Is the use of a surrogate urethra an option in prostate high-dose-rate brachytherapy?

Brand D, Lee E, Loblaw A, Tolan S, van As N, Tree AC. Developing and validating a simple urethra surrogate model to facilitate dosimetric analysis to predict genitourinary toxicity

Purpose

The urethra is a critical structure in prostate radiotherapy planning; however, it is impossible to visualise on CT. We developed a surrogate urethra model (SUM) for CT-only planning workflow and tested its geometric and dosimetric performance against the MRI-delineated urethra (MDU).

Methods

The SUM was compared against 34 different MDUs (within the treatment PTV) in patients treated with 36.25Gy (PTV)/40Gy (CTV) in 5 fractions as part of the PACE-B trial. To assess the surrogate's geometric performance, the Dice similarity coefficient (DSC), Hausdorff distance (HD), mean distance to agreement (MDTA) and the percentage of MDU outside the surrogate (UOS) were calculated. To evaluate the dosimetric performance, a paired t-test was used to calculate the mean of differences between the MDU and SUM for the D99, D98, D50, D2 and D1. The D(n) is the dose (Gy) to n% of the urethra.

Results

The median results showed low agreement on DSC (0.32; IQR 0.21-0.41), but low distance to agreement, as would be expected for a small structure (HD 8.4mm (IQR 7.1-10.1mm), MDTA 2.4mm (IQR, 2.2mm-3.2mm)). The UOS was 30% (IQR, 18-54%), indicating nearly a third of the urethra lay outside of the surrogate. However, when comparing urethral dose between the MDU and SUM, the mean of differences for D99, D98 and D95 were 0.12Gy (p=0.57), 0.09Gy (p=0.61), and 0.11Gy (p=0.46) respectively. The mean of differences between the D50, D2 and D1 were 0.08Gy (p=0.04), 0.09Gy (p=0.02) and 0.1Gy (p=0.01) respectively, indicating good dosimetric agreement between MDU and SUM.

Conclusion

While there were geometric differences between the MDU and SUM, there was no clinically significant difference between urethral dose-volume parameters. This surrogate model could be validated in a larger cohort and then used to estimate the urethral dose on CT planning scans in those without an MRI planning scan or urinary catheter.

The urethral position may shift due to urethral catheter placement in the treatment planning for prostate radiation therapy

PURPOSE

To determine the best method to contour the planning organ at risk volume (PRV) for the urethra, this study aimed to investigate the displacement of a Foley catheter in the urethra with a soft and thin guide-wire.

METHODS

For each patient, the study used two sets of computed tomography (CT) images for radiation treatment planning (RT-CT): (1) set with a Foley urethral catheter (4.0 mm diameter) plus a guide-wire (0.46 mm diameter) in the first RT-CT and (2) set with a guide-wire alone in the second CT recorded 2 min after the first RT-CT. Using three fiducial markers in the prostate for image fusion, the displacement between the catheter and the guide-wire in the prostatic urethra was calculated. In 155 consecutive patients treated between 2011 and 2017, 5531 slices of RT-CT were evaluated.

RESULTS

Assuming that ≥3.0 mm of difference between the catheter and the guide-wire position was a significant displacement, the urethra with the catheter was displaced significantly from the urethra with the guide-wire alone in > 20% of the RT-CT slices in 23.2% (36/155) of the patients. The number of patients who showed ≥3.0 mm anterior displacement with the catheter in ≥20% RT-CT slices was significantly larger at the superior segment (38/155) than at the middle (14/155) and inferior segments (18/155) of the prostatic urethra (p < 0.0167).

CONCLUSIONS

The urethral position with a Foley catheter is different from the urethral position with a thin and soft guide-wire in a significant proportion of the patients. This should be taken into account for the PRV of the urethra to ensure precise radiotherapy such as in urethra-sparing radiotherapy.

Urinary morbidity after permanent prostate brachytherapy - impact of dose to the urethra vs. sources placed in close vicinity to the urethra

BACKGROUND AND PURPOSE

The impact of the dose to the urethra and sources placed close to the urethra on urinary morbidity after permanent prostate brachytherapy (PPB) is not well known.

MATERIALS AND METHODS

Fifty-nine patients were surveyed prospectively before treatment (A), 1 month after (B) and > 1 year after PPB (C) using a validated questionnaire (Expanded Prostate Cancer Index Composite). Computed tomography (CT) postimplant scans were performed at days 1 (Foley catheter in situ) and 30 after PPB and sources within 5mm of the urethra at day 1 were identified.

RESULTS

As opposed to the urethral dose-volume histogram, a larger number of sources within 5mm of the urethra at day 1 predicted significantly larger urinary bother score changes at times B and C - with an impact on incontinence and frequency (e.g. moderate/big problem with leaking urine in 25% vs. 3%, p = 0.02; moderate/big problem with frequent urination in 33% vs. 7%, p < 0.01, at time C with vs. without ≥ 3 sources in a single strand placed close to the urethra).

CONCLUSIONS

Placement of sources with a minimum distance of a few mm to the urethra should be a major aim to avoid urinary morbidity irrespective of the urethral dose-volume histogram.

Impact of urinary catheterization on dosimetry after prostate implant brachytherapy with palladium-103 or iodine-125

PURPOSE

Postoperative dosimetry is integral to quality assurance for prostate brachytherapy. Images on Day 0 are typically obtained with a contrast-filled urinary catheter in place for urethral dose calculations. However, expansion of the urethra and perhaps the prostate by the catheter may affect target coverage. We assessed the effect of urinary catheterization on target dosimetry after implantation with palladium-103 ((103)Pd) or iodine-125 ((125)I) seeds.

METHODS AND MATERIALS

Patients were 29 consecutive men with postimplant dosimetry calculated with and without a urinary catheter after brachytherapy seed implantation; 19 patients received (103)Pd seeds and 10 patients received (125)I seeds. In each case, 14-French caude tip urinary catheters were placed before implantation, and axial CT slices of the pelvis were obtained before and after catheter removal for postimplant dosimetry. Dosimetric parameters were measured and compared with paired Student's t tests. Trends were assessed by linear regression with the Pearson correlation coefficient.

RESULTS

Removal of the urinary catheter significantly improved V(100) and D(90) for (103)Pd implants (mean±standard deviation (SD), 2.7%±4.2%; range, -0.4% to 15%; p=0.011 and mean±SD, 4.0%±3.4%; range, -0.1% to 13.8%; p<0.01, respectively). For (125)I implants, catheter removal improved D(90) (mean±SD, 1.5%±1.8%; range, -1.3% to 4.2%; p=0.027). For the (103)Pd group, the magnitude of change in V(100) correlated with prostate size (R(2)=0.16) and source number (R(2)=0.15).

CONCLUSIONS

Urinary catheterization can artificially reduce target coverage after prostate implant brachytherapy. The patients undergoing (103)Pd implantation with smaller (<30cm(3)) prostates and fewer (<90) sources are particularly susceptible to reduced D(90) and V(100) when a urinary catheter is present.

A comparison of HDR brachytherapy and IMRT techniques for dose escalation in prostate cancer: a radiobiological modeling study

A course of one to three large fractions of high dose rate (HDR) interstitial brachytherapy is an attractive alternative to intensity modulated radiation therapy (IMRT) for delivering boost doses to the prostate in combination with additional external beam irradiation for intermediate risk disease. The purpose of this work is to quantitatively compare single-fraction HDR boosts to biologically equivalent fractionated IMRT boosts, assuming idealized image guided delivery (igIMRT) and conventional delivery (cIMRT). For nine prostate patients, both seven-field IMRT and HDR boosts were planned. The linear-quadratic model was used to compute biologically equivalent dose prescriptions. The cIMRT plan was evaluated as a static plan and with simulated random and setup errors. The authors conclude that HDR delivery produces a therapeutic ratio which is significantly better than the conventional IMRT and comparable to or better than the igIMRT delivery. For the HDR, the rectal gBEUD analysis is strongly influenced by high dose DVH tails. A saturation BED, beyond which no further injury can occur, must be assumed. Modeling of organ motion uncertainties yields mean outcomes similar to static plan outcomes.