Penile bulb imaging.

PURPOSE: Because of the apparent relationship between potency loss and radiation doses to the erectile bodies, there is increasing rationale for incorporating penile bulb dosimetry into treatment planning and posttreatment evaluation. Because the location and shape of the penile bulb have not been described in detail on various imaging modalities, we herein describe the anatomic boundaries of the penile bulb on computed tomography (CT), magnetic resonance imaging (MR), and transrectal ultrasound (TRUS), before and after brachytherapy. METHODS AND MATERIALS: Nonenhanced axial CT images were taken on a CTi CT Scanner (General Electric Medical Systems, Milwaukee, WI) with the patient in the supine position. Settings were at 300 ma, 140 kvp, 4-s scan time per slice, and collimation of 3 mm with data obtained at 3-mm intervals. Nonenhanced MR images were obtained with a 1.5 Tesla Signa Horizon LX Scanner using fast spin-echo T1-weighted (TR/TE, 466/20) and T2-weighted (TR/TE, 8000/90) images, with a slice thickness of 2 mm and an interslice gap of 0.5 mm. TRUS images were obtained with a Siemens SONOLINE Prima ultrasound machine at 6.0 MHz and a Winston-Barzell stepper unit. RESULTS: The penile bulb is best visualized on T2-weighted MR images in the axial, sagittal, and coronal planes, appearing as an oval-shaped, hyperintense midline structure. On axial CT imaging, the bulb of the penis is typically readily identifiable, bounded by the paired crura laterally, the corpora spongiosum anteriorly, and the levator ani posteriorly. The penile bulb is typically well visualized on transverse TRUS, but usually only faintly seen on sagittal TRUS. The bulb is partially obscured on postimplant CT and MR images, presumably because of implant-related edema. Bulb volumes vary markedly from patient to patient, ranging from 5.6 to 12.4 cc (median: 8.1 cc). CONCLUSION: Closer attention to penile erectile tissue doses should lead to improved external beam radiation and brachytherapy delivery. It will benefit the radiation oncology community to become familiar with these imaging findings, so that penile bulb dosimetry can be incorporated into our daily practice.

The dosimetry of prostate brachytherapy-induced urethral strictures.

PURPOSE: There is a paucity of data regarding the incidence of urethral strictures after prostate brachytherapy. In this study, we evaluate multiple clinical, treatment, and dosimetric parameters to identify factors associated with the development of brachytherapy-induced urethral strictures. METHODS AND MATERIALS: 425 patients underwent transperineal ultrasound-guided prostate brachytherapy using either (103)Pd or (125)I for clinical T1b/T3a NxM0 (1997, American Joint Committee on Cancer) adenocarcinoma of the prostate gland from April 1995 to October 1999. No patient was lost to follow-up. 221 patients were implanted with (103)Pd and 204 patients with (125)I. The median patient age was 68 years (range 48-81 years). The median follow-up was 35.2 months (range 15-72 months). Follow-up was calculated from the day of implantation. Thirteen patients developed brachytherapy-induced strictures, and all strictures involved the membranous urethra. A control group of 35 patients was rigorously matched to the stricture patients in terms of treatment approach; i.e., choice of isotope, plus or minus radiation therapy, and plus or minus hormonal manipulation. Nine of the 13 stricture patients had detailed Day 0 urethral dosimetry available for review. The apex of the prostate gland and the membranous urethra were defined by CT evaluation. Urethral dosimetry was reported for the prostatic urethra, the apical slice of the prostate gland, and the membranous urethra which was defined as extending 20 mm in length. RESULTS: The 5-year actuarial risk of a urethral stricture was 5.3%, with a median time to development of 26.6 months (range 7.8-44.1 months). Of multiple clinical and treatment parameters evaluated, only the duration of hormonal manipulation (>4 months, p = 0.011) was predictive for the development of a urethral stricture. The radiation dose to the membranous urethra was significantly greater in patients with strictures than those without: 97.6% +/- 20.8% vs. 81.0% +/- 19.8% of prescribed minimum prostate dose, mPD (p = 0.031). The urethral dose 20 mm distal to the prostate apex was 57.6% +/- 23.8% vs. 31.5% +/- 13.9% of mPD for the stricture and control patients, respectively (p = 0.011). In addition, the extent of the 75% mPD and 50% mPD levels beyond the prostatic apex was also significantly greater for stricture patients, 16.6 +/- 5.3 mm vs. 11.9 +/- 4.5 mm (p = 0.010) and 19.0 +/- 3.2 mm vs. 16.0 +/- 3.4 mm (p = 0.021), respectively. Dose to the prostatic urethra was not predictive of stricture, but the magnitude and extent of high-dose regions within the prostate were predictive of stricture. Twelve of the 13 patients who developed urethral strictures were successfully managed by dilatation/transurethral incision. To date, 1 of the 12 patients has required a second dilatation. The remaining patient developed an iatrogenic induced injury and was catheter-dependent for 6 months. CONCLUSIONS: After prostate brachytherapy, the actuarial 5-year incidence of urethral strictures is 5.3% with a median time to development of 26.6 months. All strictures involved the membranous urethra and occurred within the first 44 months after brachytherapy. In most cases, membranous urethral strictures are easily managed with dilatation/incision. Factors predicting for the development of a urethral stricture included the magnitude and extent of high-dose regions within the prostate, the mean membranous urethra dose and the dose 20 mm distal to the prostatic apex, the maximum extent along the membranous urethra of certain dose levels, and the duration of hormonal manipulation.