The effect of interobserver differences in post-implant prostate CT image interpretation on dosimetric parameters.

The purpose of this study was to clarify where observers differ in their interpretation of CT scans, and to relate those differences to clinically relevant dosimetric parameters. Twenty unselected patients treated with I-125 or Pd-103 brachytherapy at the Veterans Affairs Puget Sound Health Care System (VAPSHCS) in 2001 were studied. Patients were implanted with I-125 (7 patients, 0.87 mCi/source) or Pd-103 (13 patients, 2.54 U/source). The number of I-125 sources implanted ranged from 52 to 78. The number of Pd-103 sources implanted ranged from 58-144. Post-implant 3 mm CT images were imported into a laptop running Varian Variseed and sent to the four physician investigators, who outlined the prostate independently. Investigators were not coached specifically for this study, beyond their having read prior reports regarding prostate volume determinations. There was moderate interobserver variability in CT volume determination, with the standard deviations as a percent of the mean ranging from 9% to 29% (median: 17%). An average of 14% of implants (range: 5%-20%) would have been judged inadequate based on a minimum V100 of 80%, versus 24% of implants (range: 5%-45%) being judged inadequate based on a minimum D90 of 90% of prescription dose. The greatest variability was seen in prostate length (median standard deviation: 0.57 cm), due to vagaries in base and apical localization. However, the prostatic width and thickness also varied substantially between observers, with median standard deviations of 0.24 and 0.32 cm, respectively. Treatment margin variability was greatest at the anterior border, with a median standard deviation of 0.21 cm +/- 0.10. We believe that CT-based dosimetry, while influenced by CT interpretation, still provides useful general dosimetric calculations, that are likely to be reproducible enough to provide clinically useful information between institutions. The V100 and TMs are less influenced by interobserver CT interpretation variability than is the D90, and may be better suited as interinstitutional quality indices.

Isodose patterns in patients with inadequate prostate brachytherapy coverage.

PURPOSE: The development of a practical, real-time dosimetry system should result in improved implant dose distributions and higher prostate cancer control rates. Our purpose here is to demonstrate that intraoperative isodose reconstruction in relation to the seed distribution, even without accurate registration with the prostatic volume, can likely identify an inadequate implant intraoperatively and guide corrective seed placement. METHODS AND MATERIALS: A total of 102 Pd-103 implants performed by standard techniques, using a modified peripheral loading pattern, were studied. A postimplant computed tomography (CT) scan was obtained 2-4 h after the implant. The contoured images and sources were entered into a Varian Variseed 7.0 treatment planning system. Dosimetric parameters analyzed included the percent of the postimplant prostate or rectal volume covered by the prescription dose (V100), and the dose that covers 90% of the postimplant prostate volume (D90). Isodose patterns were analyzed at midprostate, and for the entire prostate. Adverse isodose patterns were defined as gaps, holes, islands. Isodose gaps are subprescription intervals between the prostatic margin and the prescription isodose. Isodose holes are regions of subprescription dose within the prostate. Isodose islands are isolated regions > or =prescription dose inside the prostatic margins. RESULTS: Characteristic isodose patterns were predictive for V100 values. Midprostatic isodose holes were seen in 55% of patients with a V100 < 80%, 5% of patients with a V100 of 80-90%, and only 1% of patients with a V100 > 90%. When analyzing the entire prostate, isodose holes were seen in 55% of patients with a V100 < 80%, 18% of patients with a V100 of 80-90%, and 9% of patients with a V100 > 90%. Midprostatic isodose islands were seen in 55% of patients with a V100 < 80%, 5% of patients with a V100 of 80-90%, and no patient with a V100 > 90%. When analyzing the entire prostate, isodose islands were seen in all patients with V100 < 80%, 36% of patients with a V100 of 80-90%, and only 1% of patients with a V100 > 90%. The likelihood of a V100 less than 80% was best predicted by the presence of isodose holes or islands at midprostate. Patients with either finding had an 86% chance of having a V100 < 80%. CONCLUSIONS: These semiquantitative findings can provide practical guidelines for intraoperative dosimetry, to provide a more rational guide to intraoperative postimplant assessment and modification. If isodose holes or islands are seen within the implanted volume, additional seeds are added to the affected region to obtain a V100 > 80%.

Prostate brachytherapy seed identification on post-implant TRUS images.

TRUS is a conceptually appealing alternative to CT-based dosimetry, offering the substantial practical advantage of being readily available intraoperatively. To test the feasibility and reliability of seed identification on post-implant TRUS using standard two-dimensional images, ten patients treated with I-125 or Pd-103 brachytherapy were studied. A set of transverse images (6 MHz) were taken immediately following completion of the implant procedure. Original thermal images were sent to four physicians and the sources were identified independently by placing marks on a cellophane overlay, with grids to match the axial TRUS images. The number and type of seed implanted were not revealed to the investigators. Instead, they were instructed to mark the positions of what they would consider, with reasonable certainty, to be seeds. The overlays were then manually compared for source identification and agreement between observers regarding each alleged source. The actual number of implanted seeds ranged from 44 to 108 (median: 60). In contrast, the mean number of seeds allegedly identified per patients ranged from 26 to 82 (median: 43). The average percent of the seeds allegedly identified per patient ranged from 51% to 83% (mean: 74%). The four physician investigators--KW, JS, BH, and GM--identified an alleged median of 90%, 44%, 63%, and 91% of the total seeds, respectively. There were five instances in which investigators alleged more seeds than were actually implanted. The consistency of seed identification among the investigators was evaluated by noting how many investigators identified each bright spot on the images. The percent of bright spots identified by all four investigators ranged from 8% to 33% (median: 20%). Despite considerable interest among some of our clinical and commercial colleagues in developing TRUS-based intraoperative post-implant dosimetry, the use of TRUS-based seed identification for post-implant dosimetry should be viewed with skepticism.

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.