Permanent prostate seed implant brachytherapy: report of the American Association of Physicists in Medicine Task Group No. 64

Quality assurance for computed-tomography simulators and the computed-tomography-simulation process: report of the AAPM Radiation Therapy Committee Task Group No. 66

This document presents recommendations of the American Association of Physicists in Medicine (AAPM) for quality assurance of computed-tomography- (CT) simulators and CT-simulation process. This report was prepared by Task Group No. 66 of the AAPM Radiation Therapy Committee. It was approved by the Radiation Therapy Committee and by the AAPM Science Council.

Dose-volume conundrum for response of prostate cancer to brachytherapy: summary dosimetric measures and their relationship to tumor control probability

PURPOSE

Although it is known that brachytherapy dose distributions are highly heterogeneous, the effect of particular dose distribution patterns on tumor control probability (TCP) is unknown. It is unlikely that clinical results will throw light on the question in the near future, given the long follow-up and detailed dosimetry required for each patient. We used detailed dose distribution data from 50 patients combined with radiobiologic parameters consistent with what is known about TCP curves for prostate cancer to study the changes in TCP that accompany gross dosimetric measures and particular dosing irregularities (e.g., moderate underdosing of large volumes vs. extreme underdosing of small volumes).

METHODS AND MATERIALS

For each of the 50 patients with organ-confined prostate cancer who had undergone 125I prostate implants alone at our clinic, postimplant CT scans were obtained approximately 1 month after implantation. Dose distribution information was obtained from postimplant dosimetry. The percentage of the prostate volume receiving a specified dose was recorded from the respective differential dose-volume histograms in 10-Gy bins. In addition, the percentage of prostate volume underdosed at varying fractions of the prescription dose were determined, as was the minimal prostate dose. The log-normal distributions of the radiobiologic parameters [ln(initial clonogen number), alpha, and alpha/beta] were adjusted so that the predicted population parameters (steepness and location) of the dose-response curves for external beam radiotherapy agreed with the published estimates. The variability in the dose-volume details was increased by scaling the dose distributions by factors ranging from 0.7 to 1.5, thereby simulating, for each of the patients, nine new patients with different total doses but identical relative distributions of the dose over the voxels. Radiobiologic variability between the selected dose distributions was then removed by averaging >50 randomly chosen sets of radiobiologic parameters from the log-normal distributions to estimate the TCP for each of the dose distributions, giving some insight into the TCP variations with conventional dosimetric indexes and different patterns of underdosing.

RESULTS

Using the 450 dose distributions created by expanding the 50-patient data set, the volume of the prostate that was extremely underdosed (between 50% and 70% of the prescription dose) was related to the volume that was moderately underdosed (between 80% and 100% of the prescription dose). We found that the individual TCP is greatly dependent on the inhomogeneous dose distribution and the dosimetric indexes, such as the volume of prostate receiving 100% of the prescribed dose (V100) and the maximal dose received by 90% of the prostate volume (D90), which, by themselves, are not always accurate predictors of control probabilities. In a multivariate analysis of the dependence of TCP on these parameters (V100, D90, minimal dose, and moderately and severely underdosed volumes), only D90 and the minimal dose were statistically significant. Generally speaking, however, a lower minimal dose means a lower TCP.

CONCLUSION

The work described here was an hypothesis-generating study. Our results showed that even if the V100 and D90 are nearly identical for 2 patients, there can be (and frequently are) significant differences in the dose distributions in the subvolumes of the prostate. Under simulated dose-response conditions (i.e., with variations in the dose distribution), the D90 and minimal dose significantly affected the TCP but the V100 and the volumes moderately or severely underdosed did not. In general, one must consider the totality of the dose distribution to evaluate the dosimetric quality of a low-dose-rate prostate implant. TCP is not a monotonic function of extreme or moderate underdosing. In some instances, extreme underdosing of relatively small volumes may result in a greater TCP than moderate underdosing of relatively large volumes and vice versa.

The robustness of dose distributions to displacement and migration of 125I permanent seed implants over a wide range of seed number, activity, and designs

PURPOSE

To investigate the robustness of permanent prostate implant dosimetry for various (125)I seed activities and various seed models. The dosimetric impact of seed misplacement and seed migration (seed loss) is also taken into account using various standard dose indices.

METHODS AND MATERIALS

A dose-based inverse planning algorithm is used for automated dosimetric plan creation (45-60 s per plan) and provides an unbiased way to compare the robustness of various optimal dosimetric plans. Seed misplacement and seed migration are simulated by way of Monte Carlo, based on the measured displacement distributions from clinical postimplant cases. Plans were generated for seed activities between 0.2 and 1.4 mCi (0.25 to 1.78 U) and for 11 different seed models.

RESULTS

The numbers of seeds and needles are shown to decrease rapidly for a seed activity between 0.3 mCi and 0.6 mCi (0.38 and 0.76 U). The loss in V100, from 100%, because of seed misplacement is below 10% for an apparent activity ranging from 0.2 to 0.9 mCi (0.25 to 1.14 U). A minimum degradation in V100 is observed around 0.6-0.7 mCi (0.76-0.89 U). D90 increases from 150 to 170 Gy between 0.3 and 0.7 mCi (0.38 and 0.89 U) and decreases afterward to fall below 140 Gy at higher activity. V200 and D10 to the target volume both show an increase in hot spots up to 0.7 mCi, and then decrease linearly at higher activities for all seed models. V200 and D10 to the urethra remain about constant for all seed activities up to 0.8 mCi (1.02 U), at which point they start to decrease. All seed models follow this general trend.

CONCLUSIONS

Plans were shown to be robust to misplacement and migration of seeds over a wide range of seed activity and for various seed models. With a properly tuned inverse planning algorithm able to ensure the dose coverage and protection for the organs at risk in the presence of placement errors (displacement and migration), the choice of a preferred seed activity, in a range up to about 0.7 mCi (0.89 U), is open. The upper part of this range offers the opportunity to significantly reduce the number of seeds and needles, thus reducing surgical trauma to the patient, saving time in an operating room planning setting, and reducing the cost of a permanent prostate implant procedure.

Comparative study of permanent interstitial prostate brachytherapy post-implant evaluation among seven Italian institutes

BACKGROUND AND PURPOSE

The purposes of this multicentric study are (a) the evaluation of four different commercially available treatment planning systems (TPSs) and (b) to verify whether the dosimetric results are comparable, also when considering the inter-observer variabilities and the different scanning protocols used. This work is to be considered a first step to test the value of multicentric studies based on dosimetric evaluation of the quality of the implants.

PATIENTS AND METHODS

Four different TPSs were used and the following tests were performed:Comparison of the parameters and mathematical algorithms used; comparison of the dose distributions generated by three different geometries of sources based on 32 dose-points on each source geometry. An octagonal geometric phantom was used to compare volume algorithms and dose-volume histogram (DVH) calculations (V150(Gy), V100(Gy), V50(Gy) and V25(Gy)). Comparison of the post-plan source distribution performed on a prostate-phantom implanted with (125)I seeds. A CT scan of the phantom was obtained at each participating center. Both the geometrical coordinates (with respect to the most caudal one), and the spread of the geometrical distribution, were calculated. The volumes included within different isodoses were also collected. Comparison of the post-plan source distribution performed on an actual patient. Post-plan V100% and D90(Gy) derived from seed distributions obtained by different operators were calculated, using the same target delineation.

RESULTS

All the considered TPSs satisfied the AAPM dosimetric parameter recommendations. Point-dose examinations revealed differences smaller than 5%, except for one of the systems. Although the volume algorithm was not the same for all systems, no statistically significant difference was found in the volume measurements. The DVHs also presented differences smaller than 5%, except for one TPS. The distances between the seeds, based on the same CT images, showed a mean SD of 0.13 mm. The mean maximum difference of the position of each seed was 0.36 mm. The most significant errors were made in the cranio-caudal direction (mean maximal difference: 0.44 mm); here the size of the step between slices played an important role. The algorithm of source positioning of the different TPSs may also help explain this difference. The compiled DVHs showed differences smaller than 5%. Post-plans derived from different seed distributions showed a mild dependence upon operators. We obtained a mean value of 97.8 and 152.7 with a percentage of SD of 0.43 and 1.7, respectively, for V100% and D90(Gy).

CONCLUSIONS

Three-dimensional (3D) geometric reconstructions of seed distributions are slightly dependent upon the operators and the scanning protocols have little effect on the dosimetric evaluation. Some relevant discrepancies were found between one of the TPSs and the other three if few sources were used; increasing the number of seeds those differences became less pronounced. Multicentric studies on the quality of prostate implants based on post-implant dosimetry are feasible, provided an accurate step-wise evaluation of the procedure be performed.

Parametric shape modeling using deformable superellipses for prostate segmentation

Automatic prostate segmentation in ultrasound images is a challenging task due to speckle noise, missing boundary segments, and complex prostate anatomy. One popular approach has been the use of deformable models. For such techniques, prior knowledge of the prostate shape plays an important role in automating model initialization and constraining model evolution. In this paper, we have modeled the prostate shape using deformable superellipses. This model was fitted to 594 manual prostate contours outlined by five experts. We found that the superellipse with simple parametric deformations can efficiently model the prostate shape with the Hausdorff distance error (model versus manual outline) of 1.32 +/- 0.62 mm and mean absolute distance error of 0.54 +/- 0.20 mm. The variability between the manual outlinings and their corresponding fitted deformable superellipses was significantly less than the variability between human experts with p-value being less than 0.0001. Based on this deformable superellipse model, we have developed an efficient and robust Bayesian segmentation algorithm. This algorithm was applied to 125 prostate ultrasound images collected from 16 patients. The mean error between the computer-generated boundaries and the manual outlinings was 1.36 +/- 0.58 mm, which is significantly less than the manual interobserver distances. The algorithm was also shown to be fairly insensitive to the choice of the initial curve.

Treatment planning for prostate brachytherapy using region of interest adjoint functions and a greedy heuristic

We have developed an efficient treatment-planning algorithm for prostate implants that is based on region of interest (ROI) adjoint functions and a greedy heuristic. For this work, we define the adjoint function for an ROI as the sensitivity of the average dose in the ROI to a unit-strength brachytherapy source at any seed position. The greedy heuristic uses a ratio of target and critical structure adjoint functions to rank seed positions according to their ability to irradiate the target ROI while sparing critical structure ROIs. This ratio is computed once for each seed position prior to the optimization process. Optimization is performed by a greedy heuristic that selects seed positions according to their ratio values. With this method, clinically acceptable treatment plans are obtained in less than 2 s. For comparison, a branch-and-bound method to solve a mixed integer-programming model took more than 50 min to arrive at a feasible solution. Both methods achieved good treatment plans, but the speedup provided by the greedy heuristic was a factor of approximately 1500. This attribute makes this algorithm suitable for intra-operative real-time treatment planning.

An innovative dosimetric model for formulating a semi-analytical solution for the activity-volume relationship in prostate implants

An innovative (and yet simple) dosimetric model is proposed that provides a semi-analytical solution to the total activity-volume relationship in ultrasound-guided transperineal prostate implant. This dosimetric model is based on 4 simple assumptions. First, the prostate target volume is approximated as a sphere. Second, the urethra is presumed to transverse through the center of the prostate target volume. Third, peripheral loading is applied as the seed-loading technique. Fourth, as the major innovation of the proposed model, the radial dose function of the Iodine-125 125I seed is forced to fit a simple power function of the distance r. Pursuant to the third assumption, the peripherally-loaded seeds also define a spherical volume defined as the loading volume w. Also pursuant to the fourth assumption, the radial dose function is expressed as 1.139*r(-0.474) for r = 1.5 to 2.5 cm. Thereafter, a simple analytical power-law equation, A = 1.630* w(0.825), for the relationship between the total activity A in mCi and the loading volume w in cc is derived for 125I monotherapy. Isodose plans for loading volumes corresponding to r = 1.5, 1.8, 2.2, and 2.5 cm were performed. The maximal isodose coverage volume maxV100 was calculated for each case and was found to be on the average 65% larger than the loading volume w. Matching prostate target volume V to the loading volume w therefore yields a generous implant (with a margin of approximately 3.3 mm). Conversely, matching the prostate target volume V to the maxV100 yields a tight implant (with 0.0 mm or no margin). Matching the prostate target volume V to a midpoint between the loading volume w and maxV100 yields a moderate implant (with approximately 1- to 2-mm margin). Three individual equations are derived for each type of implants: A = 1.630* V(0.825), A = 1.288* V(0.825), or A = 1.078 V(0.825) for generous, tight, or moderate implants, respectively. Patient data at the Chicago Prostate Cancer Center are found to support the above dosimetric model and the 3 semi-analytically derived equations. The above equations are also compared favorably with some of the previously published equations from other authors. These results support the efficacy of the proposed dosimetric model.

Prostate implant nomograms for the North American Scientific 103Pd seed

Palladium-103-(103Pd) seed has been increasingly used in prostate implantation as either definitive or boost therapy because of its shorter half-life and higher initial dose rate. Because a growing number of radiation oncologists prefer real-time implantation in the operating room, it will be helpful if the total activity of the seeds can be determined based on the gland size before the patient is taken to the operating room. Based on our clinic data, nomograms have therefore been developed for one of the widely used 103Pd seeds, the MED3633 seed, which is produced by North American Scientific, Inc. (NASI). The total activities for implant volume ranging from 15 cc to 55 cc are provided for both seed "monotherapy" and seed boost.

Automatic post-implant needle reconstruction algorithm to characterize and improve implant robustness analyses

Post-implant analysis in permanent implant brachytherapy is an important process that provides a feedback on treatment quality. Random seed movements, edema, and needle related factors contribute to deteriorate dose coverage. For a complete study of these movements, it is important to reconstruct the post-implant seeds clusters but, up to now, this task was only possible via a long and difficult manual process. To facilitate post-implant analysis a simulated annealing algorithm was developed to perform automatic reconstructions. This process is fast (30-60 s on a 1.3 GHz pentium) and has a high level of success, even with up to 5% of seed loss. Tests on 21 clinical cases show that the algorithm yields exactly the same results as manual reconstructions. A realistic simulation tool was used to generate 58 synthetic post-implant data, in which cases the exact configuration was known. Even if some errors were found, pertinent information was extracted. For medium seed density [corresponding to seeds of 0.6 mCi (0.762 U)], 97% of seeds are matched with their correct needle and 89% are matched with their correct planned position. This method provides pertinent information that can be used to understand inhomogenous dose coverage in specific prostate quadrants; to make realistic post-implant simulations or to identify seeds belonging to a needle loaded with different seed types or activity.

Seed loss through the urinary tract after prostate brachytherapy: examining the role of cystoscopy and urine straining post implant

This study describes one institution's experience with seed retrieval through the urinary tract and makes recommendations for cystoscopy and urine straining post prostate brachytherapy (PB). 1794 patients from two separate cohorts covering different time periods (early versus late) were analyzed. All patients were preplanned with a modified peripheral loading technique and implanted with preloaded needles (125I or 103Pd) under ultrasound guidance. A catheter was used to delineate the urethra during the volume study but was not used during the implant. All patients underwent post implant cystoscopy. All patients were instructed to strain their urine for seven days post implant and return any seeds to our center. In our experience, seed loss through the urinary tract is a common event after PB, occurring in 29.7% of patients and was more common in patients from the early cohort, those implanted with 125I seeds or those patients with prior transurethral resection of the prostate. Average seed loss per case, however, represents only 0.58% of total activity. We continue to recommend routine post implant cystoscopy for seed retrieval and periprocedural management. We no longer recommend that patients strain their urine at home after documenting a low rate of seed loss after discharge.

Dosimetric consequences of increased seed strength for I-125 prostate implants

Based on the findings of an earlier planning study, we compared post-implant dose distributions for two groups of 20 consecutive patients treated to 145 Gy with 0.414 and 0.526 U I-125 seeds. Dosimetric coverage as measured by the key clinical index D(90) was significantly better for the higher-strength seeds, with no apparent deleterious effects.

A systematic study of imaging uncertainties and their impact on 125I prostate brachytherapy dose evaluation

In order to calculate the dose distribution delivered by a prostate brachytherapy implant, the seed positions and prostate volume are normally identified on post-implant CT images. We have systematically considered the impact of uncertainties in contouring the prostate, seed localization, and visualization of all the seeds on the calculated dose distributions, dose-volume histograms, and predicted radiobiological outcome. This study was done for a collection of 27 clinical 125I prostate brachytherapy implants, performed at the London Regional Cancer Centre during our early adoption of this technique. For these clinical dose distributions, the median D90 was 76% of the prescription dose, or 110 Gy, and the median V90 was 80%. We calculated the changes in these dosimetric indices (D90 and V90) and radiobiological outcome (SF2 TCP) as a function of contouring uncertainty, seed localization uncertainty, inability to localize all of the seeds, and binary combinations of these three. The results are presented for a range of uncertainties, which allows the possible application of these results to a variety of imaging modalities that have differing spatial resolutions. We found that both contouring uncertainties and seed localization uncertainties had a large impact on the predicted radiobiological outcome, but that seed localization uncertainties of 6 mm had the largest impact on the dosimetric indices. We also found that the variability in both the predicted radiobiological and dosimetric outcome was largest for contouring uncertainties of 4-8 mm. We conclude that accounting for contouring uncertainties is crucial in accurately deducing the DVHs for post-implant prostate brachytherapy, and hence enabling valid correlation with ultimate clinical outcome.

Is intraoperative nomogram-based overplanning of prostate implants necessary?

PURPOSE

Several investigators have described intraoperative planning of prostate implants based on a nomogram. The aim of this work was to investigate the adequacy of the nomogram in predicting the total activity necessary for optimal dosimetry.

METHODS AND MATERIALS

Eighty CT-based postimplant treatment plans were performed for patients who underwent ultrasound guided I-125 permanent implants alone between April 2000 and March 2001. The cohort of 40 patients had early stage (T1-T2) prostatic carcinoma and pre-treatment prostate volumes of 19-50 cc. I-125 seeds (0.391 mCi/seed) were implanted to achieve a distribution of 75% of the activity peripherally and 25% centrally. The CT studies were obtained on the day of (CT1) and at 1 month (CT2) after implant. All patients were catheterized at CT1, and 28 patients were catheterized at CT2 to visualize the urethra. For each patient, the percentage difference (dA) between the total implanted and nomogram predicted activity for a known prostate volume was calculated. The V200 (volume receiving 200% of the prescribed dose), V150, V100, V90, D100 (maximum dose received by 100% of the volume), D90, and D80 were measured for the prostate at CT1 and CT2. For the urethra, V275, V250, V200, and V150 were evaluated, and V100 and V70 were evaluated for the rectum. The Pearson test was used to correlate the dosimetric parameters with dA. Linear regression was used to fit the correlation of the volume and dose parameters with dA.

RESULTS

The median V100 at CT1 and CT2 was 91.8% and 94.2%, respectively. The Pearson test was significant for the prostate V100 and dA measured at CT1 (p = 0.005) but not at CT2 (p = 0.106). A similar correlation was found for the prostate D90 at CT1 (p = 0.002), but not at CT2 (p = 0.076). D100 (maximum dose received by 100% of volume) for prostate did not correlate with dA at CT1 (p = 0.094) and CT2 (p = 0.148). The volume of the prostate receiving higher doses (greater than 150% and 200% of the prescribed dose) correlated with dA. There were no significant correlations between V275, V250, V200, and V150 at CT1 and CT2 as a function of dA for the urethra. V100 and V70 for the rectum correlated significantly with dA; for V100, p = 0.041 at CT1 and p = 0.014 at CT2 and for V70, p = 0.041 at CT1 and p = 0.026 at CT2. A linear regression model fitted to the prostate data obtained from CT1 with the goal of achieving a V100 of 90% and D90 of 145 Gy suggests that no increase in the number of seeds may be warranted using intraoperative planning. The implants examined showed no concomitant increase of urethral doses with increase in activity relative to the nomogram, but showed an increase in the rectal doses for the same increase in activity.

CONCLUSION

The doses evaluated at CT1 represent an underestimate, whereas those obtained at CT2 represent an overestimate of the actual delivered protracted permanent implant dose. Based on these results and consideration of the dynamic nature of the dose distribution, target coverage obtained with intraoperative planning using the nomogram predicted activity is consistent with published guidelines for a quality implant and critical structure doses are within tolerance.

Automatic localization of implanted seeds from post-implant CT images

An automatic localization method of implanted seeds from a series of post-implant computed tomography (CT) images is described in this paper. Post-implant CT studies were obtained for patients who underwent prostate brachytherapy. Bright areas were segmented using binary thresholding in each CT slice, and geometrical information on these areas was collected. Large areas (possibly containing two connected seeds) were split into smaller ones by geometry-based filtering in each slice. The area connectivity along the longitudinal direction was analysed using a geometry-based connection search algorithm executed on every area slice by slice, so that the connected areas were combined into one object. The weighted centroid of each object was taken as the seed position. This method was tested on a seed-containing prostate phantom as well as using CT studies from patients. Statistical analysis demonstrates that it can achieve above 99% detection rate with reliable localization accuracy and high speed. It is reliable and convenient for localizing implanted seeds on CT and can be used to assist post-implant dosimetry for prostate brachytherapy.

[Iodine-125 transperineal prostate brachytherapy with preplanning technique: pre and post-implant dosimetry results analysis]

PURPOSE

Post-implant CT-based dosimetry is the only method of assessing the quality of permanent prostate brachytherapy. As a consequence of our permanent feedback with the preplanned technique, geometric and dosimetric criteria for optimal seed implantation are proposed and pre and post-implantation dosimetric results are presented.

PATIENTS AND METHODS

In 2000 and 2001, one hundred and twenty patients with early stage prostate cancer were treated with transperineal I-125 preplanned brachytherapy (RAPID Strand, Amersham Health). The prescription dose was 145 Gy to the planning target volume. For the pre-planning and post-implant dosimetry the Variseed 6.7 version software was used (Varian Medical Systems). The D90, V100 and V150 values, the position of the dose peak [Dose] peak) and the full width at half maximum (FWHM) on differential dose volume histogram from both planned and post-implant dosimetry were compared for all patients.

RESULTS

For preplanned dosimetry, the mean values for D90, V100, V150, [Dose] peak, FWMH were respectively of 199Gy, 100%, 70%, 220Gy, 113Gy. For post-implantation, these values became respectively of 157Gy, 90%, 62%, 220Gy, 194Gy.

CONCLUSION

In our practice, differences are noted between preplanned and post-implant dosimetry parameters that should be anticipated to assure optimal definitive result. A working methodology both for performing the preplanned dosimetry and for evaluating the post-implantation dosimetric results is proposed.

Brachytherapy dosimetry of 125I and 103Pd sources using an updated cross section library for the MCNP Monte Carlo transport code

Permanent implantation of low energy (20-40 keV) photon emitting radioactive seeds to treat prostate cancer is an important treatment option for patients. In order to produce accurate implant brachytherapy treatment plans, the dosimetry of a single source must be well characterized. Monte Carlo based transport calculations can be used for source characterization, but must have up to date cross section libraries to produce accurate dosimetry results. This work benchmarks the MCNP code and its photon cross section library for low energy photon brachytherapy applications. In particular, we calculate the emitted photon spectrum, air kerma, depth dose in water, and radial dose function for both 125I and 103Pd based seeds and compare to other published results. Our results show that MCNP's cross section library differs from recent data primarily in the photoelectric cross section for low energies and low atomic number materials. In water, differences as large as 10% in the photoelectric cross section and 6% in the total cross section occur at 125I and 103Pd photon energies. This leads to differences in the dose rate constant of 3% and 5%, and differences as large as 18% and 20% in the radial dose function for the 125I and 103Pd based seeds, respectively. Using a partially updated photon library, calculations of the dose rate constant and radial dose function agree with other published results. Further, the use of the updated photon library allows us to verify air kerma and depth dose in water calculations performed using MCNP's perturbation feature to simulate updated cross sections. We conclude that in order to most effectively use MCNP for low energy photon brachytherapy applications, we must update its cross section library. Following this update, the MCNP code system will be a very effective tool for low energy photon brachytherapy dosimetry applications.

A medical needle drive for the study of interstitial implant mechanics

We describe the construction and test performance of a computer-controlled medical needle drive. The drive represents one facet of a larger project whose aim is to investigate experimentally the mechanics of needle introduction in radioactive 'seed' prostate implants, with a view to identifying ways of making incremental improvements in needle placement accuracy. It is capable of mimicking a range of motions imparted to a needle by a clinical practitioner, and of monitoring the compressive force at the needle tip in real time via an in-line load cell. Tests involving driving needles into porcine gelatin samples using a variety of velocity profiles confirm intended operation. The drive will permit us to introduce needles in a controlled and reproducible manner into a realistic prostate phantom currently being designed.

Integral-transport-based deterministic brachytherapy dose calculations

We developed a transport-equation-based deterministic algorithm for computing three-dimensional brachytherapy dose distributions. The deterministic algorithm has been based on the integral transport equation. The algorithm provided us with the capability of computing dose distributions for multiple isotropic point and/or volumetric sources in a homogenous/heterogeneous medium. The algorithm results have been benchmarked against the results from the literature and MCNP results for isotropic point sources and volumetric sources.

Is there a preferred strength for regularly spaced 125I seeds in inverse-planned prostate implants?

PURPOSE

To determine whether a preferred seed strength exists for 125I prostate implants preplanned using a fixed intraneedle seed spacing of 1 cm and an objective needle placement strategy within the planning target volume (PTV), and incorporating explicit dose-volume constraints for the PTV and tissues at risk.

METHODS AND MATERIALS

Prostate, urethra, and rectum contours for 10 patients were obtained from transrectal ultrasound studies. The PTV was defined in accordance with Radiation Therapy Oncology Group (RTOG) 0019 protocol. Inverse planning software was used to optimally arrange seeds of strength 0.3-0.8 U to cover the PTV to D(Rx) = 145 Gy, and limit urethra and rectum doses to 150% and 100% of D(Rx), respectively. Isodose distributions and dosimetric indices were calculated: V(200), V(150), V(100), V(90), D(100), D(90) for PTV; V(150) for urethra; and V(100) for rectum. For seeds of strength 0.414 and 0.6 U and three prostate sizes, the sensitivity of V(90) and D(90) to elementary perturbations of the optimal seed arrangement were examined.

RESULTS

For our planning scenario, 125I seeds of strength 0.5-0.6 U provided the best possible PTV coverage while maintaining V(200) at approximately 25%. The source arrangement for 0.6-U seeds was only modestly more sensitive to perturbations than that for 0.414-U seeds. These findings may not be applicable to implants planned manually or that involve needle placement outside the PTV.

CONCLUSION

Given a particular source arrangement, inverse planning aimed at maximizing dosimetric coverage of the prostate while limiting doses to the urethra and rectum can be used to search for a preferred seed strength. For regularly spaced sources within the PTV, higher strength seeds can provide better dose coverage and better urethral protection than lower strength seeds.

A seed specific dose kernel method for low-energy brachytherapy dosimetry

We describe a method for independently verifying the dose distributions from pre- and post-implant brachytherapy source distributions. Monte Carlo calculations have been performed to characterize the three-dimensional dose distribution in water phantom from a low-energy brachytherapy source. The calculations are performed in a voxelized, Cartesian coordinate geometry and normalized based upon a separate Monte Carlo calculation for the seed specific air-kerma strength to produce an absolute dose grid with units of cGy hr(-1) x U(-1). The seed-specific, three-dimensional dose grid is stored as a text file for processing using a separate visual basic program. This program requires the coordinate positions of each seed in the pre- or post-plan and sums the kernel file for a three-dimensional composite dose distribution. A kernel matrix size of 81x81x81 with a voxel size of 1.0x1.0x1.0 mm3 was chosen as a compromise between calculation time, kernel size, and truncation of the stored dose distribution as a function of radial distance from the midpoint of the seed. Good agreement is achieved for a representative pre- and post-plan comparison versus a commercial implementation of the TG-43 brachytherapy dosimetry protocol.

Comparison of intraoperative dosimetric implant representation with postimplant dosimetry in patients receiving prostate brachytherapy

PURPOSE

To compare the results of intraoperative dosimetry with those of CT-based postimplant dosimetry in patients undergoing prostate seed implantation.

METHODS AND MATERIALS

Seventy-seven patients with T1-T3 prostate cancer received an ultrasound-guided permanent seed implant (36 received (125)I, 7 (103)Pd, and 34 a partial (103)Pd implant plus external beam radiation therapy). The implantation was augmented with an intraoperative dosimetric planning system. After the peripheral needles were placed, 5-mm axial images were acquired into the treatment planning system. Soft tissue structures (prostate, urethra, and rectum) were contoured, and exact needle positions were registered. Seeds were placed with an applicator, and their positions were entered into the planning system. The dose distributions for the implant were calculated after interior needle and seed placement. Postimplant dosimetry was performed 1 month later on the basis of CT imaging. Prostate and urethral doses were compared, by using paired t tests, for the real-time dosimetry in the operating room (OR) and the postimplant dosimetry.

RESULTS

The mean preimplant prostate volume was 39.8 cm(3), the postneedle planning volume was 41.5 cm(3) (p<0.001), and the 1-month CT volume was 43.6 cm(3) (p<0.001). The mean difference between the OR dose received by 90% of the prostate (D(90)) and the CT D(90) was 3.4% (95% confidence interval, 2.5-6.6%; p=0.034). The mean dose to 30% of the urethra was 120% of prescription in the OR and 138% on CT. The mean difference was 18% (95% confidence interval, 13-24%; p<0.001).

CONCLUSIONS

Although small differences exist between the OR and CT dosimetry results, these data suggest that this intraoperative implant dosimetric representation system provides a close match to the actual delivered doses. These data support the use of this system to modify the implant during surgery to achieve more consistent dosimetry results.

Ultrasonography and fluoroscopic fusion for prostate brachytherapy dosimetry

PURPOSE

To investigate the feasibility of performing postimplant and intraoperative dosimetry for prostate brachytherapy by fusing transrectal ultrasound (TRUS) and fluoroscopic data.

METHODS AND MATERIALS

Registration of ultrasound (prostate boundary) and fluoroscopic (seed) data requires spatial markers that are detectable by both imaging modalities. In this study, the needle tips were considered as such fiducials. Prostate phantoms were implanted with the seeds, and four localization needles were inserted. In the TRUS frame of reference, the longitudinal coordinate of the needle tip was determined by advancing the needle until the echo from its tip just registered at a known probe depth. The tip's transverse coordinates were determined from the associated TRUS slice. The three-dimensional needle tip positions were also calculated in the fluoroscopic coordinate system using a seed reconstruction method. The transformation between the TRUS and fluoroscopy coordinate systems was established by the least-squares solution using the singular value decomposition.

RESULTS

With three of four needle tips as fiducials and the one remaining needle as a test target, the mean fiducial registration error was 0.8 mm and the test target registration error was 2.5 mm. When all four points were used for registration, the errors decreased to 1.1 mm. A comparison between the proposed method and CT-based dosimetry yielded a percentage of prostate volume receiving 100% and 150% of the prescribed minimal peripheral dose and minimal dose received by 90% of the prostate gland that agreed within 0.4%, 2.7%, and 4.2%, respectively.

CONCLUSION

The combination of TRUS and fluoroscopy is a feasible alternative to the currently used CT-based postimplant dosimetry. Furthermore, because of online imaging capability, the method lends itself to real-time intraoperative applications.

Design of quality assurance for sonographic prostate brachytherapy needle guides

OBJECTIVE

Needles are guided to their proper anatomic locations in sonographically guided percutaneous prostate brachytherapy by a mechanical system (template). A quality assurance procedure has been designed to test this template's alignment with the needle position overlay grid displayed in the sonographic image.

METHODS

A mechanical arrangement was designed to position the needles properly with respect to the prostate probe's transducer in a liquid-filled test tank. Two liquids were tested: tap water and an ethylene glycol mixture with an acoustic velocity of 1540 m/s. Needle images with the superposed grid were analyzed for needle placement accuracy.

RESULTS

The tap water produced misregistration of the needle images. The ethylene glycol mixture yielded images of vertical and horizontal needle positions accurate to 0.3 and 1 mm, respectively. Also, the importance of selecting the lowest possible equipment echo amplitude dynamic range in these tests was shown.

CONCLUSIONS

This quality assurance test with the ethylene glycol mixture will permit accurate alignment of the brachytherapy needle position overlay grid for each separate transrectal probe used.

Impact of postimplant edema on V(100) and D(90) in prostate brachytherapy: can implant quality be predicted on day 0?

PURPOSE

To determine the effect of edema on the dosimetric parameters V(100) (percentage of prostate volume that received a dose equal to or greater than the prescribed dose) and D(90) (minimal dose delivered to 90% of prostate volume) in 125I prostate brachytherapy and to determine whether the edema can be used to predict implant quality on the day of the implant (Day 0).

METHODS AND MATERIALS

Fifty consecutive patients treated with (125)I implants who had two postimplant CT scans were selected for this study. The mean interval between the studies was 46 +/- 23 days. The implants were preplanned to deliver 150 Gy to the prostate plus a 3-5-mm symmetric dose margin using peripherally loaded 0.4-0.6-mCi (NIST-99) (125)I seeds. A dose-volume histogram was compiled for each postimplant CT scan. The V(100) and D(90) from the first and second CT scans were compared to determine the effect of edema on these parameters. A multivariate regression analysis was performed to define the linear relationships for predicting the V(100) or D(90) at 30-60 days after implant from the magnitude of the edema and the values of V(100) and D(90) on Day 0.

RESULTS

V(100) and D(90) increased by 5% +/- 6% and 15% +/- 17%, respectively, during the interval between the first and second postimplant CT scans. The mean edema was 1.53 +/- 0.20. The increases in V(100) and D(90) were found to be proportional to the edema and the values of V(100) and D(90) on Day 0. The increase in V(100) was also found to depend on the width of the preplan dose margin. Linear relationships were derived that predict the V(100) and D(90) at 30-60 days after implant with a standard error of +/-4% and +/-24 Gy, respectively.

CONCLUSION

V(100) and D(90) increased by 5% +/- 6% and 15% +/- 17%, respectively, during the first 30-60 days after implant. The results of a multivariate linear regression analysis showed that the increases in V(100) and D(90) were proportional to both the magnitude of the edema and the values of these parameters on Day 0. The relationships derived by linear regression analysis predict V(100) and D(90) at 30-60 days after implant to within +/-4% and +/-24 Gy, respectively. However, predicting the 30-60-day V(100) and D(90) on Day 0 is a poor substitute for obtaining a 30-60-day CT scan, because the uncertainty in the predicted values is greater by a factor of > or =2. Nevertheless, on average, the predicted values should provide a more reliable estimate of the actual V(100) and D(90) than the Day 0 values that ignore the effect of edema altogether. The increase in V(100) was also found to depend on the width of the preplan dose margin; therefore, our results for V(100) are only valid for implants planned with a 3-5-mm margin.

Comparison of methods for calculating rectal dose after (125)I prostate brachytherapy implants

PURPOSE

To compare several different methods of calculating the rectal dose and examine how accurately they represent rectal dose surface area measurements and, also, their practicality for routine use.

METHODS AND MATERIALS

This study comprised 55 patients, randomly selected from 295 prostate brachytherapy patients implanted at the Vancouver Cancer Center between 1998 and 2000. All implants used a nonuniform loading of 0.33 mCi (NIST-99) 125I seeds and a prescribed dose of 144 Gy. Pelvic CT scans were obtained for each patient approximately 30 days after implantation. For the purposes of calculating the rectal dose, several structures were contoured on the CT images: (1) a 1-mm-thick anterior rectal wall, (2) the anterior half rectum, and (3) the whole rectum. Point doses were also obtained along the anterior rectal surface. The thin wall contour provided a surrogate for a dose-surface histogram (DSH) and was our reference standard rectal dose measurement. Alternate rectal dose measurements (volume, surface area, and length of rectum receiving a dose of interest [DOI] of > or =144 Gy and 216 Gy, as well as point dose measures) were calculated using several methods (VariSeed software) and compared with the surrogate DSH measure (SA(DOI)).

RESULTS

The best correlation with SA(144 Gy) was the dose volumes (whole or anterior half rectum) (R = 0.949). The length of rectum receiving > or =144 Gy also correlated well with SA(144 Gy) (R > or =0.898). Point dose measures, such as the average and maximal anterior dose, correlated poorly with SA(144 Gy) (R < or =0.649). The 216-Gy measurements supported these results. In addition, dose-volume measurements were the most practical (approximately 6 min/patient), with our surrogate DSH the least practical (approximately 20 min/patient).

CONCLUSION

Dose-volume measurements for the whole or anterior half rectum, because they were the most practical measures and best represented the DSH measurements, should be considered a standard method of reporting the rectal dose when calculating the DSH is not practical. Average or maximal anterior rectal doses are not reliable indicators of surface area dosimetry.

The Anderson nomograms for permanent interstitial prostate implants: a briefing for practitioners

PURPOSE

The objective of this report is to re-evaluate the role of the Anderson nomograms in treatment planning for permanent prostate implants. The incentive for revisiting this topic concerns three issues: (1) Although nomograms continue to be used in many centers for ordering seeds, few centers use them during treatment planning; (2) Whereas nomograms were designed to deliver a minimum peripheral dose for a uniform distribution of seeds in the gland, many practitioners use peripheral seed loading patterns to reduce urethral toxicity; and (3) As preoperative and intraoperative treatment planning is becoming standard, the apparent role of nomograms is diminished. The nomogram method is reviewed in terms of: (1) total activity predicted, (2) target coverage (as planned in the operating room and as calculated from postimplant computed tomography studies), and (3) reproducibility (i.e., patient-to-patient and planner-to-planner variability). In each case, the computer-optimization system for intraoperative planning currently in use at our institution was taken as the "gold standard."

METHODS AND MATERIALS

We compared for the same patient the results of nomogram planning to those yielded by genetic algorithm (GA) optimization in terms of total activity predicted (n = 20 cases) and percent target coverage (n = 5 cases). Furthermore, we examined retrospectively the dosimetry of 61 prostate implants planned with the GA (n = 27) and the current implementation of Anderson nomograms (n = 34).

RESULTS

Nomogram predictions of the total activity required are in good agreement (within 10%) with the GA-planned activity. However, computer-optimized plans consistently yield superior plans, as reflected in both pre- and postimplant analyses. We find also that user (specifically, treatment planner) implementation of the nomograms may be a major source of variability in nomogram planning-a difficulty to which robust computer optimization is less prone.

CONCLUSIONS

Nomograms continue to be useful tools for predicting the total required activity for volume implants, and thus for performing an independent check of this quantity. Not unexpectedly, computer optimization remains the preferred planning method. Generally, nomogram-guided implants do not incorporate structures other than the treatment volume into the planning process. Further yet, they deliver a lower dose than that prescribed and result in greater variability among plans than computer-optimized treatments. In summary, nomograms (1) remain an efficient quality assurance tool for computer-generated plans, (2) serve as a good predictor of the number of seeds required for ordering purposes, and (3) provide a simple and dependable backup planning method in case the intraoperative planning system fails.

Dosimetric impact of the variation of the prostate volume and shape between pretreatment planning and treatment procedure

PURPOSE

The goal of this study is to evaluate the dosimetric impact on a pretreatment planning of prostatic volume and shape variations occurring between the moment of the volume study (preplanning) and just before a transperineal permanent seed implant procedure. Such variations could be an obvious source of misplacement of the seeds relative to the prostate gland and organs at risk. Other sources of dosimetric uncertainties, such as misplacement due to the procedure itself or edema, are eliminated by looking at these variations before the implant procedure.

METHODS AND MATERIALS

For 35 clinical cases, prostate contours were taken at preplanning time as well as in the operating room (OR) minutes before the procedure. Comparison of shape and volume between the two sets was made. The impact on V100 was evaluated by placing the seeds in their planned positions in the new volume (clinical situation) and also by performing a new plan with the second set of contours to simulate an intraoperative approach.

RESULTS

The volume taken in the OR remained unchanged compared to the pretreatment planning volume in only 37% of the cases. While on average the dose coverage loss from pretreatment planning due to a combination of variations of volume and shape was small at 5.7%, a V100 degradation of up to 20.9% was observed in extreme cases. Even in cases in which no changes in volume were observed, changes in shape occurred and strongly affected implant dosimetry.

CONCLUSIONS

Variations of volume and shape between pretreatment planning and the implant procedure can have a strong impact on the dosimetry if the planning and the implant procedure are not performed on the same day. This is an argument in favor of performing implant dosimetry in the OR.

Impact of prostate volume evaluation by different observers on CT-based post-implant dosimetry

PURPOSE

An analysis of computed tomography (CT)-based dosimetry was performed to evaluate the variability of different observers' judgements in marking the prostate gland on CT films, and its effect on the parameters that characterise the prostate implantation quality. Accuracy of data entry by the first author in the process of dosimetry procedure has also been evaluated.

MATERIALS AND METHODS

Four observers were asked to evaluate the prostate volume on CT films for six different patients. Each observer repeated the evaluation six times. The sample of patients has a prostate volume in the range of 21.4-42.0 cc derived from transrectal ultrasound volume study. After an average period of 6 weeks of the I-125 implantation, all patients had CT scans. CT-based post-implant dosimetry was performed and the dose volume histograms DVHs were calculated to report the re-constructed prostate volume, Vp100, Vp150, Vp90 and D90. Comparison between the four observers' output was performed.

RESULTS

Comparison between the four observers shows that each observer has a different way of estimating the prostate on CT films. Observers' precision also varies according to the prostate volume and the image quality. This can cause a variation in the resulting D90 value by up to 50%. Analysis of data entry shows a high degree of accuracy. The error of digitizing the prostate is +/-0.19 cc. This is correlated to an error of +/-0.78 Gy of the D90.

CONCLUSION

The evaluation of prostate gland volume on CT films varies between different observers. This has an effect on the dosimetric indices that characterise the implant quality in particular the D90.

The effect of seed orientation deviations on the quality of 125I prostate implants

We quantified the effect of seed orientation deviations on five prostate seed implant cases at our institution. While keeping their positions fixed, the iodine-125 seeds were assigned orientations sampled from a realistic probability distribution derived from the post-implant radiographs of ten patients. Dose distributions were calculated with both a model that explicitly includes anisotropy (TG43 anisotropy function) and a point source model (TG43 anisotropy factor). Orientation deviations had only a small influence on prostate dose-volume histograms: the 95% confidence intervals on the volumes receiving 100%, 150% and 200% dose were at most +/-0.8%, +/-1.1% and +/-0.6% of the prostate volume, respectively. The dose-volume histograms of anisotropic seed distributions were marginally better than those with isotropic point-source seeds. Anisotropy caused a displacement of cold spots (regions receiving <100% of the prescribed dose) in <1% of the prostate volume. Our results indicate no net benefit to prostate dosimetry in using more isotropic seeds. Furthermore, we propose a new 'weighted anisotropy function' to better account for the effects of anisotropy when seed orientation is unknown. Conceptually, the TG43 anisotropy factor described in AAPM TG43 averages the effect of anisotropy over all solid angles, with the implicit assumption that all seed orientations are equally probable. In prostate implants, however, seeds are preferentially oriented parallel to the needle axis. The proposed weighted anisotropy function incorporates this non-uniform probability.

An algorithm for automatic, computed-tomography-based source localization after prostate implant

Permanent implant of the prostate using I-125 and Pd-103 seeds is a popular choice of treatment for early-stage prostate cancer in the United States. Evaluation of the quality of the implant is best based on the calculated dose distribution from postimplant computed tomography (CT) images. This task, however, has been time-consuming and inaccurate. We have developed an algorithm for automatic source localization from postimplant CT images. The only requirement of this algorithm is knowledge of the number of seeds present in the prostate, thus minimizing the need for human intervention. The algorithm processes volumetric CT data from the patient, and pixels of higher CT numbers are categorized into classes of definite and potential source pixels. A multithresholding technique is used to further determine the number of seeds and their precise locations in the CT volume data. A graphic user interface was developed to facilitate operator review of and intervention in the calculation and the results of the algorithm. This algorithm was tested on two phantoms containing nonradioactive seeds, one with 20 seeds in discrete locations and another with 100 seeds with small distances between seeds. The tests showed that the algorithm was able to identify the seed locations to within 1 mm of their physical locations for discrete seed locations. It was further able to separate seeds at close proximity to each other while maintaining an average seed localization error of less than 2 mm, with no operator intervention required.

Dosimetry of the I-Plant Model 3500 iodine-125 brachytherapy source

125I brachytherapy sources have been widely used for interstitial implants for a number of years in several tumor sites, especially the prostate. The design of the new I-Plant Model 3500 iodine source is novel, yet its characteristics are similar to those of two existing designs, Model 6711 and the Symmetra. Dosimetry parameters (including dose rate constant, radial dose function, and anisotropy function, as defined by AAPM Task Group 43) were measured with LiF thermoluminescent dosimeters in water-equivalent plastic phantoms. The dose rate constant was found by direct comparison of calibrated I-Plant Model 3500 and Model 6711 seeds in a solid water phantom, to be 1.01 (cGy/h)/U. The radial dose function and anisotropy function are similar to those of the Model 6711 and Symmetra seeds.

A theoretical derivation of the nomograms for permanent prostate brachytherapy

This study calculates the required minimum radioactivity to deliver a prescribed dose of radiation to a target using radioisotopes in permanent prostate brachytherapy. Assuming the radioactivity to be in a continuous form, an integral equation--Fredholm equation of the first kind, can be formulated with the radioactivity density used as the variable. The density distribution to produce a uniform volume dose rate is determined using a quadrature method and the radial profile behaves smoothly from the zero radius, and peaks sharply approaching the volume boundary. The density for Pd-103 is about 1.5 times that of I-125 due to its higher spatial attenuation. A nomogram is the relationship between the total activity per unit dose (A) and the dimension of the volume (d). Expressing the nomogram as A=c X dn U/Gy, then (c,n)= [(0.0098, 2.09) I-125] and [(0.031, 2.25) Pd-103]. Compared with the Memorial nomogram, (c,n)=[(0.011,2.2) I-125] and [(0.036,2.56) Pd-103], or that quoted by AAPM TG64, (c,n)=[(0.014,2.05) I-125] and [(0.056,2.22) Pd-103], our calculation determined an average 33% and 35% decrease for I-125, and 89% and 77% decrease for Pd-103, respectively. Two reasons for the extra total activity found in the Memorial and AAPM nomograms are: (a) An imperfect clinical situation limited by the restraints of implant techniques (e.g., use of templates) associated with the presence of adjacent normal organs, and (b) source discretization into seeds. When radioactivity is clumped as discrete seeds, higher activity is needed because of "wastage" in two aspects: (a) Dose cold-spots at intersource spaces, (b) hot-spots around the sources. Thus in theory, use of lower activity seeds will require less total activity to deliver a prescribed dose. Based on our study, Pd-103 delivers a higher therapeutic ratio and a lower integral dose to the patient compared to I-125.

The effect of seed anisotrophy on brachytherapy dose distributions using 125I and 103Pd

We have evaluated the effect of the anisotropy of individual seeds on dose distributions for permanent prostate implants using 125I and 103Pd. The dose distributions were calculated for various implants using both the line source and point source calculational formalisms, for two different models of 125I and 103Pd seeds. The dose distributions were compared using cumulative dose volume histograms (DVH) and cumulative difference dose volume histograms (deltaDVH) for the prostate target volume and for the rectum surface. The DVHs could not distinguish between the dose distributions from isotropic and non-isotropic seeds. However, the deltaDVHs were useful in determining the fraction of the target volume for which the difference between the dose distribution for line sources and for point sources exceeded a threshold value. The dose distributions were calculated (1) for all the seeds oriented co-linearly, along either the x-, y-, or z-axis, and (2) for the seeds at randomized orientations, more closely resembling the clinical situation. For all cases, there was a significant difference in the effect of seed anisotropy from the different seed types. For the geometrically simpler test cases with a small number of seeds, the effect of anisotropy on the dose distribution was too large to ignore for any of the seed types investigated. For the idealized pre-plan case, the effect was much smaller. For clinical prostate implants, the calculations done with seeds oriented co-linearly along the z-axis (needle implant axis) were a reasonable approximation for those from simulations of seeds with randomized orientations. Again, the effect of anisotropy varied drastically between different seed models, and also between different clinical cases. However, the effect of anisotropy must be considered in the context of all the other uncertainties in clinical brachytherapy treatments.

Ultrasound imaging in three and four dimensions

Three-dimensional (3D) reconstruction of ultrasound images was first demonstrated nearly 15 years ago, but only now is becoming a clinical reality. In the meantime, methods for 3D reconstruction of CT and MRI images have achieved an advanced state of development, and 3D imaging with these modalities has been applied widely in clinical practice. 3D applications in ultrasound have lagged behind CT and MRI, because ultrasound data is much more difficult to render in 3D, for a variety of technical reasons, than either CT or MRI data. Only in the past few years has the computing power of ultrasound equipment reached a level adequate enough for the complex signal processing tasks needed to render ultrasound data in three dimensions. At this point in time, the clinical application of 3D ultrasound is likely to advance rapidly, as improved 3D rendering technology becomes more widely available. This article is a review of the present status of 3D ultrasound imaging. It begins by comparing the characteristics of CT, MRI, and ultrasound image data that either make these data amenable or not amenable to 3D reconstruction. The article then considers the technical features involved with acquiring an ultrasound 3D data set and the mechanisms for reconstructing the images. Finally, the article reviews the literature that is available regarding clinical application of 3D ultrasound in obstetrics, ultrasound, the abdomen, and blood vessels.

Phase II prospective study of the use of conformal high-dose-rate brachytherapy as monotherapy for the treatment of favorable stage prostate cancer: a feasibility report

PURPOSE

To evaluate the technical feasibility and tolerance of image-guided transperineal conformal high-dose-rate (C-HDR) brachytherapy as the sole treatment modality for favorable, localized cancer of the prostate, and to analyze possible intrafraction and interfraction volume changes in the prostate gland which may affect dosimetric quality.

METHODS AND MATERIALS

Patients were eligible for this prospective Phase II trial if they had biopsy proven adenocarcinoma of the prostate with favorable prognostic factors (Gleason score < or =7, PSA < or =10 ng/ml and Stage < or =T2a). The technique consisted of a transperineal implant procedure using a template with transrectal ultrasound (TRUS) guidance. An interactive on-line real-time planning system was utilized with geometric optimization. This allowed dosimetry to be generated and modified as required intraoperatively. Prescription was to the minimum dose point in the implanted volume, assuring conformal coverage of the prostate at its widest dimension with no margin. Total dose was 3800 cGy in 4 fractions of 950 cGy each, delivered twice a day over 2 days. The dose to any segment of rectum and urethra was limited to < or =75% and < or =125% of the prescription dose, respectively. Before each fraction, needle positions were verified under fluoroscopy and adjusted as required. For the last 10 patients, the adjustments required were measured in a prospective fashion in representative extrema of the gland. TRUS images were recorded for all patients before any needle manipulation, again just before delivering the first fraction and immediately after the last fraction. This typically meant approximately 36 h to pass between the first and last measurements. Implant quality was assessed via dose-volume histograms (DVH).

RESULTS

Between 3/99 and 6/00, 41 patients received C-HDR interstitial brachytherapy as their only treatment for prostate cancer at our institution. Median age was 64 years (range 51-79). Stage distribution was 27 T1c patients and 14 T2a patients. Three patients had Gleason score (GS) of 5; 34 had GS of 6; 4 patients had GS of 7. Median pretreatment PSA was 4.7 ng/ml (range 0.8-13.3). All patients tolerated the treatment well with minimal discomfort. For 23 patients, data on volume changes in the gland during the implant were tabulated. They demonstrated a mean prostate volume of 30.7 cc before any manipulation with needles, 37.0 cc at the end of fraction 1, and 38.2 cc at the end of fraction 4. In addition, for those 10 patients prospectively evaluated for required adjustments, the overall mean adjustment between fraction 1 and fraction 2 was 2.0 cm, between fraction 2 and 3 was 0.4 cm, and between fractions 3 and 4 was 0.4 cm. For 10 consecutive patients, the average prescriptions dose -D90 for fractions 1 and 4 were 104% and 100%, respectively. The corresponding average urethral D10 for fractions 1 and 4 were 122% and 132%.

CONCLUSION

Our protocol using C-HDR interstitial brachytherapy as monotherapy for early cancer of the prostate was feasible and well tolerated by 41 patients treated. Changes in interfraction prostate volume do not appear to be significant enough to warrant modification of dosimetry for each fraction. Both excellent dose coverage of the prostate gland and low urethral dose are achieved as measured by DVH. However, paramount attention should be given to needle displacement before each fraction. Needle movement is most significant between fractions 1 and 2. Acute toxicity (RTOG) has been modest. Late toxicity and tumor control rates will be reported as longer follow-up allows.

Postimplant dosimetry for (125)I prostate implants: definitions and factors affecting outcome

OBJECTIVE

An analysis of CT-based dosimetry was performed to assess the efficacy of the real time method of prostate implantation, explore the relationship of various dose descriptions and determine implant factors affecting outcome.

METHODS AND MATERIALS

Between 7/95 and 8/99, 297 patients underwent (125)I implants for T1-T2 prostate cancer and had CT-based dosimetry performed (TG43 formalism). Dosimetry was performed 1 month postimplant. Using a dose-volume histogram, doses delivered to 100%, 95%, 90%, and 80% of the prostate (D100, D95, D90, D80, respectively) as well as percentages of the gland receiving 240 Gy, 160 Gy, 140 Gy (V240, V160, V140, respectively) were reported. Correlations between the various dose parameters and D90 were generated. The effect of the number of seeds implanted, seeds/volume, prostate volume, experience as assessed by time (8/01/99-date of implant), ultrasound probe (mechanical sector vs. dual phased electronic), and the ratio of the CT dosimetry prostate volume/ultrasound implant volume (CT/US vol) were analyzed.

RESULTS

The median D100, D95, D90, and D80 values were 10,200 cGy, 15,655 cGy, 17,578 cGy, and 19,873 cGy, respectively. The median V240, V160, and V140 were 56%, 94%, and 98%, respectively. Correlations of dose descriptions found a close relationship of D95, D80, V240, V160, and V140 with D90 with r values of 0.928, 0.973, 0.911, 0.816, and 0.733, respectively. D100 correlated poorly with D90 (r = 0.099). Using a stepwise regression analysis, CT/US vol ratio, prostate volume, and seed number were the only significant factors affecting D90 with CT/US vol ratio having the greatest effect. The dual-phased electronic probe was associated with fewer D90 values of less than 140 Gy (2%) compared to the mechanical sector probe (14%) (p = 0.02).

CONCLUSION

CT-based dosimetry results reveal the real-time implant technique to be an effective method of prostate implantation. Factors associated with more precise implantation, such as decreased postimplant edema, new technology, and increased number of seeds will lead to higher D90 values.

Well-ionization chamber response relative to NIST air-kerma strength standard for prostate brachytherapy seeds

The response of well-ionization chambers to the emissions of 103Pd and 125I radioactive seed sources used in prostate cancer brachytherapy has been measured. Calibration factors relating chamber response (current or dial setting) to measured air-kerma strength have been determined for seeds from nine manufacturers, each with different designs. Variations in well-ionization chamber response relative to measured air-kerma strength have been observed because of differences in the emitted energy spectrum due to both the radionuclide support material (125I seeds) and the mass ratio of 103Pd to 102Pd (103Pd seeds). Obtaining accurate results from quality assurance measurements using well-ionization chambers at a therapy clinic requires knowledge of such differences in chamber response as a function of seed design.

A dose-volume histogram based optimization algorithm for ultrasound guided prostate implants

The task of treatment planning for prostate implants is to find an optimal seed configuration, comprising the target coverage and dosimetric consideration of critical structures such as the rectum and urethra. An efficient method to accomplish this is to use an inverse planning technique that derives the optimized solution from a prescribed treatment goal. The goal can be specified in the voxel domain as the desired doses to the voxels of the target and critical structures, or in the dose volume representation as the desired dose volume histograms (DVHs) of the target and critical structures. The DVH based optimization has been successfully used in plan optimization for intensity-modulated radiation therapy (IMRT) but little attention has been paid to its application in prostate implants. Clinically, it has long been known that some normal structure tolerances are more accurately assessed by volumetric information. Dose-volume histograms are also widely used for plan evaluation. When working in the DVH domain for optimization one has more control over the final DVHs. We have constructed an objective function sensitive to the DVHs of the target and critical structures. The objective function is minimized using an iterative algorithm, starting from a randomly selected initial seed configuration. At each iteration step, a trial position is given to a randomly selected source and the trial position is accepted if the objective function is decreased. To avoid being trapped in a less optimal local minimum, the optimization process is repeated. The final plan is selected from a pool of optimized plans obtained from a series of randomized initial seed configurations.

Dose-volume histograms computation comparisons using conventional methods and optimized fast Fourier transforms algorithms for brachytherapy

In anatomy based optimization procedures for large volume implants the calculation of dose-volume histograms (DVH) accounts for the major part of the time involved and can be as long as a few hours. This time is proportional to the number of seeds or source dwell positions required for the implant. A procedure for the calculation of brachytherapy seed dose distribution calculation employing fast Fourier transforms (FFT) and the convolution theorem has been described by others and was supposed to significantly improve the speed of the dose distribution computation. Using new significantly improved FFT algorithms and various other optimization techniques we have compared the calculated differential and integral DVHs in high dose rate (HDR) brachytherapy with a single stepping source using actual clinical implants. This is so that we could assess the efficiency and accuracy of the FFT method with that of conventional methods. Our results showed that the FFT based method of calculating DVHs in brachytherapy is comparable in speed with conventional dose calculation methods, but only for implants with more than 287 sources. It is therefore of limited practical use even for large implants. This result is in direct opposition to the claim by other authors.

The American Brachytherapy Society recommendations for permanent prostate brachytherapy postimplant dosimetric analysis

PURPOSE

The purpose of this report is to establish guidelines for postimplant dosimetric analysis of permanent prostate brachytherapy.

METHODS

Members of the American Brachytherapy Society (ABS) with expertise in prostate dosimetry evaluation performed a literature review and supplemented with their clinical experience formulated guidelines for performing and analyzing postimplant dosimetry of permanent prostate brachytherapy.

RESULTS

The ABS recommends that postimplant dosimetry should be performed on all patients undergoing permanent prostate brachytherapy for optimal patient care. At present, computed tomography (CT)-based dosimetry is recommended, based on availability cost and the ability to image the prostate as well as the seeds. Additional plane radiographs should be obtained to verify the seed count. Until the ideal postoperative interval for CT scanning has been determined, each center should perform dosimetric evaluation of prostate implants at a consistent postoperative interval. This interval should be reported. Isodose displays should be obtained at 50%, 80%, 90%, 100%, 150%, and 200% of the prescription dose and displayed on multiple cross-sectional images of the prostate. A dose-volume histogram (DVH) of the prostate should be performed and the D90 (dose to 90% of the prostate gland) reported by all centers. Additionally, the D80, D100, the fractional V80, V90, V100, V150 and V200 (i.e., the percentage of prostate volume receiving 80%, 90%, 100%, 150%, and 200% of the prescribed dose, respectively), the rectal, and urethral doses should be reported and ultimately correlated with clinical outcome in the research environment. On-line real-time dosimetry, the effects of dose heterogeneity, and the effects of tissue heterogeneity need further investigation.

CONCLUSION

It is essential that postimplant dosimetry should be performed on all patients undergoing permanent prostate brachytherapy. Guidelines were established for the performance and analysis of such dosimetry.