Elsevier

Brachytherapy

Volume 16, Issue 4, July–August 2017, Pages 728-733
Brachytherapy

Magnetic Resonance Imaging-Based Brachytherapy
Permanent prostate brachytherapy pubic arch evaluation with diagnostic magnetic resonance imaging

https://doi.org/10.1016/j.brachy.2017.02.001Get rights and content

Abstract

Purpose

Pubic arch interference (PAI), when it occurs, is often a limiting factor for patients pursuing brachytherapy treatment of prostate cancer. Pre-brachytherapy pubic arch evaluation is often performed by CT or transrectal ultrasound (TRUS), but MRI has increasingly replaced these modalities for prostate cancer evaluation. The purpose of this study was to determine if staging MRI could be used to evaluate PAI and compare it with these other imaging methods.

Methods and Materials

Forty-one consecutive patients undergoing brachytherapy evaluation had pelvic MRI-, CT-, and TRUS-based brachytherapy simulation. Pubic arch overlap on T2-weighted MRI and CT was determined by contouring the prostate gland on its largest axial slice and superimposing this contour onto the pubic arch bones. The largest degree of overlap of the prostate gland on MRI and CT was used to predict the existence of PAI as determined by TRUS-based simulation. The correlation between prostate contour overlap was also compared between MRI and CT.

Results

Nineteen patients (48%) exhibited PAI on TRUS brachytherapy simulation evaluation. The average (±standard error) amount of prostate contour overlap on the pubic arch on CT was 2.9 ± 0.6 mm and on MRI was 2.0 ± 0.6 mm (linear correlation, R, of 0.783, p < 0.001). CT and MRI were equally predictive of PAI on TRUS evaluation (area under the curve = 0.75).

Conclusion

Pre-brachytherapy pubic arch assessment with diagnostic MRI provides similar predictability of PAI compared with CT, potentially obviating the need for additional pre-brachytherapy CT in the setting of staging MRI.

Introduction

Permanent prostate brachytherapy is one treatment option available to men diagnosed with prostate cancer which is able to provide excellent therapeutic outcomes with relatively minimal morbidity [1], [2], [3]. Unfortunately some men are precluded from this treatment modality secondary to anatomic or patient specific factors such as prostate size or the presence of pubic arch interference (PAI) [4], [5]. The assessment for PAI has been traditionally performed before brachytherapy implantation using CT and/or transrectal ultrasound (TRUS) [6], [7]. For CT, a common method of PAI determination consists of outlining the prostate on the largest axial slice and superimposing this over the pelvic bones (6). Overlap of the prostate with the pubic rami of ≤1 cm has been proposed as a threshold value suitable for brachytherapy consideration (8). These methods are generally able to provide an approximate assessment of PAI before brachytherapy implantation but can be subject to operator variability and often require additional testing/procedures in addition to the routine staging and workup for prostate cancer in the modern era.

In recent years, MRI of the pelvis has often replaced CT as the modality of choice for imaging of the prostate and seminal vesicles because of its improved soft tissue delineation and improvements in MRI capabilities over the past decade. MRI is able to provide good sensitivity and specificity for detection of extracapsular extension or seminal vesicle involvement especially with using multiparametric MRI [9], [10], which can lead to altered treatment recommendations especially with regard to brachytherapy candidacy. The use of MRI in the pre-prostatectomy setting has been an area of active on-going research and implementation (11), but its routine clinical adoption in permanent prostate brachytherapy outside staging purposes remains limited (12). The purpose of this study was, therefore, to investigate whether diagnostic MRI performed for the staging of prostate cancer is able to predict PAI in patients being considered for permanent prostate brachytherapy and compare these MRI results to traditional CT- and TRUS-based methods.

Section snippets

Methods and materials

Forty-one consecutive patients’ charts being considered for brachytherapy at our institution received staging MRI, pre-brachytherapy CT, and TRUS-based simulation. Clinical information, such as demographics, prostate cancer stage, prostate gland size on TRUS, and imaging data, were collected. All patients underwent pelvic MRI with a T2-weighted sequence with endorectal coil (inflated to 30–60 cubic centimeters) as part of their cancer staging. Patients are considered eligible at our institution

Results

The average age for patients at the time of brachytherapy evaluation was 66 years old. All patients but one were classified as having intermediate-risk prostate cancer. The patient with high-risk prostate cancer was evaluated for brachytherapy implant in addition to external beam radiation. The average prostate size (±standard error) on pre-brachytherapy ultrasound was 32.6 ± 2.33 mL. Full details of patient characteristics are summarized in Table 1.

Nineteen patients (46%) exhibited evidence of

Discussion

This study is the first to our knowledge to evaluate PAI before permanent prostate brachytherapy implantation using MRI. Our results show that both MRI and CT provide similar estimates of PAI as determined by TRUS-based brachytherapy simulation, and there was a strong linear correlation between quantitative values of MRI- and CT-based PAI. Furthermore, the ability of MRI to reliably predict lack of PAI was excellent if prostate contour showed no evidence of pubic arch overlap but was less

References (20)

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  • Low dose rate brachytherapy for primary treatment of localized prostate cancer: A systemic review and executive summary of an evidence-based consensus statement

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    Special consideration should be made for patients with large prostate glands, large median lobes, or prior history of transurethral resection of the prostate (TURP). Patients with large prostate volumes (>60 cc) may benefit from a TRUS (71,72), CT (73), or MRI-guided (74) volume study to ensure minimal pubic arch interference. Such patients may also be at greater risk of urinary retention (75) and late urinary toxicity (76).

  • Comparison of prostate distortion by inflatable and rigid endorectal MRI coils in permanent prostate brachytherapy imaging

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    We believe that MRI provides multiple advantages over ultrasound or CT due to its improved soft tissue delineation, ability to reduce fusion uncertainties, and decrease in the total number of scans/procedures that need to be performed for brachytherapy implantation. For example, in the preimplant setting, MRI has been shown to decrease the number of prostate ultrasound procedures for planning and can replace axial CT imaging for pubic arch interference assessment (31, 32). In the postimplant setting, MRI-only–based dosimetry may also decrease the dose uncertainty arising from CT-MRI fusion (33), and the improved anatomic detail allows for better identification of organs at risk such as the bladder neck or external urinary sphincter, both of which have been associated with prostate brachytherapy dose-related toxicity (34, 35).

Funding/Disclaimer: This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors. The authors declare no conflicts of interest.

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