Abstract
OBJECTIVE
To investigate the use of microwave ablation (MWA) with cooling urethral perfusion and with no perfusion (MWA-UP and MWA-NP, respectively) for prostate gland ablation in canine cadavers.
ANIMALS
Cadavers of 18 sexually intact male dogs.
PROCEDURES
After technique refinement in 2 cadavers, laparotomy with ultrasound-guided MWA-UP (n = 8) or MWA-NP (8) of the prostate gland was performed in 16 cadavers. Normograde cystourethroscopy was performed before and after treatment; recorded images were reviewed in a blinded manner for scoring of urethral mucosal discoloration and loss of integrity. Difficulty with cystoscope insertion was recorded if present. Excised prostate glands were fixed for serial sectioning, gross measurements, and calculation of percentage ablation. Percentages of prostate tissue necrosis from MWA, denuded urethral mucosa, and depth of epithelial surface loss in an adjacent section of the colon were estimated histologically. Variables of interest were statistically analyzed.
RESULTS
Difficulty with cystoscope insertion after treatment was significantly more common and scores for urethral mucosal discoloration and loss of integrity were significantly higher (indicating more severe lesions) for the MWA-NP group than for the MWA-UP group. The histologically assessed percentage of denuded urethral mucosa was also greater for the MWA-NP group. Overall median percentage prostate gland ablation was 73%; this result was not associated with prostate gland volume or chronological order of treatment.
CONCLUSIONS AND CLINICAL RELEVANCE
MWA-UP induced subtotal thermal necrosis of prostate glands in canine cadavers while limiting urethral mucosal injury. Further study is required to optimize the technique and evaluate its safety and efficacy in vivo as a future curative-intent treatment for prostatic tumors in dogs.
Introduction
Prostatic carcinoma is an uncommon but aggressive urogenital cancer in dogs with a locally invasive and highly metastatic nature.12 Limited curative-intent treatment options are available, and most canine patients die or are euthanized shortly after diagnosis because of the local extent of disease, advanced disease stage, and grave prognosis. Radical excision through total prostatectomy has been described.2–5 Although more positive outcomes have been reported for dogs that underwent this procedure in a recent retrospective study,5 case selection likely contributed to those results, and some findings, including the prevalence of postoperative urinary incontinence (8 of 23 [35%]) and the overall median survival time of 231 days, may suggest limited clinical application. Definitive radiation therapy options have been associated with high complication rates, including chronic colitis and cystitis, pelvic limb edema, and more life-threatening urethral, ureteral, or colonic stricture formation and colonic perforation.2,6,7 The advance of IMRT allows for the protection of radiosensitive organs such as the colon, urethra, and rectum, with median survival times of 563 to 654 days recently reported for dogs receiving this treatment for prostatic carcinoma.8,9 However, a moderate risk of toxicosis, incomplete tumor response, limited availability, and the cost of treatment may still restrict the application of IMRT to select cases.6–8
Thermal ablation is a widely used organ-sparing technique for the treatment of liver, kidney, lung, and bone tumors in human patients and can be performed through open surgery, minimally invasive surgery, or image-guided percutaneous methods.10 Several modalities of thermal ablation have been described, including RFA and MWA. Microwave ablation relies on the oscillation of an electromagnetic field that causes agitation of water molecules. This effect produces frictional heat, which is responsible for coagulative necrosis of the tissue.10,11 The use of MWA in veterinary medicine is nascent, and its clinical application has been described for the treatment of nonresectable hepatic tumors and metastatic pulmonary nodules.12–14
Microwave ablation may represent an attractive minimally invasive option for the treatment of prostatic carcinoma in dogs. Transrectal and transurethral MWA have been anecdotally reported for the treatment of localized prostatic carcinoma in people, mainly as a palliative procedure or before total prostatectomy.15–17 A few experimental studies18–21 have also evaluated focal MWA in prostatic tissue of dogs as a preliminary assessment of its potential for treatment of human disease. Lesion-targeted ablation methods have been used for treatment of a selected group of men with localized prostatic carcinoma.15 However, the disease is commonly diffuse in dogs, and complete prostate gland ablation is necessary for a curative-intent treatment.2 Limited trauma to the urethra develops as a result of localized prostatic carcinoma ablation in men.15,16 However, protection of delicate surrounding anatomic structures, including the urethra, is required during complete organ ablation. This is essential to avoid thermal damage that may lead to urethral stricture or perforation resulting in urinary obstruction or uroperitoneum, respectively.
Ureteral cooling by means of retrograde or antegrade pyeloperfusion has been described as a method that prevents thermal damage to the collecting ducts and ureter during MWA and RFA of small renal tumors in close proximity to the renal pelvis.22–29 One experimental study30 investigated the use of urethral and neurovascular cooling during ultrasound-guided RFA of the prostate glands of dogs in vivo. Microwave ablation carries numerous advantages, compared with RFA, including shorter ablation time and higher temperatures independent of tissue electric conductivity, the possibility of creating larger ablation zones, and a limited heat-sink effect secondary to perfusion.10,20 To our knowledge, no study investigating the use of a cooling perfusion system for protection of the urethra during MWA of the entire prostate gland in dogs has been reported. The purpose of the study reported here was to evaluate the use of ultrasound-guided MWA-UP versus MWA-NP for prostate gland ablation in canine cadavers as a first step in the evaluation of MWA-UP as a curative-intent treatment for future clinical applications in dogs with prostatic neoplasia. We hypothesized that MWA-UP would result in predictable and complete thermal ablation of prostatic tissue in canine cadavers while preventing substantial MWA-induced damage to the urethra and adjacent colon.
Materials and Methods
Animals
The cadavers of 18 medium-sized sexually intact male dogs that were euthanized at a local shelter for reasons unrelated to the study were used. Cadavers were collected immediately after euthanasia, and the assigned treatment was performed within 2 to 6 hours after euthanasia.
Experimental design
Two cadavers that were not used for other parts of the study were used to evaluate and improve the MWA technique (with and without cooling perfusion); the MWA treatments were performed on the prostate glands of both dogs, and the results were evaluated as described for the main study. The technique was deemed appropriate to start collecting data.
For the main study, 16 cadavers were assigned to 2 groups by order of collection without randomization (first to MWA-NP [n = 5] and then to MWA-UP [5], with the remainder distributed evenly between groups [3 each]) so that 8 cadavers underwent MWA-NP and 8 underwent MWA-UP of the prostate gland. Each cadaver was weighed, and prostate gland size was evaluated and recorded in 3-D (height × length × width) with ultrasonographya in the transverse and sagittal planes performed by 1 board-certified veterinary radiologist (GSS) using an 11-MHz microconvex transducer that was placed directly on the surface of the organ prior to the start of the assigned treatment.
MWA-NP
A caudal ventral abdominal incision was created, and MWA-NP was performed with the antenna inserted directly into the prostate gland after it was exposed by laparotomy. Ultrasonography was used to guide placement of the ablation antenna tip and to monitor changes during the treatment, with the covered ultrasound probe placed directly on the surface of the gland. One board-certified veterinary surgeon (MT) and 1 board-certified veterinary radiologist (GSS) performed the treatments with an MWA systemb and energy from a 100-W generator. Periprostatic fat was dissected away from the ventral aspect of the gland to facilitate prostate gland exposure, and a 15-cm MWA antennac was inserted sequentially into the center of each prostatic lobe or separately in the cranial and caudal aspects of each lobe to create 2 to 4 overlapping ablation regions. Briefly, the dimensions obtained by ultrasonography were used to determine generator ablation settings (power and time) required to obtain ablation zones of equal dimensions. The manufacturer-provided ablation charts for renal, hepatic, and pulmonary tissue that are used to calculate the ablation power and time needed to obtain given ablation zone dimensions in people were reviewed.31 Generator ablation settings required for each prostatic lobe or quadrant were extrapolated from the closest liver and kidney reference charts provided because no ablation chart for prostatic tissue was available. The technique was adjusted according to the size and shape of the organ so that 1 ablation zone was created in the left and right lobes with the antenna directed in a ventrodorsal-to-craniocaudal oblique orientation for smaller prostate glands (determined by subjective assessment on consensus of both investigators), and 2 ablation zones/lobe (1 each in the cranial and caudal aspects) were created with the antenna directed in a ventrodorsal orientation for larger prostate glands. A fiberoptic thermosensord was inserted immediately adjacent to the MWA antenna to record peak temperatures obtained during thermal ablation. The position of the antenna and the progression of thermal ablation were monitored with ultrasonography throughout the treatment and adjusted accordingly. The total ablation cycle was recorded for each prostate gland, and final ultrasonographic images were obtained after the treatment was complete. Treatment duration was measured for each ablation region, and the total was calculated for each gland from the start of ablation in the first zone to the end of ablation in the last zone.
MWA-UP
For MWA-UP, a 7F, 60-cm double-lumen vascular cathetere was inserted up to the level of the apex of the prostate gland; the use of a double-lumen catheter and its insertion location allowed for concomitant infusion and evacuation of saline (0.9% NaCl) solution through the prostatic portion of the urethra, preventing excessive inflation of the bladder. The vascular catheter was connected through an IV fluid line and 10-drop IV set to a 1-L bag of saline solutionf that had been refrigerated at 2°C to 4°C for 24 hours prior to the treatment and was suspended from a pole positioned 1 m above the level of the organ. The solution was kept cool by placement of the bag in an ice pack-filled container, and the temperature of the solution was monitored with a thermometer placed in direct contact with the fluid bag throughout MWA. Cooled saline solution was instilled by gravity-dependent open flow into the prostatic portion of the urethra ≥ 3 minutes prior to MWA and continuously throughout the treatment, which was otherwise performed as described for MWA-NP. A second, similarly prepared fluid bag was used when needed to ensure continuous perfusion. A total of 1 to 1.5 L of saline solution was delivered into and collected from the lower urinary tract. Total treatment duration for MWA-UP was calculated in the same manner as for MWA-NP.
Urethrocystoscopic examination
Normograde apical rigid urethrocystoscopy was performed before and after MWA-NP or MWA-UP by 2 board-certified veterinary internal medicine specialists (AK and SV) who were blinded to the treatment group. A 2.9-mm forward-oblique 30° rigid telescopeg was used. The urinary bladder was initially emptied and flushed with saline solution by use of a urethral catheter passed through the urethra in a retrograde manner to clear any debris present prior to cystoscopic evaluation. The saline solution was kept and used at room temperature, which was not recorded. A purse-string suture was placed at the apex of the bladder, and a stab incision was made in the center of the purse string to facilitate cystoscope insertion; the suture was used to create and maintain a seal to prevent leakage of fluid during the procedure. Saline solution was instilled into the urinary bladder for the examination as needed to facilitate visualization. Video recordings of the proximal portion of the urethra were obtained before and after the procedure and were reviewed later for grading of lesions. In addition, anatomic abnormalities, difficulty with cystoscope passage, and difficulty with completion of the examination (each scored as present vs absent) were recorded. Saline leakage from the prostatic portion of the urethra was indicated if observed during urethrocystoscopy. On completion of the procedure, diluted methylene blue solution was instilled into the prostatic portion of the urethra by retrograde catheterization to reassess for the presence of leakage.
Recorded video and still images were reviewed together by 2 board-certified veterinary internal medicine specialists (AK and SV) who were blinded to the treatment group. Urethral mucosa discoloration and loss of mucosal integrity were graded separately after a consensus score was obtained from the 2 observers according to a custom-made 6-point grading system adapted from scoring systems for intestinal32,33 and ureteral34 lesions (Appendix).
Gross and histologic examination of tissues
Prostate glands were harvested en bloc with a portion of the ventral colonic wall and placed in neutral-buffered 10% formaldehyde solution at an organ-to-solution (vol:vol) ratio of 1:10. Transverse slices of 5-mm thickness were created in a craniocaudal direction throughout the prostate gland and were digitally photographed (Figure 1).
Photograph of multiple cross sections of the prostate gland from 1 of 16 canine cadavers in a preliminary study to compare the use of MWA-UP and MWA-NP for thermal ablation of the prostate gland (n = 8 dogs/group). The prostate gland shown was collected from a cadaver in the MWA-UP group after the treatment was completed. The technique was adjusted according to size, so that smaller prostate glands had 1 ablation zone created in each lobe in an oblique orientation (shown) and larger prostate glands had 2 ablation zones created in each lobe (1 at the cranial aspect and 1 at the caudal aspect) in ventrodorsal orientation. The excised gland was fixed in neutral-buffered 10% formaldehyde solution prior to sectioning. The 2 thermal ablation zones (1 outlined in 1 section) are evident as well-delimited halos surrounding ablation antenna insertion artifacts (indicated with an arrow in another section).
Citation: American Journal of Veterinary Research 82, 5; 10.2460/ajvr.82.5.395
Ablated tissue was visually identified as a discolored zone with clear delimitation margins radiating around the probe insertion site on the transverse sections. Total prostate gland volume and volume of the ablated tissue were determined by measuring the respective cross-sectional areas in each section with an open-source image processing program35,36,h and multiplying the sum of all sections for each tissue type by the slice thickness (5 mm) as previously described.30 Percentage ablation of the entire prostate gland was calculated by dividing the volume of ablated tissue by the total volume and multiplying by 100.
Sections of large prostate glands were then halved, with caution taken to preserve the urethral section while the cut was performed in a periurethral zone. Slices were embedded in paraffin blocks, sectioned at 5- to 10-μm thickness, and stained with H&E stain. Histologic analysis was performed by 1 board-certified veterinary pathologist (KAS) who was blinded to the treatment group. The percentage of denuded and necrotic prostatic urethral mucosa around the circumference of the urethra was estimated for each section, with the median value for all slides recorded for each specimen, and a maximum percentage depth of colonic epithelial surface loss was evaluated on the colonic section in closest contact with the prostate gland with a standard light microscope.i Qualitative assessment of the prostatic parenchyma was performed by use of light microscopy with particular focus on estimation of the percentage tissue necrosis resulting from MWA.
Statistical analysis
Owing to the novel nature of the study, no preliminary data were available to define the appropriate sample size required to detect a meaningful difference in urethral lesions between groups. Recording preliminary data for power analysis was not practical because of the lack of availability of the MWA system and appropriate canine cadavers. Therefore, we estimated that a minimum of 5 cadavers/treatment group would be sufficient on the basis of a previous study design30 for an α level of 0.05 and statistical power (1 - β) of 80%.
Statistical analysis was performed with the aid of commercial statistical software.j Intergroup (WMA-NP vs MWA-UP) homogeneity for cadaver prostate gland volume and body weight was assessed with the Welch t test or the Wilcoxon rank sum test, respectively, with the decision for testing made on the basis of visual inspection of histograms for violation of assumptions such as strong asymmetry or excessive outliers and skew. Homogeneity of prostate gland dimensions was assessed by fitting a logistic regression model with treatment group as the response and dimensions as the predictors and comparing it with the intercept-only model. A Wilcoxon rank sum test was used to compare urethral discoloration and loss of integrity scores on urethrocystoscopy between the 2 treatment groups, and Pearson correlation analysis was used to assess the association of necrosis with volume ablated and urethral denuding percentage in the entire study population. Linear models were used to analyze the association between the percentage of prostate gland ablation and the variables of total power and total prostate gland volume in the entire study population. Potential impacts of the learning curve on treatment duration and percentage ablation were assessed by fitting a linear model with the chronological order of treatment for each gland and a binary variable for the first 8 versus the last 8 prostate glands treated. Differences in treatment duration between the MWA-NP and MWA-UP groups were assessed with the Welch t test; differences in treatment variables were reflected as a summation of power × time of each ablation zone and compared between treatment groups with the Welch t test. The Fisher exact test was used to assess independence between difficulty of passing the cystoscope through the urethra and the treatment group, with an alternative hypothesis that the cooling urethral perfusion would result in a lower frequency of difficulty with this procedure, compared with that when perfusion was not performed.
Results
Animals
Body weight did not differ significantly (P = 0.279) between groups; the median result for all cadavers was 25.2 kg (range, 10.7 to 34.5 kg). Prostate gland dimensions and volume also did not differ significantly (P = 0.832 and P = 0.125, respectively) between groups. Median dimensions for a single prostate gland lobe were 27.0 mm (range, 16.4 to 37.9 mm) high × 36.1 mm (range, 18.7 to 55.4 mm) long × 21.0 mm (range, 11.4 to 26.9 mm) wide and 22.6 mm (range, 10.6 to 26.2 mm) high × 30.1 mm (range, 17.9 to 43.3 mm) long × 17.4 mm (range, 7.2 to 20.7 mm) wide for the MWA-NP and MWA-UP groups, respectively. Median total prostate gland volume including both lobes was 23.4 cm3 (range, 4.7 to 54.8 cm3) and 13.3 cm3 (range, 3.0 to 22.8 cm3) for the MWA-NP and MWA-UP groups, respectively. No anatomic abnormalities were noted on gross examination.
Technical feasibility and percentage ablation of the prostate gland
The median maximum temperature during MWA by either method was 100°C (range, 100°C to 100°C) for all ablation zones. Subjectively, ultrasound guidance allowed for precise placement of the antenna and active monitoring of ablation (Figure 2). Median treatment duration was 5.5 minutes (range, 2 to 12 minutes). Treatment duration was significantly associated with prostate gland volume (P = 0.001) and chronological treatment category (P = 0.032), with greater prostate gland size and earlier experiments requiring greater treatment duration. Treatment variables varied according to the desired dimensions of each ablation zone, with a power range of 45 to 100 W/ablation zone for an ablation time of 60 to 300 seconds. The combination of settings extrapolated from the manufacturer's chart for liver ablation subjectively corresponded to the ablation zones obtained in prostatic tissues; however, small adaptations in power and time settings and angulation of the antenna were made over time according to ultrasonographic observations and the experience gained with earlier experiments; these changes were made at the surgeon's discretion. There was no significant difference in total treatment duration (P = 0.626) or total ablation power × time (P = 0.285) between treatment groups.
Long-axis ultrasonographic images of the lateral aspect of a prostate gland lobe in a canine cadaver of the MWA-UP group in dorsal recumbency (with the head to the left of the images) during and after treatment. A—Ultrasound-guided MWA antenna placement. B—Evidence of thermal ablation in the prostatic tissue after MWA. Thermal artifact characterized by elliptic echogenic foci and air bubbles were observed in a progressive and centrifugal pattern surrounding the MWA antenna throughout the ablation treatment. Notice that the dorsal aspect of the prostatic wall cannot be visualized after the treatment. Numbers on the left in both images represent distance in cm.
Citation: American Journal of Veterinary Research 82, 5; 10.2460/ajvr.82.5.395
The median total percentage prostate gland ablation was 73% (range, 55% to 99%), with median values of 77% (range, 67% to 99%) and 67% (range, 55% to 87%) for the MWA-NP and MWA-UP groups, respectively. No association was found between percentage ablation and prostate gland volume (P = 0.994), chronological treatment category (first 8 vs last 8 treatments, used to assess learning curve effects; P = 0.978), or total ablation power × time (P = 0.942). Histologic evaluation of prostatic tissue necrosis resulting from MWA was complicated by the presence of autolysis. The overall median percentage necrosis was estimated at 30% (43% for MWA-NP and 29% for MWA-UP), and there was no significant (P = 0.431) correlation between the percentage necrosis and total percentage ablation calculated from macroscopic measurements (r = −0.212).
Assessment of MWA-induced damage to other tissues
During pretreatment evaluations, no difficulty was noted during normograde passage of the cystoscope into the proximal portion of the urethra, and no lesions were detected in cadavers of either treatment group (ie, all had a score of 0). After treatment, urethral narrowing characterized by difficulty with cystoscope passage was present in 8 of 16 cadavers (6/8 in the MWA-NP group and 2/8 in the MWA-UP group; P = 0.032). The median mucosal discoloration grade was 1 (range, 0 to 4) and 0 (range, 0 to 0; P = 0.035) and median decreased mucosal integrity grade was 4 (range, 1 to 5) and 0 (range, 0 to 1; P = 0.002) for the MWA-NP and MWA-UP groups, respectively (Figure 3). No leakage from the prostatic portion of the urethra was detected in either group during urethrocystoscopy or instillation of methylene blue solution after the procedure.
Urethrocystoscopic images of the prostatic portion of the urethra in a canine cadaver of the MWA-NP group before (A) and after (B) treatment. In panel B, areas of coalescing as well as pinpoint discoloration (arrow) and loss of integrity (asterisk) of the urethral mucosa are evident (both graded 4 on a scale of 0 [no evidence of abnormality] to 5 [severe, circumferential abnormality affecting 75% to 100% of the mucosal surface]).
Citation: American Journal of Veterinary Research 82, 5; 10.2460/ajvr.82.5.395
On histologic examination, denuded urethral mucosa was found for a median of 98% and 72% of the circumference for the MWA-NP and MWA-UP groups, respectively (P = 0.019), with differentiation observed between thermal and autolytic necrosis (Figure 4). Colonic lesions noted on histologic examination were localized within the epithelium and showed centrifugal expansion from the colonic lumen; these locations did not appear to be associated with areas of highest exposure to MWA and were considered to likely represent autolytic changes. The median percentage colonic mucosal loss was estimated at 50% (range, 0% to 75%) overall, with no significant (P = 0.938) difference between the MWA-NP (median, 25%; range, 0% to 50%) and MWA-UP (median, 50% range, 0% to 75%) groups.
Photomicrograph of a histologic section of the prostate gland from a canine cadaver of the MWA-NP group after the treatment was performed. Thermal necrosis of the urethral epithelium (asterisk) is distinguishable from autolysis (dagger) in this image by evidence of loss of the basophilic staining of the nuclei. Epithelium not affected by microwave energy has well-stained nuclei, suggesting autolytic disaggregation rather than thermal necrosis. H&E stain; bar = 200 μm.
Citation: American Journal of Veterinary Research 82, 5; 10.2460/ajvr.82.5.395
Discussion
In the present study, MWA-UP induced macroscopic subtotal thermal necrosis (ablation) in prostate glands ranging in size from 3.0 to 54.8 cm3 in canine cadavers while limiting urethral mucosal injury. Furthermore, we found that ultrasound guidance aided placement of the MWA antenna and enabled visualization of thermal damage to prostatic tissue to guide the progression of MWA treatment. Although in vivo investigation is required to confirm its benefit, our preliminary results supported that MWA-UP may have utility as a treatment for neoplasia of the prostate gland in dogs.
Although both MWA procedures were performed with the prostate gland exposed by laparotomy in this preliminary study, the opportunity of delivering a curative-intent minimally invasive or interventional treatment is appealing, compared with the invasive and radical nature of a total prostatectomy. Other strategies have been explored to decrease undesirable effects associated with prostatectomy in dogs, with the goal of relieving signs of obstruction while preserving urinary continence. Subcapsular partial prostatectomy by means of laser dissection,37 ultrasonic aspiration,38,39 transurethral electrocoagulation,40 high-intensity focused ultrasound,41 and adjunct photodynamic treatment42,43 have been reported, with variable success rates. However, prostatic excision remained subtotal and incomplete; therefore, those treatments are considered strictly palliative, and residual disease may warrant adjuvant definitive treatment. The advance of IMRT allows for the protection of radiosensitive organs such as the colon, urethra, and rectum, which have historically limited the total dose of radiation delivered and its efficacy for treatment of prostatic carcinoma in dogs.8,9 However, 2 recent retrospective studies8,9 found grade 1 or grade 2 acute adverse radiation effects in 9 of 18 and 10 of 21 canine patients with prostatic and (general) urogenital carcinomas, respectively, that were treated with total radiation doses ranging from 48 to 58 Gy. These effects were most commonly characterized by colitis, cystitis, and local dermatitis.8,9 More severe late adverse radiation effects were reported in 3 of 18 and 4 of 21 treated dogs, respectively, including development of hind limb edema and formation of urethral, ureteral, or colonic strictures that required surgical correction or stenting.8,9 Local progression was also noted in up to 7 of 18 dogs at a median interval of 241 days after IRMT in one study8 and in 7 of 21 dogs in the other study.9 The severity of some late adverse effects as well as the absence of complete response to treatment despite prolonged survival times warrant investigation of novel curative-intent treatment methods.
The utility of MWA for ablation of prostatic tissue in canine patients with prostatic carcinomas would rely on the absence of trauma to the prostatic portion of the urethra and surrounding organs. Although our study findings were limited by the use of cadavers for method evaluation, we demonstrated a clear benefit of the use of cooling urethral perfusion and confirmed that it has a protective effect on the urethral mucosa during MWA of the prostate gland, compared with MWA-NP. It is important to note that temperature checks of the prostatic tissue were performed throughout the entire ablation period with a fiberoptic thermosensor placed near the ablation antenna for both MWA procedure types and with a standard thermometer placed in contact with the ice-cooled bag of saline solution for MWA-UP. However, no temperature measurement was performed within the urethra, and this could have allowed confirmation of cooler temperatures at that level. We considered that the short saline circulation time within the urethral lumen would limit temperature fluctuations by conduction. Furthermore, obtaining exact temperature measurements within the urethral lining in direct contact with cooling saline without inducing any trauma to the tissue might have been challenging. Unfortunately, the study design did not include assessment of the urothelium for hemorrhage, edema, and inflammation during cystoscopy. It is also important to note that some degree of urethral narrowing, characterized subjectively by difficult passage of the cystoscope, and a high percentage of denuded urethral mucosa on histologic examination were found after MWA by either method, although urethral narrowing was significantly more frequent and the percentage of denuded urethral mucosa was significantly greater in cadavers that underwent MWA-NP; it is unknown whether the degree of mucosal changes observed histologically in the MWA-UP group would correspond to clinical problems. Our results corroborated those of an experimental study30 performed to evaluate the use of urethral cooling during in vivo ultrasound-guided RFA of the prostate glands of dogs. Circumferential urethral inflammation and necrosis, if present in vivo, could lead to complete urethral stricture and therefore substantially impact clinical recovery. A relatively straightforward and minimally invasive measure that could potentially palliate this risk would be to leave a urinary catheter in place for 3 to 5 days after MWA-UP, allowing the urethral mucosa to heal while preventing the development or worsening of luminal narrowing and stricture formation. However, evaluation of these procedures in vivo is required to assess the safety of MWA-UP in dogs prior to its clinical use. An in vivo study can also provide reliable information regarding the effects on surrounding organs, including the colon and rectum, which could not be adequately assessed in our study owing to the presence of advanced autolysis of these tissues in the cadavers.
In vivo evaluation is also needed to assess the functional, macroscopic, and microscopic integrity of the neurovascular bundles located 5 to 10 mm dorsolateral to the prostate gland.44 The integrity of the hypogastric nerve is particularly essential to the maintenance of urethral sphincter function and urinary continence.37 A recent literature review15 found that urinary incontinence and erectile dysfunction have been reported to affect quality of life in up to 48% and 50% of men, respectively, after various types of focal ablation therapy. In the aforementioned study30 of RFA of the prostate gland in dogs, protection of the associated neurovascular bundles was attempted by catheterization of the iliac arteries for infusion of cold saline solution in a small subset of animals; however, intra-arterial cooling did not result in a significant improvement in neurologic tissue preservation. Palliative subcapsular partial prostatectomy performed with laser dissection,37 ultrasonic aspiration,38,39 or transurethral electrocoagulation40 has been shown to allow neurovascular preservation and maintenance of urinary continence in dogs.
Other potential complications of MWA include hemorrhage and abscess formation due to the presence of a large amount of necrotic tissue. These complications have been reported in < 2% to < 3% of human patients treated with percutaneous MWA for hepatic, renal, or pulmonary lesions.45–47 Prophylactic antimicrobial administration may be used to mitigate the risk of infection, although this is considered low. Tumor seeding of thoracoabdominal wall needle tracks has also been reported in a small proportion of people affected by hepatocellular carcinoma and treated by percutaneous MWA or RFA.48 This late complication is significantly associated with presurgical percutaneous fine-needle aspiration or biopsy and has been successfully treated by local resection, MWA, high-intensity focused ultrasound, or radiation therapy.48 Repositioning the ablation antenna without applying heat treatment has also been proposed as a cause of tumor seeding, as viable neoplastic cells may adhere to the electrode during retraction.48 Prophylactic measures such as applying heat when repositioning or withdrawing the ablation needle may prevent this long-term complication.
As previously mentioned, the eventual success of MWA as a curative-intent treatment option for prostatic carcinoma in dogs would depend on the ability to achieve thermal ablation of the entire prostate gland. In the study reported here, calculations based on macroscopic posttreatment measurements in all dogs indicated that the ablated tissue comprised 55% to 99% of the total prostate gland volume despite creation of 2 to 4 overlapping ablation regions in an effort to adjust the technique to the size of each gland. Owing to these and other small adjustments, ablation time and power varied according to prostate gland volume and shape and MWA antenna placement without standardization among cadavers. Although the procedures were technically demanding, the chronological treatment category selected as a measure of the learning curve for MWA was not significantly associated with the percentage prostate gland ablation. Understandably, this study reports pioneering efforts in the development of a new treatment method for prostatic carcinoma in dogs, and further technical adjustments will be required before it can be recommended for clinical use. Ensuring adequate correspondence between predetermined ablation zone dimensions, pretreatment image-guided observations, and posttreatment measurements of ablated tissue is essential to develop a repeatable and reliable MWA technique for total thermal ablation of the gland. Although real-time monitoring of tissue ablation was enabled by the use of ultrasound, the presence of gas artifact prevented complete visualization of the dorsal aspect of the gland. Investigators of the previously described experimental study30 performed RFA of the prostate gland in dogs in vivo with contrast-enhanced pulse-inversion harmonic ultrasound guidance and obtained mean ablation volumes ranging from 91.9% to 96.6%. The extent of ablation achieved may also differ in vivo, compared with results found in cadavers, and in abnormal versus healthy tissue.20,49 The utility of hyperthermia in antineoplastic treatment has been demonstrated in many ways, and its results are influenced by locoregional blood supply.17 It is thought that neoplastic cells are intrinsically more sensitive to thermal treatment than normal cells as a result of poorer locoregional vascularization, which limits heat dissipation. Variability in locoregional perfusion among or within tumors may therefore influence the response to MWA. Thermal treatment is also known to potentiate the effects of radiation therapy.17 Thus, MWA may have a role in improving local tumor control obtained after IMRT, or it may be used to decrease the total radiation dose needed and thereby help to limit adverse effects. Finally, the possibility must be considered that attenuation of thermal microwave energy in the periurethral zone by intraluminal cooling may impede local tumor control at the level of that interface, as neoplastic cells classically extend in close association with the urothelium in dogs with prostatic carcinoma.2 A terminal in vivo study incorporating histologic analysis after MWA treatment would provide additional information regarding the microscopic transition to necrotic tissue at the periurethral interface. To our knowledge, no previous study has evaluated this specific outcome. However, investigation in dogs with prostate gland tumors will ultimately be needed to truly characterize the impact of MWA-UP on neoplastic cells intimately associated with the prostatic urothelium.
The largest limitation of our study was the use of canine cadavers, which was elected for ethical reasons in this preliminary investigation. Despite the fact that the treatments were performed within a few hours after euthanasia, it was difficult to differentiate autolysis from thermal necrosis on histologic sections. As expected, mucosal layers were particularly affected by autolytic changes. True coagulative necrosis was also not directly observed in parenchymal sections that were found to be affected by MWA on gross examination. High temperatures reached during MWA were thought to contribute to the fixation and preservation of prostatic tissue in another study,50 and we suspected that areas not directly exposed to microwave energy may have continued to autolyze and subsequently lack staining detail (autolytic necrosis), whereas areas affected by MWA remained more microscopically intact. Those changes were only occasionally observed on microscopic analysis and did not apparently correspond to the location of ablation antenna insertion. Changes observed macroscopically, however, were obvious and corresponded anatomically to alterations expected to result from thermal treatment as the visible ablation zone radiated in a centrifugal pattern surrounding the site of ablation antenna placement. The low correlation index between the percentage of tissue with thermal necrosis estimated on histologic examination and percentage of ablated tissue calculated from measurements on macroscopic specimens supported the low reliability of histologic assessment for this variable in tissue from cadavers.
The use of urethrocystoscopy before and after MWA treatment for each cadaver allowed for gross assessment of posttreatment changes to mucosal integrity that resulted from thermal energy delivery and confirmed significant differences in this outcome measure between the MWA-NP and MWA-UP groups. We believe that the cystoscopy grading scheme created for the present study was a reliable indicator of the benefit of using cooling urethral perfusion for the preservation of urethral epithelium. Future in vivo study is required to assess the full extent of thermal coagulative necrosis of prostate gland parenchyma and confirm the absence of thermal lesions in the surrounding tissues suggested by our findings. These results provide a foundation for future studies to evaluate the in vivo efficacy and safety of MWA-UP for prostate gland ablation in dogs.
Acknowledgments
The MWA system used in the study was loaned and the antennas were donated by Medtronic. The company had no involvement in the study design, data analysis and interpretation, or writing and publication of the manuscript.
The authors declare that there was no conflict of interest.
The authors thank James B. Robertson for performing the statistical analysis, J. Ashley Ezzell and Kara A. Clissold for processing the histologic samples, Jonathan Hash for technical assistance, and Patty Secoura and Jocelyn Burkit for assistance with urethrocystoscopy and ultrasonography.
Abbreviations
| IMRT | Intensity-modulated radiation therapy |
| MWA | Microwave ablation |
| MWA-NP | Microwave ablation with no perfusion |
| MWA-UP | Microwave ablation with urethral perfusion |
| RFA | Radiofrequency ablation |
Footnotes
Aplio 500, Canon Medical Systems USA Inc, Tustin, Calif.
Covidien Emprint ablation system with thermosphere technology, Medtronic, Boulder, Colo.
Covidien Emprint SX short navigation antenna with thermosphere technology, Medtronic, Boulder, Colo.
Covidien RF ablation remote temperature probe (E series, 20 cm), Medtronic, Boulder, Colo.
Small animal long-term venous catheterization set (24 in), Mila International Inc, Florence, Ky.
Hospira, Lake Forest, Ill.
Hopkins forward-oblique telescope, Karl Storz Endoscopy-America Inc, El Segundo, Calif.
ImageJ, version 1.52, NIH, Bethesda, Md.
Leica DM 3000, Buffalo Grove, Ill.
R, version 3.6.2, R Foundation for Statistical Computing, Vienna, Austria. Available at: www.R-project.org. Accessed Jan 29, 2020.
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Appendix
Subjective grading system used for assessment of mucosal discoloration and loss of mucosal integrity on urethrocystoscopic examination of the prostatic portion of the urethra in a study to investigate the use of MWA-NP and MWA-UP for prostate gland ablation in canine cadavers.
| Grade | Definition |
|---|---|
| 0 | No evidence of abnormality |
| 1 | Mild; pinpoint abnormalities; 1%–15% of mucosal surface affected |
| 2 | Focal; 16%–30% of mucosal surface affected |
| 3 | Moderate; patchy abnormalities; 31%–50% of mucosal surface affected |
| 4 | Coalescing; 51%–75% of mucosal surface affected |
| 5 | Severe; circumferential abnormalities; 76%–100% of mucosal surface affected |
Images (video recordings) were obtained before and after each MWA treatment and reviewed by 2 observers who were blinded to the treatment group. Mucosal discoloration and mucosal integrity were each graded separately, and a total organ score for each feature was assigned by consensus between the 2 observers.
