropy function, as formulated in the spherical coordinate system. Absorbed dose at a spatial point is calculated by applying bilinear interpolation for the anisotropy function and linear interpolation for the radial dose function. The geometry factor for a finite line source is used. 3D dose calculations and optimization were performed for 20 treatments of 10 patients. The absorbed dose to critical structures, bladder and rectum, was compared by applying both the 2D table currently in use and the new tables.

RESULTS

The new 2D table for the intrauterine tandem yields doses different by less than 10% from those with the current table. The 3D table for the shielded ovoids shows as large as a factor of 4 reduction of dose behind the shield compared with the present 2D table. This shielding effect leads to 21.6 +/- 9.3% and 20.0 +/- 6.6% dose reduction at rectum and bladder, respectively, for actual treatments.

CONCLUSION

Our analysis indicates a need for patient-specific 3D dosimetry to permit more accurate dosimetric evaluation of HDR cervix treatments using shielded applicators. We have also shown that a Monte Carlo simulation code enabled us to derive the lookup tables necessary for 3D planning.

Experimental and Monte Carlo dosimetry of the Henschke applicator for high dose-rate 192Ir remote afterloading

We have performed extensive computational and experimental dosimetry of the Henschke applicator with respect to high dose-rate 192Ir brachytherapy using a GAMMAMED remote afterloader. Our goal was to generate clinically useful two- and three-dimensional look-up tables. Dose measurements of the Henschke applicator involved using TLD chips placed in a polystyrene phantom. Monte Carlo simulations were performed using the MCNP code. The computational models included the detailed geometry of 192Ir source, tandem tube, and shielded ovoid. The measured dose rates were corrected for the dependence of TLD sensitivity on the distance of measurement points from the source. Transit dose delivered during source extension to and retraction from a given dwell position was estimated by Monte Carlo simulations, and a correction was applied to the experimental values. For the applicator tandem, the ratio of dose rates obtained by MCNP to those measured by TLD chips ranges from 0.92 to 1.10 with an average of 0.98 and a standard deviation of 0.02. The measured and calculated dose rates at 1 cm on the transverse axis are 1.10 cGy U-1 h-1. For the shielded ovoid, the ratio ranges from 0.88 to 1.16 with an average of 1.00 and a standard deviation of 0.07. Causes of the discrepancy between the Monte Carlo and TLD results were identified. We found that the combined uncertainty of measured dose rates due to these causes is 5.6% for the applicator tandem and 8.4% for the shielded ovoid. Therefore, the results of the Monte Carlo simulation are considered to have been validated by the measurements within the uncertainty involved in the calculation and measurements.

Code of practice for brachytherapy physics: report of the AAPM Radiation Therapy Committee Task Group No. 56. American Association of Physicists in Medicine

Recommendations of the American Association of Physicists in Medicine (AAPM) for the practice of brachytherapy physics are presented. These guidelines were prepared by a task group of the AAPM Radiation Therapy Committee and have been reviewed and approved by the AAPM Science Council.

An analysis of the effect of ovoid shields in a selectron-LDR cervical applicator on dose distributions in rectum and bladder

PURPOSE

A disadvantage of ovoid shields in a Fletcher-type applicator is that these shields cause artifacts on postimplant CT images. CT images, however, make it possible to calculate the dose distribution in the rectum and the bladder. To be able to estimate the possible advantage of having CT information over the use of ovoid shields without having CT information, we investigated the influence of shielding segments in a Fletcher-type Selectron-LDR applicator on the dose distribution in rectum and bladder.

METHODS AND MATERIALS

Contours of rectum and bladder were delineated on transaxial CT slices of 15 unshielded applications. Of the volumes contained within these structures dose-volume histograms (DVHs) were calculated. In a similar way, DVHs of simulated shielded applications were calculated. The reduction, due to shielding, of the dose to the 2 cm3 (D2) and 5 cm3 (D5) volume of the cumulative DVHs of rectum and bladder, were determined. An isodose pattern in the sagittal plane through the center of each applicator was plotted to compare the location of the shielded area with the location of maximum dose in rectum and bladder in the unshielded situation. In two cases local dose reductions to the rectal wall were determined by calculating the dose in points at 10-mm intervals on the rectal contours.

RESULTS

For the rectum, the reduction of D2 ranged from 0 to 11.1%, with an average of 5.0%; the reduction of D5 ranged from 2.3 to 12.1%, with an average of 6.4%. The reduction of D2 and D5 for the bladder ranged from 0 to 11.9% and from 0 to 11.6%, with average values of 2.2 and 2.6%, respectively. In 8 out of 15 cases the rectal maximum dose was located inferior to the shielded area. In all cases except one the bladder maximum dose was located superior to the shielded area. Local dose reductions on the rectal wall can be as high as 30% or more in an optimally shielded area.

CONCLUSIONS

Reductions of D2 and D5 to rectum and bladder due to shielding are rather small, because the shielded area does usually not coincide with the high dose region and even if it does, the shielded area is too small to result in large reductions of these values. Because local dose reductions vary largely, one should proceed with caution when calculating the dose in just one rectal or bladder reference point. Because large overall dose reductions cannot be achieved with shielding, it is safe to use an unshielded applicator when post implant CT images are used to realize optimized dose distributions.

Treatment planning for carcinoma of the cervix: a patterns of care study report

PURPOSE

The Patterns of Care Study (PCS) of patients treated in 1988-89 included "patterns of treatment planning" for radiotherapy of carcinoma of the uterine cervix. A Consensus Committee of radiation physicists and oncologists established current guidelines and developed questionnaires to assess the treatment planning process (i.e., the general structure, methodology, and tools) of institutions involved in the Patterns of Care Study. This paper reports the findings of the assessment.

METHODS AND MATERIALS

The PCS surveyed 73 radiotherapy facilities, of which 21 are academic institutions (AC), 26 hospital-based facilities (HB), and 26 free-standing centers (FS). In total, 242 cases were assessed with 39% from academic centers, 33% from hospital-based centers, and 28% from free-standing centers. The survey collected treatment planning information such as the use of computed tomography (CT), simulation procedure, contouring of patient outline, tumor or target delineation, identification of critical structures, method of dose prescription (point or isodose), etc. Data was also obtained concerning implant boosts, e.g., radioisotope used, use of midline block for external beam treatment, availability of remote afterloader, practice of interstitial implants, combination with hyperthermia, etc.

RESULTS

There is a high degree of compliance relative to the basic treatment planning standards. For example, 171 cases (out of 173) from AC and HB institutions included simulation and 169 used port film; for cases from FS centers, 61 out of 69 involved simulation and 66 out of 69 included port film. Most institutions used linacs (231 out of 242); in five cases, Co-60 units and in six cases betatron was used. In terms of treatment planning, 53% used skin contours, but only 14% had target volume delineation, with AC and HB being slightly more conscientious in these efforts. Critical organs did not appear to be explicitly considered in external beam treatment planning, with only 3% outlining the bladder, 5% the rectum, and less than 1% the small bowel. Only 11% of the centers used CT in treatment planning, and none reported the use of magnetic resonance imaging (MRI). For patients receiving implants, about 40% had midline blocking during external beam treatment, of which one out of three were shielded by standard blocks and two out of three with customized ones. About 11% of the patients receiving implants were treated with remote afterloading devices, 5% received interstitial implants, and none were treated in combination with hyperthermia.

CONCLUSION

The treatment planning aspects of radiotherapy of carcinoma of the cervix have been established by this Patterns of Care Study Survey. There is a high level of uniformity in the approach. Some variations exist among centers in the different strata.

Rapid two-dimensional dose measurement in brachytherapy using plastic scintillator sheet: linearity, signal-to-noise ratio, and energy response characteristics

Because of the large dose gradients encountered near brachytherapy sources, an efficient, accurate, low-atomic number areal detector, which can record dose at many points simultaneously, is highly desirable. We have developed a prototype of such a system using thin plates of plastic scintillator as detectors. A micro-channel plate (MCP) image intensifier was used to amplify the optical scintillation images produced by radioactive 125I and 137Cs sources in water placed 0.5-5.7 cm distance from the detector. A charge-coupled device (CCD) digital camera was used to acquire 2-D light-intensity distributions from the image intensifier output window. For both isotopes, a small area (2 x 3 mm2) PVT detector yields a CCD net count rate that is linear with respect to absorbed dose rate within +/- 3% out to 5.7 cm distance. Acquisition times range from 1.5-400 sec with a reproducibility of 0.5-5.5%. If a large-area (6 x 20 cm2) PVT detector is used, a four-fold increase in count rate and large deviations from linearity are observed, indicating that neighboring pixels contribute light to the signal through diffusion and scattering in PVT and water. A detailed noise analysis demonstrates that the image intensifier reduces acquisition time 10000-fold, reduces noise relative to signal 200-fold, and reduces amplifier gain noise as well.

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.

Dose calculation and measurements for a CT compatible version of the Fletcher applicator

Experimental measurements in a water phantom were made for a new shielded plastic Fletcher applicator. A dose calculation algorithm which allows the description of asymmetrically shielded source containers is presented. The traditional primary plus build-up factor description of the dose in a homogeneous volume is corrected by using effective attenuation coefficients measured in a water phantom for all the source-shield-container materials. Comparison of the theoretical results to the experimental data shows excellent agreement.

CT-assisted assessment of bladder and rectum dose in gynecological implants

Eight patients who received gynecological implants with Fletcher-Suit type applicators were involved in this study. An orthogonal pair of films and computed tomographic scans were obtained for each patient. In the CT study, judicious use of contrast materials and selective window and level settings permitted clear delineation of the bladder and the rectum boundaries relative to the implanted applicators. In comparison to reference organ doses derived from the orthogonal film pair method, the maximum organ doses estimated from the CT-assisted evaluation were considerably higher, by approximately twofold on the average. The differences between the values estimated from the two methods vary from patient to patient, being highly dependent on the individual anatomy and the geometry of the implanted sources. These preliminary results point to the inaccuracy of the conventional method of estimating organ doses. CT-assisted evaluation may be necessary to accurately calculate organ doses in gynecological applications.

Using measured dose distribution data of the Fletcher-Suit-Delclos colpostat in brachytherapy treatment planning

A program to use the measured dose distribution of a shielded ovoid has been written and interfaced with a commercial system. The processing of the data and the method of reconstructing isodose curves in any desired plane is described. The effect of the shielding on calculated dose distributions is evaluated.

Computation of radiation dose distributions for shielded cervical applicators

While cervical applicators with shielded ovoids are used widely in brachytherapy, we know of no system for calculating dose distributions for them. For shielded sources, because of a lack of symmetry and because of a rapid variation of dose as a function of position relative to the source, extensive measured data in three dimensions are required. In the method we have developed, the dose at a given point from a source in a shielded ovoid is calculated by multiplying the dose from an unshielded source by the "effective attenuation factor" of the shields. The latter quantity is obtained by linear-interpolation in a three-dimensional table generated from measurements described in an earlier paper. The unshielded-source dose is calculated as the product of source strength, time of implant, distance-dependent geometry factor and a tabulated quantity called the "relative dose rate factor". Relative dose rate factor is obtained by dividing measured dose rate by the product of geometry factor and source strength. Division by the geometry factor reduces the amount of data required with respect to accuracy in linear-interpolation. Input localization data must include not only the position of the end points defining the source but also a third reference point to define the orientation of the shields.