Impact of selection of post-implant technique on dosimetry parameters for permanent prostate implants

Is intraoperative real-time dosimetry in prostate seed brachytherapy predictive of biochemical outcome?

Purpose

To analyze intraoperative (IO) dosimetry using transrectal ultrasound (TRUS), performed before and after prostate low-dose-rate brachytherapy (LDR-BT), and compare it to dosimetry performed 30 days following the LDR-BT implant (Day 30).

Material and methods

A total of 236 patients underwent prostate LDR-BT using ¹²⁵I that was performed with a three-dimensional TRUS-guided interactive inverse preplanning system (preimplant dosimetry). After the implant procedure, the TRUS was repeated in the operating room, and the dosimetry was recalculated (postimplant dosimetry) and compared to dosimetry on Day 30 computed tomography (CT) scans. Area under curve (AUC) statistics was used for models predictive of dosimetric parameters at Day 30.

Results

The median follow-up for patients without BF was 96 months, the 5-year and 8-year biochemical recurrence (BR)-free rate was 96% and 90%, respectively. The postimplant median D₉₀ was 3.8 Gy lower (interquartile range [IQR], 12.4-0.9), and the V₁₀₀ only 1% less (IQR, 2.9-0.2%) than the preimplant dosimetry. When comparing the postimplant and the Day 30 dosimetries, the postimplant median D₉₀ was 9.6 Gy higher (IQR [–] 9.5-30.3 Gy), and the V₁₀₀ was 3.2% greater (0.2-8.9%) than Day 30 postimplant dosimetry. The variables that best predicted the D₉₀ of Day 30 was the postimplant D₉₀ (AUC = 0.62, p = 0.038). None of the analyzed values for IO or Day 30 dosimetry showed any predictive value for BR.

Conclusions

Although improving the IO preimplant and postimplant dosimetry improved dosimetry on Day 30, the BR-free rate was not dependent on any dosimetric parameter. Unpredictable factors such as intraprostatic seed migration and IO factors, prevented the accurate prediction of Day 30 dosimetry.

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

PURPOSE

To 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 MATERIALS

Seven 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.

RESULTS

Averaged over three patients, the interobserver variability in CT-based seed reconstruction was 1.1 mm (1 SDref, 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 SDref) for CT and CT + MRI-T2, respectively. The mean interobserver variability in MRI-based seed reconstruction was 3.0 mm (1 SDref), and the impact of this variability on D90 was 6.6% for MRI-T1 + MRI-T2.

CONCLUSIONS

Seed 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.

Is prostate-specific antigen percentage decrease predictive of clinical outcome after permanent iodine-125 interstitial brachytherapy for prostate cancer?

OBJECTIVE

To determine the usefulness of prostate-specific antigen (PSA) percentage (vs. pretreatment value assumed as 100%) in prediction of biochemical relapse, after iodine-125 ((125)I) permanent brachytherapy for prostate cancer, to employ a parameter independent by the initial PSA amount and by the individual prostatic volume.

METHODS AND MATERIALS

Our study included 133 patients, 102 still disease free (Group A) and 31 who experienced proven biochemical recurrence (Group B). PSA levels before and after (125)I brachytherapy were recorded, and PSA percentage vs. pretreatment values were calculated. Cox regression model, receiver operating characteristic curves, and Kaplan-Meier regression model with log-rank test were calculated.

RESULTS

We observed that, in patients submitted to brachytherapy for prostate cancer, a PSA percentage >20% of pretreatment value is highly associated with relapse risk (p<0.0001) and that this association is strongly present since t=6 months of followup (p<0.0001), with a hazard ratio near to five times (4.965), a sensitivity of 72.4%, and specificity of 79.8% related to the chosen cutoff.

DISCUSSION

Despite the amount of PSA is the only parameter that the clinicians can deploy to monitor patient's followup after permanent interstitial brachytherapy for prostate cancer, its evolution in time seems unable to predict early biochemical relapse as it is influenced by prostatic volume and initial PSA amount.

CONCLUSIONS

Our data suggest that a PSA percentage >20% of pretreatment value at 6 months might represent an early, inexpensive, and useful predictive tool of bad outcome in patients after permanent brachytherapy.

A magnetic resonance-based seed localization method for I-125 prostate implants

This study was performed to develop and evaluate a semi-automatic seed localization algorithm from magnetic resonance (MR) images for interstitial prostate brachytherapy. The computerized tomography (CT) and MR images (3 mm-slice thickness) of six patients who had received real-time MR imaging-guided interstitial prostate brachytherapy were obtained. An automatic seed localization method was performed on CT images to obtain seed coordinates, and an algorithm for seed localization from MR images of the prostate was developed and tested. The resultant seed distributions from MR images were then compared to CT-derived distribution by matching the same seeds and calculating percent volume receiving 100% of the prescribed dose and the extent of the volume in 3-dimensions. The semiautomatic seed localization method made it possible to extract more than 90% of the seeds with either less than 8% of noises or 3% of missing seeds. The mean volume difference obtained from CT and MR receiving 100% of the prescribed dose was less than 3%. The maximum extent of the volume receiving the prescribed dose were 0.3, 0.6, and 0.2 cm in x, y, and z directions, respectively. These results indicate that the algorithm is very useful in identifying seeds from MR image for post-implant dosimety.

Prostate volume contouring: a 3D analysis of segmentation using 3DTRUS, CT, and MR

PURPOSE

This study evaluated the reproducibility and modality differences of prostate contouring after brachytherapy implant using three-dimensional (3D) transrectal ultrasound (3DTRUS), T2-weighted magnetic resonance (MR), and computed tomography (CT) imaging.

METHODS AND MATERIALS

Seven blinded observers contoured 10 patients' prostates, 30 day postimplant, on 3DTRUS, MR, and CT images to assess interobserver variability. Randomized images were contoured twice by each observer. We analyzed length and volume measurements and performed a 3D analysis of intra- and intermodality variation.

RESULTS

Average volume ratios were 1.16 for CT/MR, 0.90 for 3DTRUS/MR, and 1.30 for CT/3DTRUS. Overall contouring variability was largest for CT and similar for MR and 3DTRUS. The greatest variability of CT contours occurred at the posterior and anterior portions of the midgland. On MR, overall variability was smaller, with a maximum in the anterior region. On 3DTRUS, high variability occurred in anterior regions of the apex and base, whereas the prostate-rectum interface had the smallest variability. The shape of the prostate on MR was rounder, with the base and apex of similar size, whereas CT contours had broad, flat bases narrowing toward the apex. The average percent of surface area that was significantly different (95% confidence interval) for CT/MR was 4.1%; 3DTRUS/MR, 10.7%; and CT/3DTRUS, 6.3%. The larger variability of CT measurements made significant differences more difficult to detect.

CONCLUSIONS

The contouring of prostates on CT, MR, and 3DTRUS results in systematic differences in the locations of and variability in prostate boundary definition between modalities. MR and 3DTRUS display the smallest variability and the closest correspondence.

Tumour and target volumes in permanent prostate brachytherapy: A supplement to the ESTRO/EAU/EORTC recommendations on prostate brachytherapy

The aim of this paper is to supplement the GEC/ESTRO/EAU recommendations for permanent seed implantations in prostate cancer to develop consistency in target and volume definition for permanent seed prostate brachytherapy. Recommendations on target and organ at risk (OAR) definitions and dosimetry parameters to be reported on post implant planning are given.