Proposal of a method to assess and report the extent of residual neurovascular tissue present in radical prostatectomy specimens

Authors


Correspondence: Vipul R. Patel, Global Robotics Institute, Florida Hospital Celebration Health, 410 Celebration Pl, suite 200, Celebration, FL 34747, USA.

e-mail: vipul.patel@flhosp.org

Abstract

Objective

  • To propose a method to assess and report the amount of neurovascular tissue present in radical prostatectomy (RP) specimens.

Patients and Methods

  • The data of 133 consecutive patients who underwent robot-assisted RP by a single surgeon (V.R.P.) were prospectively collected.
  • Degree of nerve sparing (NS) was graded intraoperatively by the surgeon independently at either side as complete, partial or none.
  • A pathologist who was ‘blinded’ to the surgeon's classification measured the following parameters at the posterolateral aspect of the apex, base and mid prostate at either side of the RP specimen: length, width and area of neural tissue, number of nerves per high-power field and number of total slides containing neural tissue.
  • Measurements were correlated to the surgeon's intraoperative perception.

Results

  • All measurements correlated significantly with surgeon's intent of NS at all locations (P = 0.001).
  • Among them, the cross-sectional area had the highest correlation coefficient (–0.550 at apex, –0.604 at mid prostate and –0.606 at the base).

Conclusions

  • The cross-sectional area of nerve tissue showed the highest correlation with surgeon's intent of NS at all locations.
  • Having a standardised method of assessing and reporting residual nerve tissue allows the surgeon to objectively evaluate the quality of nerve preservation and to compare the progress of his NS technique over time.
Abbreviations
ECNVT

extracapsular neurovascular tissue

IQR

interquartile range

NS

nerve sparing

NVB

neurovascular bundle

RP

radical prostatectomy

Introduction

As a consequence of advances in screening and the early detection of prostate cancer, radical prostatectomy (RP) is being increasingly performed in younger patients at an earlier disease stage [1]. Post-RP erectile dysfunction is a significant concern for this young population and a determinant of postoperative quality of life [2]. The quantity and quality of cavernous nerve preservation is probably the most determinant factor influencing recovery of erectile function in preoperatively potent patients. An optimal nerve sparing (NS) technique should consistently avoid mechanical traction, thermal energy and respect tissue planes. To date, evaluation of NS has been done exclusively intraoperatively by the surgeon, relying on his subjective perception of the amount of nerves spared at the time of surgery. However, a comprehensive evaluation of the quality of NS cannot obviate the histopathological assessment of the amount of extracapsular neurovascular tissue (ECNVT) present in the RP specimen, because ultimately, regardless of the surgical technique, optimal NS could not possibly have been performed if the pathologist finds a large amount of nerves in the area of the neurovascular bundles (NVBs).

The amount of ECNVT objectively denotes the degree of NS performed at the time of surgery and could potentially influence postoperative erectile function. Despite this, there are currently no reports on methods to assess the amount of ECNVT in RP specimens. Moreover, although RP specimens harbour a variable amount of ECNVT, this is not part of a regular histopathological report [3, 4]. We propose a method to assess and report the amount of ECNVT present in RP specimens.

Patients and Methods

The study was performed with the approval of our Institutional Internal Review Board. Data of 136 consecutive patients who underwent robot-assisted RP by a single surgeon (V.R.P.) between January and February of 2011 were prospectively collected and entered into a customised database by a collaborator unrelated to the study. Three patients who underwent a salvage procedure were excluded from the analysis.

NS Technique and Grading System

Our preferred NS technique is athermal, retrograde, with minimal traction on the NVB and based on intraoperative anatomical landmarks [5-8]. To achieve optimal visualisation and control of the NVB, the posterior plane between the prostate and the rectum must be widely developed and the periprostatic veins running along the lateral border of the prostate must be decompressed. Venous decompression is achieved by previously ligating the dorsal venous complex, bladder attachments to the prostate and anterior portion of the prostatic pedicle. The prostate is then rotated and the overlying lateral pelvic fascia is opened sharply. The NVBs are identified at the level of mid prostate and approached from above according to our previously described technique [5-8].

In all cases, the surgeon graded the amount of NS intraoperatively independently for either side as complete, partial or none. A complete NS was assigned when the NS was performed interfascially through the areolar plane existing between the prostatic fascia and the NVB. NS was graded as partial when it was performed through the NVB resulting in different degrees of nerve preservation, whereas a non-NS grading was assigned when the NVB was resected lateral to the lateral pelvic fascia [7].

Histopathological Assessment of Residual ECNVT

The RP specimen is inked to aid with orientation of microscopic sections and assessment of the surgical margins. Next, the apex and bladder neck are amputated and serially cross-sectioned. The seminal vesicles and vasa deferentia are likewise amputated and sections are taken, with an attached portion from the prostatic base. The remaining prostate is cross-sectioned in the axial plane from apex to base at 3–4 mm intervals. A pathologist who was unaware of the surgeon's intraoperative grading of NS, measured on the posterolateral aspect of the prostate the length, width and cross-sectional area of ECNVT using low magnification and also counted the number of nerve fibres present on a representative spot using high-power field (×40). The length of ECNVT was measured in a straight line parallel to the prostatic capsule between the two farthest nerve fibres, whereas the width was measured in a straight line from the capsule to the most lateral nerve fibre. The area was calculated as the width times the length (Fig. 1). Measurements were conducted at the apex, mid part and base independently on either side of the RP specimen for a total of six slides per patient.

Figure 1.

Method for measuring the cross-sectional area of neural tissue. Length (blue line): The distance between the two most distant nerve fibres (*) is measured with a straight line running parallel to the prostatic capsule. Width (red line): The distance between the capsule and the most lateral nerve fibre (x) is measured with a straight line perpendicular to the previous one. Area of cross-sectional neural tissue = length × width.

Statistical Analysis

Continuous parametric and non-parametric data are presented as the mean (sd) and median (interquartile range, IQR), respectively. Categorical data are presented as frequencies. Correlations between the type of NS and histolopathological assessments were made using Spearman's ρ, while Kruskal–Wallis and Mann–Whitney U tests were used to compare the values of the different histopathological assessments stratified by the type of NS. All tests are two-sided with a P ≤ 0.05 considered to indicate statistical significance.

Results

In all, 133 patients were included. The mean (sd) age was 60 (8) years. The median (IQR) body mass index, preoperative Sexual Health Inventory for Men (SHIM) score, AUA symptom score, PSA level, and prostate volume were 28 (26–31) kg/m2, 20 (13–25), 7 (3–14), 5.2 (4.2–7) ng/mL and 48 (39–61) mL, respectively. Overall, 52.6% (140/266) of the operated sides had complete NS, 45.5% (121/266) had partial NS and 1.9% (5/266) had non-NS.

Positive surgical margins were found in 9.02% of the patients (12 of 133 patients), with 8.3% (nine of 108) for stage pT2 and 12% (three of 25) for pT3. When considering side specific positive surgical margins according to the intraoperative grading of NS, surgical margins were positive in 3.6% (five of 140) of the sides having complete NS, in 7.4% (nine of 121) of those having a partial NS, and in none of those having complete resection of the NVB (zero of five).

There were no side differences in the amount of ECNVT at a same location, so both apices, mid-prostates and bases were combined for statistical analysis. The values for the different histopathological variables measured at the apex, mid prostate and base are detailed in Table 1. All of them showed significant differences according to the type of NS. We then studied the association between the histopathological variables and the type of NS, and found that all of the histopathological variables showed significant correlation with the type of NS (Table 2). However, among all of them the cross-sectional area of ECNVT showed the highest correlation with the NS at each of the studied locations.

Table 1. Value of histopathological variables stratified by type of NS
Median (IQR) measurementNo NSType of NSP
Partial NSFull NS
  1. HPF, high-power field.
Apex:    
Length, mm10 (4–10)2 (0–5)0 (0–2)<0.001
Width, mm2 (1–2)1 (0–1)0 (0–1)<0.001
Area, mm212 (4–24)2 (0–5)0 (0–2)<0.001
Nerves/ HPF2 (2–3)2 (0–2)0 (0–2)<0.001
Mid prostate:    
Length, mm10 (6–12)4 (0–8.5)0 (0–4)<0.001
Width, mm2 (1–3)1 (0–1)0 (0–1)<0.001
Area, mm220 (10–30)4 (0–10)0 (0–3)<0.001
Nerves/HPF3 (2–3)2 (0–3)0 (0–2)<0.001
Base:    
Length, mm12 (10–15)6 (3–10)3 (0–7)<0.001
Width, mm3 (2–4)1 (1–2)1 (0–1)<0.001
Area, mm232 (14–60)7 (3–15)3 (0–8)<0.001
Nerves/HPF3 (2–3)2 (2–3)2 (0–2)<0.001
Table 2. Spearman's ρ correlations between surgeon's grading of NS and histological assessment of periprostatic nerve tissue in RP specimens
Correlations atType of NSLength of ECNVTWidth of ECNVTNerves per HPFArea of ECNVT
  1. HPF: high-power field.
Apex:     
Type of NS     
Correlation coefficient1.000−0.529−0.538−0.471−0.550
P 0.0010.0010.0010.001
Mid prostate:     
Type of NS     
Correlation coefficient1.000−0.572−0.554−0.473−0.604
P 0.0010.0010.0010.001
Base:     
Type of NS     
Correlation coefficient1.000−0.568−0.568−0.288−0.606
P 0.0010.0010.0010.001

Discussion

With younger and healthier patients seeking surgical treatment of prostate cancer, NS during RP is becoming increasingly important, as these contemporary patients expect not only long-term cancer control, but also complete functional recovery [1]. In this context, postoperative erectile dysfunction has been shown to substantially affect the quality of life of men undergoing RP [2]. An optimal NS should comply with appropriate surgical technique (i.e. avoidance of mechanical traction, thermal injury and respect for tissue planes) and demonstrate absence of residual ECNVT on histopathological evaluation of the surgical RP specimen.

The first step in assessing the amount of ECNVT is to define the location of the NVBs and cavernous nerves. The original description by Walsh and Donker [9] stated that the cavernous nerves run in a well-defined NVB located posterolaterally on the prostate. Costello et al. [10] have confirmed these findings and even suggested the existence of functional compartments within the NVB. Conversely, others have reported a broader distribution of periprostatic nerves around the prostatic capsule [11-14]. In fact, a study by Kyoshima et al. [15] found a distinct NVB in only 52% of RP specimens. Similar conclusions were obtained by Lee et al. [16] after analysing specimens of 95 non-NS RPs, where a bundle-like formation was found in only 51.6% of the RP specimens. Several anatomical studies indicate that these nerves located outside the traditional NVBs are mainly located anteriorly on the prostate and have proposed a higher detachment of the NVB [11-14]. Electrophysiological studies have suggested that these anteriorly located nerves may contribute to erectile function [17, 18]. However, methodological limitations may limit the validity of this conclusion. Unless cadaveric dissections can trace these anterior nerve fibres entering the corpora cavernosa, speculation on the function of these anterolateral nerves cannot be made solely by intraoperative electrical nerve stimulation during RP. Costello et al. [19] performed an immunohistochemical study of the cavernous nerves in the periprostatic region using cadavers and found that parasympathetic nerves above the horizontal axis of the prostate accounted for only 4%, 5% and 6.8% of the total number of nerves at the base, mid-prostate and apex, respectively. Similarly, Ganzer et al. [20] found that only 1.5% of the nerve content at the anterior aspect of the apex was parasympathetic. Considering the lack of consistent evidence supporting the parasympathetic nature of these nerves located outside the posterolateral aspect of the prostate and the satisfactory functional outcomes obtained with a technique that is already mature and standardised [21], we have measured the amount of ECNVT at the posterolateral aspect of the prostate exclusively.

After deciding on the location of the NVBs, the next step is to quantify the amount of ECNVT. Ganzer et al. [14], Clarebrough et al. [22] and Lee et al. [16] counted the extraprostatic nerves by digitisation and computerised planimetry, while Eichelberg et al. [11] and Sievert et al. [23] counted them manually. In the present study, we manually counted the number of nerve fibres per high-power field as a surrogate of nerve density and found significant correlation with the surgeon's grading of NS. The periprostatic cross-sectional area has also been used to measure the amount of ECNVT [14, 22]. We measured the cross-sectional area of ECNVT on the posterolateral aspect of the prostate and found that it correlated best with the amount of NS intended by the surgeon. Unlike the nerve count, the cross-sectional area of ECNVT can be appreciated intraoperatively as a fatty strip over the prostate and tailored by the surgeon intraoperatively [7, 24]. We think this is why it showed the highest correlation with the surgeon's intent of NS. As pathologists are used to performing measurements on microscopic slides, i.e. for reporting the extent of positive surgical margins or extraprostatic extension, measuring the length and width of the ECNVT can be easily done during routine microscopic evaluation without requiring significant extra time (Fig. 1).

Deciding how to measure the amount of ECNVT was the first step in validating our five-point NS grading system [24]. In the present study, we tested four histopathological variables that were easy to measure in a standard RP specimen (i.e. length, width and area of nerve tissue, and number of nerve fibres per high-power field) and found that the area of ECNVT had the highest correlation with the extent of NS intended by the surgeon. Hence, we then tested these results in our anatomical grading of NS. Briefly, this grading system consists of a five-category grading of NS done by the surgeon based on intraoperative anatomical landmarks [7]. The cross-sectional area of ECNVT was measured as described above and significantly differed among the different NS groups, especially for those with higher NS scores [7, 24]

This exercise of developing a standardised form of measurement of ECNVT has great importance for every surgeon regardless of his level of expertise or caseload. It is important to assess and report the amount of ECNVT for several reasons. If a surgeon would like to evaluate the progress of his NS technique, it is critical use a standardised method of assessing the amount of ECNVT. For example, a decrease in the area of ECNVT over time for an intended complete NS confirms an improvement in his NS technique. We think this feedback is important for every surgeon, as a way to refine his NS technique and improve patient outcomes. Robotic technology allows performance of a finer and more precise operation, to the point that complete excision of the NVB is now rarely performed, and most patients are candidates to either a complete or different degrees of partial NS. In the present series, only 1.9% of the operated sides had a non-NS procedure. These different degrees of NS can be defined as a function of the area of ECNVT and proper NS can be tailored according to preoperative and intraoperative patient characteristics. The cross-sectional area of the ECNVT has been a simple and useful tool for us in assessing our precision during NS and for developing our anatomical approach to NS [7, 24].

Reviewing the amount of ECNVT is also useful in addressing preoperatively potent patients who do not recover erectile function, despite an intended complete or nearly complete NS. There is evidence that surgeon's perception of NS is not always accurate in determining the quantity of nerve preservation when confronted with the pathological analysis of the RP specimen [23-26]. In this context, the presence of a significant amount of ECNVT confirms a suboptimal NS, regardless of surgeon's intent.

A limitation of the present study is that the correlation between surgeon's intent of NS and cross-sectional area of ECNVT is dependent on the surgeon's experience and surgical technique. An additional limitation is the lack of functional outcomes reported for this cohort, as the ability to predict recovery of erectile function is the ultimate outcome of any NS classification. The aim of the present study was to describe a method for reporting ECNVT that is simple to perform, standardised and useful for the surgeon. It is well known that recovery of erectile function is multifactorial. In this context, the amount of ECNVT could provide additional information for models and algorithms aimed to predict postoperative recovery of potency.

In conclusion, we report a method to assess and report the amount of ECNVT that is simple to perform and useful for the surgeon, as it allows him to objectively evaluate the quality of nerve preservation and to compare the progress of his NS technique over time.

Conflict of Interest

None declared.

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