High-dose gynecologic endocavitary brachytherapy. Technical and dosimetric aspects

The technical and dosimetric aspects are presented of high-dose intracavitary brachytherapy in gynecology. Fifty-five patients (203 insertions) were examined over two years with a remote loading Selectron HDR 60Co unit installed in a dedicated bunker. The dose to the rectal and bladder markers on AP and LL films was calculated before every irradiation, in order to allow the necessary corrections to be made. Uniform irradiation conditions were obtained at each treatment set-up for both tumoral target and bladder and rectal doses. High-dose intracavitary brachytherapy proved to be a safe, reliable and versatile method from the technical and dosimetric point of view both in the treatment of unoperated gynecological malignancies and in postoperative therapy.     

Prostate brachytherapy postimplant dosimetry: a comparison of suture-embedded and loose seed implants

PURPOSE: To compare postimplant dosimetry and seed embolization rates for prostate brachytherapy implants using suture-embedded and loose seeds. METHODS AND MATERIALS: Dosimetric analysis of the whole prostate, prostate quadrants, rectum, and surrogate urethra was performed on 54 loose seed and 81 RAPIDStrand (RS) patients. Seed embolization rates were determined from chest radiographs. RESULTS: Whole prostate V100 and D90 did not differ significantly for the loose seed (V100 = 90.5%, D90 =153.2 Gy) and RS groups (V100 = 91.5%, D90 = 151.6 Gy) (p = 0.43 and 0.65, respectively), but V150, V200, and contiguous V200 were higher (p < or = 0.003) for the RS group (59.9%, 28.3%, and 23.2%, respectively) than the loose seed group (52.5%, 22.8%, and 16.1%, respectively). Extraprostatic measures (conformity index and external index) were also different at the p < 0.05 level. The embolization rate was 40% in the loose seed group and 14% in the RS group. CONCLUSIONS: The most significant difference between the two study groups was a decrease in the embolization rate. Although some statistically significant changes in postimplant dosimetry were observed, they were nevertheless small.     

CT- and MRI-based seed localization in postimplant evaluation after prostate brachytherapy

PurposeTo compare the uncertainties in CT- and MRI-based seed reconstruction in postimplant evaluation after prostate seed brachytherapy in terms of interobserver variability and quantify the impact of seed detection variability on a selection of dosimetric parameters for three postplan techniques: (1) CT, (2) MRI-T1 weighted fused with MRI-T2 weighted, and (3) CT fused with MRI-T2 weighted.Methods and MaterialsSeven physicists reconstructed the seed positions on postimplant CT and MRI-T1 images of three patients. For each patient and imaging modality, the interobserver variability was calculated with respect to a reference seed set. The effect of this variability on dosimetry was calculated for CT and CT + MRI-T2 (CT-based seed reconstruction), as well as for MRI-T1 + MRI-T2 (MRI-T1–based seed reconstruction), using fixed CT and MRI-T2 prostate contours.ResultsAveraged over three patients, the interobserver variability in CT-based seed reconstruction was 1.1 mm (1 SD_ref, i.e., standard deviation with respect to the reference value). The D₉₀ (dose delivered to 90% of the target) variability was 1.5% and 1.3% (1 SD_ref) for CT and CT + MRI-T2, respectively. The mean interobserver variability in MRI-based seed reconstruction was 3.0 mm (1 SD_ref), and the impact of this variability on D₉₀ was 6.6% for MRI-T1 + MRI-T2.ConclusionsSeed reconstruction on MRI-T1–weighted images was less accurate than on CT. This difference in uncertainties should be weighted against uncertainties due to contouring and image fusion when comparing the overall reliability of postplan techniques.     

CT-based postimplant dosimetry of prostate brachytherapy: comparison of 1-mm and 5-mm section CT

Purpose The aim of this study was to compare the outcomes between 1-mm and 5-mm section computed tomography (CT)-based postimplant dosimetry. Materials and methods A series of 21 consecutive patients underwent permanent prostate brachytherapy. The postimplant prostate volume was calculated using 1-mm and 5-mm section CT. One radiation oncologist contoured the prostate on CT images to obtain the reconstructed prostate volume (pVol), prostate V₁₀₀ (percent of the prostate volume receiving at least the full prescribed dose), and prostate D₉₀ (mean dose delivered to 90% of the prostate gland). The same radiation oncologist performed the contouring three times to evaluate intraobserver variation and subjectively scored the quality of the CT images. Results The mean ±1 SD postimplant pVol was 20.17 ± 6.66 cc by 1-mm section CT and 22.24 ± 8.48 cc by 5-mm section CT; the difference in the mean values was 2.06 cc (P < 0.01). The mean postimplant prostate V₁₀₀ was 80.44% ± 7.06% by 1-mm section CT and 77.33% ± 10.22% by 5-mm section CT. The mean postimplant prostate D₉₀ was 83.28% ± 10.81% by 1-mm section CT and 78.60% ± 15.75% by 5-mm section CT. In the evaluation of image quality, 5-mm section CT was assigned significantly higher scores than 1-mm section CT. In regard to intraobserver variation, 5-mm section CT revealed less intraobserver variation than 1-mm section CT. Conclusion Our current results suggested that the outcomes of postimplant dosimetry using 1-mm section CT did not improved the results over those obtained using 5-mm section CT in terms of the quality of the CT image or reproducibility.     

Interobserver variation in postimplant computed tomography contouring affects quality assessment of prostate brachytherapy

PURPOSE: Permanent seed implants are accepted treatment of early stage prostate cancer. Implant quality is assessed by post implant CT-based dosimetry but prostate contours on CT images are obscured by metallic seed artefact and edema. Outcome depends on implant quality, but perceived implant quality depends on accurate prostate contouring. This study documents inter observer variation in prostate contouring on post implant CT scans. METHODS AND MATERIALS: Ten patients had implant dosimetry calculated on 4 copies of the post implant CT scan. Prostate contours from MRI-CT fusion were the gold standard for prostate edge identification. CTs were contoured by an experienced prostate brachytherapist matching CT images to the pre implant TRUS, and by 2 GU radiation oncologists experienced in conformal radiotherapy planning. Dosimetry was compared to that obtained using MRI-CT fusion in terms of D90 and V100. RESULTS: Contours and dosimetry were not reproducible among the 3 observers. The V100's of the experienced brachytherapist differed from that of MRI-CT fusion by a mean of 2.4% compared to 9.1% and 4.4% for observers 1 and 2, and the D90 by a mean of 9.3 Gy compared to 30.3 and 14.4 Gy for observers 1 and 2. CONCLUSIONS: Quality assessment of prostate brachytherapy based on 1 month post implant CT is difficult. This may obscure the dose-response relationship in brachytherapy as well as create problems for quality assurance in multicentre trials evaluating brachytherapy against standard modalities. Whenever possible, MRI-CT fusion should be employed to verify prostate contours post implant.     

Defining a magnetic resonance scan sequence for permanent seed prostate brachytherapy postimplant assessment

PurposeWe describe a magnetic resonance (MR) scan sequence for prostate brachytherapy postimplant assessment.Methods and MaterialsOne brachytherapy team at the British Columbia Cancer Agency has incorporated MR–CT fusion into their permanent seed prostate brachytherapy quality assurance procedure. Several attempts were required to ensure that the diagnostic MR scanner at the adjoining general hospital performed the desired sequence, providing many examples of suboptimal scans and underlining the pitfalls for a center trying to incorporate the use of MR scanning into their brachytherapy program.ResultsThe recommended sequence (Fast Spin Echo T2-weighted, repetition time [TR]/echo time [TE] 4500/90, echo train length [ETL] 10, 20 × 20 field of view [FOV], 80 bandwidth [BW]) is associated with superior edge detection when compared with those images in which a typical diagnostic sequence was used. The use of a low bandwidth sequence does not compromise edge detection or seed identification when compared with a higher bandwidth.ConclusionsWe have defined a magnetic resonance imaging sequence, which appears to optimize both prostate delineation and identification of seeds, lending itself to straightforward fusion with CT images and allowing for less uncertainty in permanent seed prostate brachytherapy quality assurance.     

Technical aspects and perspectives of the vaginal mold applicator for brachytherapy of gynecologic malignancies

PurposeThe importance of the quality of cervical cancer brachytherapy applicators has been reported, suggesting a direct influence of competent technical implant performance on outcome. In our institute, an original brachytherapy technique based on the use of a molded applicator for genital tract brachytherapy has been applied routinely in clinical practice. Here, we report the technical aspects of this customized applicator and perspectives on its use.Technical AspectsThe first step consists of a vaginal impression that accurately shows the topography and extension of the tumor as well as the anatomy of the vagina and cervix. From this impression, an acrylic applicator is made. Then, the intended positions of the vaginal catheters are drawn on the surface of the mold by the radiation oncologist. Two plastic vaginal catheters are introduced and fixed on the internal surface of the molded applicator. A hole for the cervical os is made through which the uterine probe will be positioned.PerspectivesThis method allows for high specificity within the framework of a modern brachytherapy procedure, integrating the tumor topography, anatomy of the patient, and internal movements of target and critical volumes. This technique has been successfully extended to other tumor locations, such as genital tract rhabdomyosarcoma in children and postoperative endocavitary brachytherapy in patients with endometrial cancer.ConclusionCustomization of a vaginal brachytherapy applicator allows for the maintenance of morphologic optimization throughout the treatment course, which better takes into account a fourth dimension: internal organ motion during the course of brachytherapy.     

Comparison of 3 different postimplant dosimetry methods following permanent 125I prostate seed brachytherapy

Postimplant dosimetry (PID) after Iodine-125 (¹²⁵I) implant of the prostate should offer a reliable qualitative assessment. So far, there is no consensus regarding the optimum PID method, though the latest literature is in favor of magnetic resonance imaging (MRI). This study aims to simultaneously compare 3 PID techniques: (1) MRI-computed tomography (CT) fusion; (2) ultrasound (US)-CT fusion; and (3) manual target delineation on CT. The study comprised 10 patients with prostate cancer. CT/MR scans with urinary catheters in place for PID were done either on day 0 or day 1 postimplantation. The main parameter evaluated and compared among methods was target D90. The results show that CT-based D90s are lower than US-CT D90s (median difference,?6.85%), whereas MR-CT PID gives higher D90 than US-CT PID (median difference, 4.25%). Manual contouring on CT images tends to overestimate the prostate volume compared with transrectal ultrasound (TRUS) (median difference, 23.33%), whereas on US images the target is overestimated compared with MR-based contouring (median difference, 13.25%). Although there are certain differences among the results given by various PID techniques, the differences are statistically insignificant for this small group of patients. Any dosimetric comparison between 2 PID techniques should also account for the limitations of each technique, to allow for an accurate quantification of data. Given that PID after permanent radioactive seed implant is mandatory for quality assurance, any imaging method–based PID (MR-CT, US-CT, and CT) available in a radiotherapy department can be indicative of the quality of the procedure.     

Feasibility of an MRI-only workflow for postimplant dosimetry of low-dose-rate prostate brachytherapy: Transition from phantoms to patients

PurposeThe lack of positive contrast from brachytherapy seeds in conventional MR images remains a major challenge toward an MRI-only workflow for postimplant dosimetry of low-dose-rate brachytherapy. In this work, the feasibility of our recently proposed MRI-only workflow in clinically relevant scenarios is investigated and the necessary modifications in image acquisition and processing pipeline are proposed for transition to the clinic.Methods and MaterialsFour prostate phantoms with a total of 321 I-125 implanted dummy seeds and three patients with a total of 168 implanted seeds were scanned using a gradient echo sequence on 1.5 T and 3T MR scanners. Quantitative susceptibility mapping (QSM) was performed for seed visualization. Before QSM, the seed-induced distortion correction was performed followed by edge enhancement. Seed localization was performed using spatial clustering algorithms and was compared with CT. In addition, feasibility of the proposed method on detection of prostatic calcifications was studied.ResultsThe proposed susceptibility-based algorithm generated consistent positive contrast for the seeds in phantoms and patients. All the 321 seeds in the four phantoms were correctly identified; the MR-derived seeds centroids agreed well with CT-derived positions (average error = 0.5 ± 0.3 mm). The proposed algorithm for seed visualization was found to be orientation invariant. In patient cases, all seeds were visualized and correctly localized (average error = 1.2 ± 0.9 mm); no significant differences between dose volume histogram parameters were found. Prostatic calcifications were depicted with negative contrast on QSM and spatially agreed with CT.ConclusionsThe proposed MRI-based approach has great potential to replace the current CT-based practices. Additional patient studies are necessary to further optimize and validate the workflow.     

Impact of MRI-based postimplant dosimetric assessment in prostate brachytherapy using contrast-enhanced T1-weighted images

PurposeTo compare contrast-enhanced T1-weighted (CE-T1WI) magnetic resonance imaging (MRI) with computed tomography (CT) for postimplant dosimetry and seed recognition in prostate brachytherapy.Methods and MaterialsA total of 245 patients who received ¹²⁵I prostate brachytherapy with or without external beam radiotherapy were enrolled. For postimplant analysis, CT and MRI scans were obtained at 1 month after seed implantation. For MRI-based dosimetry, T2-weighted images were fused with the CE-T1WI; the prostate was delineated on the T2-weighted images, and the seed detection was performed manually on the CE-T1WI. In CT-based dosimetry, the seed detection was essentially performed automatically. The dosimetric results obtained by MRI-based and CT-based dosimetry were compared.ResultsThe mean prostate D₉₀ (the minimum dose received by 90% of the prostate volume) estimated by MRI-based and CT-based dosimetry were 113% and 115%, respectively, with no significant difference. The mean prostate V₁₀₀ (the percent volume of the postimplant prostate receiving 100% of the prescribed dose) estimated by MRI-based and CT-based dosimetry were 95.2% and 95.8%, respectively, again with no significant difference. The mean prostate V₁₅₀ (the percent volume of the postimplant prostate receiving 150% of the prescribed dose) estimated by MRI-based and CT-based dosimetry were 52.8% and 57.0%, respectively (p < 0.01). In all of the 35 patients (14%) in whom the MRI-based V₁₅₀ were at least 10% lower than the CT-based results, the seed detection by CT-based dosimetry was overestimated in highly seed-clustered areas or in the areas close to calcifications because of reconstruction artifacts in CT images.ConclusionsMRI-based dosimetry using CE-T1WI appears to be acceptable. Our results suggest that MRI-based dosimetry is a practical method for estimation of the higher dose distribution, especially if seeds are clustered together or when they are close to calcifications.     

Comparison of intraoperative ultrasound with postimplant computed tomography--dosimetric values at Day 1 and Day 30 after prostate brachytherapy

PurposeTo compare the results of intraoperative dosimetry with those of postimplant computed tomography (CT)-based dosimetry after ¹²⁵I prostate brachytherapy.Methods and materialsWe treated 412 prostate cancer patients with ¹²⁵I prostate brachytherapy, with or without external beam radiotherapy at our institution. Neoadjuvant hormone therapy was administered to 331 patients (80.3%). Implantation was performed using an intraoperative interactive technique. Postimplant dosimetry was performed on Day 1 and Day 30 using CT imaging. The dosimetric results for the prostate, urethra, and rectum were compared among intraoperative ultrasound, and CT scans of Day 1 and Day 30.ResultsThe mean intraoperative minimal dose received by 90% of the prostate volume (D₉₀) was 118.8% of the prescribed dose vs. 106.4% for Day 1 (p < 0.01) and 119.2% for Day 30 (p = 0.25). There were no significant correlations between the intraoperative D₉₀ and the postimplant D₉₀ values (intraclass correlation coefficients = 0.42 and 0.33 for Day 1 and Day 30, respectively). Prostatic edema at Day 1 had the largest effect on the Day 1 D₉₀ (p < 0.01). The factor significantly affecting the Day 30 D₉₀ was neoadjuvant hormone therapy (p < 0.01). The mean Day 30 D₉₀ for the hormone-treated patients was 117.9%, compared with 124.6% for those who remained hormone naïve. The intraoperative and postimplant dosimetric values differed significantly for the urethra and rectum.ConclusionsOur results demonstrate that there are no significant differences between the D₉₀ assessments obtained intraoperatively and at Day 30 postoperatively. Furthermore, there are no definite correlations between intra- and postimplantation dosimetric values. Other D₉₀ values differed significantly between the intraoperative and postimplant dosimetry. This study suggests that dosimetry has negligible clinical utility for informing patients, at discharge, of whether or not their implants are adequate.     

Parallel imaging compressed sensing for accelerated imaging and improved signal-to-noise ratio in MRI-based postimplant dosimetry of prostate brachytherapy

PurposeTo investigate the feasibility of using parallel imaging compressed sensing (PICS) to reduce scan time and improve signal-to-noise ratio (SNR) in MRI-based postimplant dosimetry of prostate brachytherapy.Methods and MaterialsTen patients underwent low-dose-rate prostate brachytherapy with radioactive seeds stranded with positive magnetic resonance-signal seed markers and were scanned on a Siemens 1.5T Aera. MRI comprised a fully balanced steady-state free precession sequence with two 18-channel external pelvic array coils with and without a rigid two-channel endorectal coil. The fully sampled data sets were retrospectively subsampled with increasing acceleration factors and reconstructed with parallel imaging and compressed sensing algorithms. The images were assessed in a blinded reader study by board-certified care providers. Rating scores were compared for statistically significant differences between reconstruction types.ResultsImages reconstructed from subsampling up to an acceleration factor of 4 with PICS demonstrated consistently sufficient quality for dosimetry with no apparent loss of SNR, anatomy depiction, or seed/marker conspicuity when compared to the fully sampled images. Images obtained with acceleration factors of 5 or 6 revealed reduced spatial resolution and seed marker contrast. Nevertheless, the reader study revealed that images obtained with an acceleration factor of up to 5 and reconstructed with PICS were adequate-to-good for postimplant dosimetry.ConclusionsCombined parallel imaging and compressed sensing can substantially reduce scan time in fully balanced steady-state free precession imaging of the prostate while maintaining adequate-to-good image quality for postimplant dosimetry. The saved scan time can be used for multiple signal averages and improved SNR, potentially obviating the need for an endorectal coil in MRI-based postimplant dosimetry.     

Evaluation of interobserver differences in postimplant dosimetry following prostate brachytherapy and the efficacy of CT/MRI fusion imaging

Purpose  

Interobserver differences in postimplant dosimetry based on computed tomography (CT) and CT/magnetic resonance imaging (MRI) fusion images were assessed to evaluate the efficacy of the fusion image. In addition, the part of the prostate contour responsible for the interobserver differences in CT was identified.     

Comparison between postoperative TRUS-CT fusion with MRI-CT fusion for postimplant quality assurance in prostate LDR permanent seed brachytherapy

PURPOSE/OBJECTIVE Permanent seed Low-Dose-Rate brachytherapy is planned and delivered using transrectal ultrasound (TRUS). Post-implant evaluation for quality assurance is usually performed using Computed Tomography (CT). Registration of the CT images with MRI reduces subjectivity in contouring by improving prostate edge detection. We hypothesized that a set of TRUS images post procedure may provide the same benefit.MATERIAL/METHODS Consecutive patients undergoing Low-Dose-Rate prostate brachytherapy were recruited. TRUS images were recorded under anesthesia at completion of their implant. In addition, all patients underwent standard post-implant quality assurance including prostate CT and MRI at day 30. These were co-registered, contoured and seeds were identified. Three independent observers contoured and registered the post implant TRUS images to the Day 30 CT using seed matching. Prostate volumes and dosimetric parameters were compared through Intraclass Correlation Coefficient (ICC) to evaluate the concordance between MRI and ultrasound (US).RESULTS 26 patients were recruited from 10/17 to 01/18. Mean prostate volume was 34.5 (SD 10.8) cm³ at baseline on planning TRUS images, 37.4 (SD 11.3) cm³ on Day 0 post implant TRUS and 36.7 (SD 11.7) cm³ on Day 30 MRI. D90 was 112.6% (SD 9.3) on CT-MRI and 112.9% (SD 11.1) on CT-US. V100 was 94.6% (SD 3.8) for CT-MRI, 95.1% (SD 4.3) for CT-US. Student t-tests were used to compare groups. No significant differences were noted.CONCLUSION Post implant TRUS may be useful for quality assurance for post-implant dosimetry particularly if access to an MRI is limited.     

Examining the relationship between pre- and postimplant geometry in prostate low-dose-rate brachytherapy and its correlation with dosimetric quality using the similarity concept

PurposeThis is a retrospective study in which we define multiple metrics for similarity and then inquire on the relationship between similarity and currently used dosimetric quantities describing preimplant and postimplant plans.Methods and MaterialsWe analyzed a unique cohort of 94 consecutively performed prostate seed implant patients, associated with excellent dosimetric and clinical outcomes. For each patient, an ultrasound (US) preimplant and two CT postimplant (Day 0 and Day 30) studies were available. Measures for similarity were created and computed using feature vectors based on two classes of moments: first, invariant to rotation and translation, and the second polar–radius moments invariant to rotation, translation, and scaling. Both similarity measures were calibrated using controlled perturbations (random and systematic) of seed positions and contours in different size implants, thus producing meaningful numerical threshold values used in the clinical analysis.ResultsAn important finding is that similarity, for both seed distributions and contours, improves significantly when scaling invariance is added to translation and rotation. No correlation between seed and contours similarity was found. In the setting of preplanned prostate seed implants using preloaded needles, based on our data, similarity between preimplant and postimplant plans does not correlate with either minimum dose to 90% of the volume of the prostate or analogous similarity metrics for prostate contours.ConclusionsWe have developed novel tools and metrics, which will allow practitioners to better understand the relationship between preimplant and postimplant plans. Geometrical similarity between a preplan and an actual implant, although useful, does not seem to be necessary to achieve minimum dose to 90% of the volume of the prostate-good dosimetric implants.     

Preliminary study of correction of original metal artifacts due to I-125 seeds in postimplant dosimetry for prostate permanent implant brachytherapy

Purpose

We investigated a subtraction-based reprojection approach to reduce CT metal artifacts due to I-125 seeds and evaluated the clinical implications in postimplant dosimetry for prostate permanent implant brachytherapy.

Materials and Methods

The raw projection data were used to reduce metal artifacts due to I-125 seeds. CT images of the metal parts only were separated from the original CT images by setting the threshold for pixel value to that of the I-125 seeds. Using these images, sinograms of CT images with and without seeds were obtained by inverse Radon transform (iRT), and the sinogram of the metal image was subtracted from that of the original image. Finally, the image was reconstructed using the sinogram by Radon transform (RT). This technique was applied to a prostate phantom and to a patient undergoing prostate permanent implant brachytherapy.

Results

Metal artifacts from I-125 seeds were reduced in both the phantom and patient studies. This technique decreased the density of the inner region of seeds but enhanced the density of the seed edge, thereby facilitating the identification of seed number, orientation, and location.

Conclusion

This method reduces metal artifacts from I-125 seeds, and has potential for decreasing the time required for and improving the accuracy of postimplant dosimetry.