Histopathologic Changes After Bipolar Resection of the Prostate: Depth of Penetration of Bipolar Thermal Injury

Introduction

B enign prostatic hyperplasia (BPH) is a common pathologic entity encountered by urologists throughout the world and becomes increasingly prevalent in the elderly population. More than 50% of patients aged 60 years or older will be affected by hypertrophy of the prostate, with this number approaching 90% in those older than 85 years. ¹ Given that BPH is a diagnosis of the aging male population, those afflicted tend to have significant medical comorbidities. Many advances in medical therapy have been made since the initial descriptions of BPH as a major contributing factor of bladder outlet obstruction. Despite these improvements in medical management, therapy will fail in up to 25%, and surgical treatment will be needed. ²

Transurethral resection of the prostate (TURP) was developed in the early 20th century and remains the gold standard of surgical treatment of patients with BPH. Although TURP is considered a relatively safe procedure, in many situations it may result in significant patient morbidity, such as bleeding, infection, and dilutional hyponatremia. Contributing factors to the risk associated with TURP include an elderly patient population with many comorbidities and the method of energy delivery. To limit these undesirable attributes of monopolar TURP, considerable urologic research has focused on alternative methods of managing the prostatic adenoma.

Various technologies such as microwave and laser therapy have emerged. Recently, there has been a tremendous surge in the use of bipolar technology in TURP. The popularity of this technique is because of its similarities to traditional TURP. Bipolar TURP has been reported to decrease bleeding and catheter time. ³ The most important advantage of the bipolar TURP is its ability to be performed in physiologic saline, eliminating the risk for hyponatremia and transurethral resection (TUR) syndrome. ³

Essential to both monopolar and bipolar electrosurgery are a generator, an active and return electrode, and a medium between the electrodes (target tissue) to complete the circuit. With monopolar devices, the active electrode and return electrode (skin pad) are separated by significant distances and thus require higher voltages during resection. In contrast, bipolar devices incorporate the active and return electrode into the resectoscope design limiting tissue impedance and thus allowing for resections to occur with lower sustained voltages after an initial high voltage spike to create the plasma arc. ³

Because of the lower voltages necessary for resection, bipolar TURP has theoretical advantages of decreased charring, lower tissue temperatures, and collateral tissue injury; however, its histopathologic effect on human prostate tissue is not well understood. The objective of this study is to prospectively assess histopathologic changes in prostatic tissue after bipolar transurethral vaporization of the prostate.

Patients and Methods

Institutional Review Board approval and informed consent were obtained before enrollment. This prospective study was performed over a 14-month period (1/2010–3/2011). Inclusion criteria were male patients older than 40 years with bladder outlet obstruction secondary to BPH. Medical therapy had failed in all patients, and they had diminished quality of life measures (American Urological Association symptom score) documented. Exclusion criteria from the study were those men who had previously undergone any therapy of the prostate, including surgery, radiation, or hormonal therapy for prostate cancer.

A brief description of the procedure is provided. At the beginning of each procedure standard cystoscopy was performed, and anatomic assessment of the area to be sampled was defined. The Olympus™ vaporizing button electrode was used with settings of 290 W cutting and 145 W coagulation current in the defined sampling area. After vaporization of the prostatic tissue with the button electrode using cutting current only, a loop electrode using the same settings was used to take a deep resection sample. The vaporized and loop resected surfaces of the specimen were then uniquely inked for identification and sent in formalin for pathologic review.

The bipolar TURP was then completed independent of the study using either the bipolar button or loop, depending on the anatomy of the prostate. Typically at our institution, if only bilobar hypertrophy is encountered, the button is used to complete the procedure. In instances where there is a large median lobe or extremely large gland, resection is performed using the bipolar cautery loop.

All specimens were reviewed by a single pathologist (SM). Sections were processed for routine histopathologic examination. Samples were fixed in neutral buffered formalin and underwent processing for paraffin embedded sections with hematoxylin and eosin stains. The depth of thermal effect was assessed by correlating the maximum measurement from the blue ink (vaporized surface) to yellow ink (resected surface) on microscopic examination at magnifications of 200X and 400X. Presence of thermal effect was identified by distortion of glandular and stromal cells in keeping with thermal injury.

All data were accrued and analyzed using Microsoft Excel 2001 (1985–2001 Microsoft Corporation, Redmond, WA). A Mann-Whitney-Wilcoxon test was used to determine statistical significance of the difference in operative times between those who underwent vaporization only and those with loop resections.

Results

All patients underwent successful bipolar TURP without intraoperative complications. Mean operative time was 48.7±21.1 minutes (range 28–89 min). Mean patient age was 70±10.2 years (range 56–88 years) with a mean American Society of Anesthesiologists score of 2.4±0.9 (range 1–4). No patients needed blood transfusion. Bipolar TURP was performed as an outpatient procedure in seven patients. Five patients were admitted for observation.

Of the 12 patients in the study, 8 patients exclusively underwent button vaporization of the prostate using the loop electrode only to study histopathologic changes. In four patients, the bipolar loop was used for completion of resection. The average operative time was similar for those patients undergoing loop resection vs those who only had button vaporization (65.8 vs 40.1 min, P=0.089).

The mean depth of thermal injury was 2.4±0.84 mm (range 0.3–3.5 mm). No char artifact was noted in any of the specimens (Fig. 1). On final pathology analysis of the prostate chips, one patient had evidence of prostate cancer in the specimen, and one patient had foci of atypical glands. The remainder of the patients had benign pathology results. Table 1 shows the patient, surgical, and histopathologic characteristics of the study population.

OPEN IN VIEWER

FIG. 1.

(A) 20× magnification of a specimen showing the inked margins of the vaporized and resected surface; (B) 100× magnification of a specimen showing the measured depth of thermal penetration from the vaporized surface.

Discussion

Monopolar TURP remains the gold standard for surgical intervention for patients with BPH. The endoscopic nature of the procedure makes it a minimally invasive therapy compared with the open surgical alternatives. Monopolar TURP, however, has significant potential perioperative morbidities, including hemorrhage, perforation, and TUR syndrome as a result of dilutional hyponatremia. ⁴ The latter results because the medium used to perform the procedure is hypotonic irrigation solution necessary for monopolar current conductivity. Because of these defined deleterious effects of monopolar TURP, improvements in energy used to resect or ablate the prostate adenoma have been sought. Bipolar TURP represents an advancement in this arena because of its use of localized energy necessitating no grounding of the patient in addition to an energy that is delivered in physiologic saline media.

Bipolar circuitry allows prostate resection to be performed in a physiologic saline environment. This essentially eliminates the most severe complication of the TUR syndrome even in lengthy resections of large glands. ⁵ In a review article, Issa ³ compared monopolar with bipolar technology and highlights some of the advantages of bipolar electrocautery including: Lack of energy traveling through the patient from both active and return electrodes incorporated into the resectoscope, less energy and voltage requirement with decreased resistance, ability to function in physiologic saline, decreased temperatures and thus minimization of tissue charring, and simultaneous hemostatic properties during resection.

Advantages are obvious, including more controlled hemostasis, less physiologic fluid shifts, and the ability to safely increase resection times. In addition, most procedures may be performed in an outpatient setting. The procedural volumes using this technology have increased despite a paucity of data describing its long-term clinical outcomes. The local tissue effects and depth of thermal injury using bipolar technology are not well reported or understood in an in vivo human model.

In this report, we assessed the histopathologic effect of bipolar energy and compared it with the limited historical data available in the literature. Our desire to study this parameter was based on a curiosity of whether localized bipolar energy may result in deeper and possible dangerous levels of tissue injury. We found that the depth of thermal injury for bipolar vaporization exceeded the depth of previously published studies on bipolar resections with an average penetration of 2.4 mm.

Few studies of various designs have reported on the histopathologic effect of bipolar resection within the realm of urologic surgery. Wendt-Nordahl and associates ⁶ initially described their results of monopolar vs bipolar resection over a range of power settings in an ex-vivo porcine kidney model. They found a coagulation depth of 163 μm with bipolar resection at the highest output power compared with 287 μm using a conventional monopolar resectoscope. In addition to the coagulation depths, a decreased bleeding rate was observed with the bipolar resectoscope. Further supporting these findings is Ko and colleagues ⁷ who demonstrated in a canine prostate model an increased tissue temperature with monopolar resection as well as an increased depth of thermal artifact (0.59 mm monopolar depth vs 0.07 and 0.15 mm, respectively, for the Gyrus and Vista bipolar resections). ⁷

These comparisons between bipolar and monopolar penetration depths, however, have not been uniform in the literature. Two studies by Huang and coworkers ^8,9 showed in a canine and human prostate model a conflicting histologic pattern. This group studied the evolution of the coagulation zone after monopolar and bipolar resections of canine prostates at 0, 7, 14, and 60 days postoperatively. The initial coagulation zone was deeper for bipolar resections compared with monopolar (237.73 and 200.75 μm, respectively), while at 7 days postoperative the monopolar group was found to have a deeper coagulation zone (129.46 vs 113.03 μm). ⁸ When applied to a human prostate model, the bipolar group demonstrated a significantly deeper coagulation zone of 141.35 μm compared with 122.54 μm in the monopolar group. Prostate microvessel diameters were also measured, which revealed that the bipolar and not the monopolar coagulation zone was deeper than the maximum vessel diameter. ⁹ These reported depths compare favorably to the published depths for potassium-titanyl-phosphate and holmium laser tissue penetration of 2 mm and 0.4 mm, respectively. ^10,11

All of the previously mentioned studies on histologic effects of bipolar electrosurgery were performed with a bipolar loop electrode. To our knowledge, no study has investigated the depth of penetration of the bipolar button vaporization electrode. It is thought that tissue penetration is affected by not only contact time of the electrode and target tissue, but also the surface area of the electrode itself. Given the larger surface area of the button vaporization electrode, it was postulated that the extent of thermal injury would be accentuated. Therefore, we sought to prospectively investigate the depth of thermal penetration of the vaporizing electrode to compare it with previously published depths with other modalities. In our study, there was a mean depth of thermal injury of 2.4 mm, which is 10 times deeper than the maximum depth reported in the literature for bipolar loop resections. This may have important implications with respect to hemostatic potential and possible perioperative morbidity.

There is a general consensus that the depth of coagulation plays an integral role in at least hemostasis and possibly other postoperative outcomes. It has been extrapolated that with increased depth of bipolar tissue penetration, there is an increased coagulative effect resulting in less bleeding. ^6,8,9 In studies with conflicting depth measurements, authors have suggested that deeper penetration with monopolar devices may lead to more sloughing and thus more postoperative hemorrhage. ⁷ Clinical studies have demonstrated the advantage of decreased blood loss with bipolar TURP compared with monopolar TURP, but given the paucity of histopathologic studies, the explanation for this improved hemostasis remains speculative.

Theoretically, with deeper penetrations comes the increased risk of periprostatic injury including injury to the neurovascular bundle. The bipolar device, however, allows for resection at lower temperatures, which may negate the effect of deeper penetration. We feel that with our results of deeper penetration with the bipolar vaporization electrode, this risk may be increased, and urologists should take care when performing deep ablations of the prostate. In our experience, use of coagulation is limited to spot coagulation when nearing the prostate capsule during deep resections to limit the potential for periprostatic injury.

Despite our findings, the use of the button vaporization electrode did not preclude pathologic tissue diagnoses in our study. Two of the 12 patients were found to have lesions within the prostate that needed further urologic attention. Wang and colleagues ¹² similarly found that the use of bipolar energy for transurethral resection of bladder tumors did not affect the ability to make an accurate pathologic diagnosis. Although the primary objective of TURP is not tissue diagnosis, it is reassuring that bipolar electrocautery does not appear to interfere with regard to pathologic identification of incidental disease.

One limitation of our study was the low enrollment of patients as well as a lack of a monopolar control arm in the protocol. Our goal was to investigate the thermal injury caused by the button electrode; therefore, it seemed unnecessary to have a monopolar control. Other limitations included a limited ability to standardize cautery speed and depth of resection. We tried to control for this by only including patients of two surgeons as opposed to all eligible patients undergoing bipolar TURP at our institution. Finally, the method of loop resecting previously vaporized prostate tissue may have obscured the ability to delineate the depth of penetration. This did not seem to have an effect on the pathologists' assessment of thermal injury from the vaporized surface, however, because there was a normal area of prostate tissue between the two resected margins.

Conclusions

We found an average depth of thermal injury of 2.4 mm with use of the bipolar button electrode on histolopathologic review. Because of the larger depth of penetration in our study, urologists should use caution during deep vaporizations of prostatic adenomas to avoid injury to the neurovascular bundle.