Introduction

Tumor of the bladder and urethra accounts for 0.5% to 1% of all canine neoplasia.¹ Of all the canine bladder neoplasia, 89% has urothelial origin, whereas the remaining 11% is mesenchymal. Smooth muscle neoplasia (SMN) can be divided into benign leiomyoma and malignant leiomyosarcoma based on histological findings. Canine urinary bladder leiomyosarcoma is rare and only represents 2% of the primary canine urinary bladder tumors.¹ Articles describing the imaging findings of SMN in canine urinary bladder are scarce.^2–11 To the authors’ knowledge, this is the first report with detailed documentation of imaging findings of different modalities.

Case Report

A 9 yr old intact male miniature poodle (3.4 kg, body condition score 4–5/9) was initially presented with acute vomiting and diarrhea not responsive to symptomatic treatment. Abdominal radiography and ultrasonography discovered a large peritoneal mass. The dog was referred for computed tomography (CT) to investigate the origin of the mass and for treatment planning. On physical examination, the dog was quiet, alert, and responsive with normal hydration status. The body temperature, pulse rate, and respiration rate were within normal limits. The pulses were strong and synchronous, and auscultation of the chest was unremarkable. A firm mass was palpated in the caudal abdomen. Complete blood count revealed slightly reduced hemoglobin of 12.7 g/dL (reference: 13.1–20.5 g/dL) and reticulocyte-hemoglobin of 20.7 pg (reference: 22.3–29.6 pg). Blood chemistry tests (glucose, creatinine, blood urea nitrogen, phosphorus, calcium, total protein, albumin, alanine aminotransferase, alkaline phosphatase, gamma-glutamyl transferase, total bilirubin, and electrolytes) were within reference range.

Imaging Findings, Diagnosis, and Outcome

Owner consent was obtained, and the contemporary standard of care was provided to the patient. Three-view radiography^a,b of the abdomen showed a gigantic elliptical soft tissue mass (11.9 × 6.6 cm) in the ventral two-thirds of the abdomen, extending from the level of T12 to L7. The cranial aspect of the mass was caudal to the liver and stomach and its caudal margin superimposed with the urinary bladder. The dorsal margin was not well visualized due to superimposition with the small intestines and colon. The mass displaced the colon dorsally and most of the small intestines dorsally and laterally (Figure 1A–C). In the caudoventral abdomen, a triangular fat region was composed by the cranioventral margins of the prostate gland, caudoventral margins of the urinary bladder and abdominal wall, indicative of prostatomegaly (Figure 1A). Three-view thoracic radiographs were unremarkable.

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Ultrasound^c revealed an overall smooth and well-defined margin of the mass with the caudal aspect intimately associated/adjacent to the cranioventral aspect of the bladder (Figure 1D). The mass exhibited mixed echogenicity, had no cavitation (Figure 1E), and lacked blood flow upon color Doppler interrogation. Origination of the mass was still uncertain. The bladder wall was smooth and normal in thickness (not highlighted by the image).

Pre- and postcontrast CT^d,e,f of the thorax and abdomen (1-mm slice thickness, 120 kVp, 121 mAs [thorax], 150 mAs [abdomen], 512 × 512 matrix, and 0.75 s/rotation) was performed. A large, mildly heterogeneous, soft-tissue attenuating, mildly and homogeneously contrast enhancing mass (11 × 6.2 × 5.5 cm) was present mostly in the right side of the peritoneal cavity (Figure 2A–C). The mass had a well-defined and smooth margin. The mass intimately associated with the right cranioventral aspect of the urinary bladder and mildly compressed and displaced the bladder caudodorsally. There was a concave appearance of the urinary bladder (Figure 2B). The caudal margin of the mass was in confluence with the urinary bladder wall. The exact origin of the mass was unknown. It might be a large urinary bladder mass such as SMN extending into the peritoneal cavity or a peritoneal neoplasia such as hemangiosarcoma. The prostate gland was moderately enlarged and heterogeneously contrast-enhancing compatible with a benign prostatic hyperplasia. Other findings, probably incidental, included chronic extrusion of mineralized discs at T13-L1 and L2-L3. The remainder of the thorax and abdomen were unremarkable.

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The dog underwent exploratory laparotomy 2 wk immediately following the CT scan. The firm mass was confirmed to originate and extend from the cranial aspect of the bladder (Figure 2D). The mass and the attached cranial aspect of the bladder were removed (Figure 2E). The final diagnosis was leiomyosarcoma of the urinary bladder with a mitotic figure of 2 per high-power field. Postsurgical mild hematuria was reported for <2 wk, and the dog has been doing well without any clinical signs 8 mo after surgery.

Discussion

In the last two decades, there have been few reports of histologically confirmed SMN (nine leiomyoma, five leiomyosarcoma) of canine urinary bladder with diagnostic imaging descriptions.^2–9,11 These cases were compiled and summarized in Supplementary Table 1. SMN of the urinary bladder has been reported in various breeds with most dogs being male (7/10). Median age at the time of diagnosis was 10 yr (range: 4–18 yr). The dog presented here was also male and was diagnosed with leiomyosarcoma of the urinary bladder at 8 yr old, consistent with previous reports.

Of the 13 cases in which the location of the mass was reported,¹⁰ 6 had an extraluminal mass that was largely ovoid with regular or irregular margins similar to the findings presented in this case.^2,6,8,11 The remaining seven dogs had a broad base intraluminal mass that was rounded with a well-defined smooth margin.^3–5,7,9 Due to the firm and solitary nature of SMN, both intraluminal and extraluminal masses might be palpated when they are large.^2,3,9,10 It was initially speculated that the small mass originates intraluminally and then extends extraluminal. However, the finding of a small extraluminal SMN by Ferrante et al.⁶ in 2017 suggests the possibility of growth extraluminally.

Survey radiography detected a large extraluminal SMN as an ovoid soft-tissue mass in the mid and caudal abdomen. Intraluminal SMN and small extraluminal SMN were not seen on survey radiographs. Ultrasonography, radiographic contrast studies, or CT were necessary.^3–7,9 Ultrasound showed intraluminal SMN usually located at the more cranial aspect of the bladder (5/7)^4,5,9 or at the dorsal aspect of the bladder (2/7).^3,7 The degree of bladder wall involvement was not documented in these reports possibly due to poor sonographic detail and edge shadowing artifact at these areas.⁹ In contrast, ultrasonography only identified the origin of a relatively small extraluminal SMN in one dog⁶ but not the large extraluminal SMN causing marked abdominal distension in the other two dogs.^2,8 Similarly, the extremely large size of the mass in our dog precluded confident identification of its origin from the bladder on ultrasound. The exact origin of all the reported large extraluminal masses was confirmed in surgery.

Regardless of its intra- or extraluminal growth, the SMN of the urinary bladder were described to be hypoechoic or of mixed echogenicity (Supplementary Table 1). Only one (1/7) intraluminal SMN, a leiomyosarcoma, showed central cavitation on ultrasound examination. The mass in our dog was heterogeneous and had no cavitation. Vascularity was assessed in six (6/7) intraluminal SMN using color Doppler and vascular flow was detected in only two cases.^5,7 Vascularity assessment of extraluminal SMN has not been reported in previous cases. In our dog, the large extraluminal SMN showed no detectable blood flow with color Doppler.

CT with contrast enhancement in dogs with intraluminal SMN (3/7) showed moderate to marked heterogeneous contrast enhancement in two leiomyosarcomas and postcontrast hyperattenuation in one leiomyoma.^3–5 Four (4/7) dogs with extraluminal SMN underwent pre- and postcontrast CT.¹¹ It is postulated that extraluminal leiomyoma (3/4) had round and smooth margins and showed homogeneous attenuation and contrast enhancement, whereas the one extraluminal leiomyosarcoma had irregular margins and heterogeneous contrast uptake.¹¹ In comparison, the extraluminal leiomyosarcoma in our dog had a slightly irregular contour and mild inhomogeneous contrast enhancement which, at first inspection, might resemble the reported features of an extraluminal leiomyoma.

At the time of diagnosis, the most common clinical signs were urinary incontinence and hematuria in dogs with intraluminal SMN. Clinical signs including abdominal distension and hematuria were documented in only two (2/7) dogs with extraluminal SMN.^2,8 One dog with a large mass in the right cranial aspect of the bladder resulted in abdominal distension, and the other dog with a large mass in the trigone area showed abdominal distension, hematuria, ureteral obstruction, and hydronephrosis.^2,8 Our dog had a similar clinical picture to the first dog in which the mass grew eccentrically and cranially from the cranial aspect of the bladder and only lead to abdominal distension and possibly gastrointestinal discomfort due to mass effect. Extraluminal SMN not involving the trigone area appears to be well tolerated in the dogs without causing urinary discomfort.

All 13 dogs (including this case) with a premortem diagnosis of urinary bladder SMN underwent cystectomy. Complete removal of the mass without ureteral reimplantation was attainable in 11 dogs. Similar to the nine dogs with longer-term follow-up (ranging from 2 to 29 mo),^4,5,9,10 our dog was free of clinical signs and showed no evidence of recurrence or metastasis at 8 mo follow-up.

In humans, SMN of the urinary bladder is also relatively rare with leiomyoma accounting for 0.43% of bladder tumors, and the incidence of leiomyosarcoma is reported to be 0.23 cases per million.^12,13 At the moment there is no reported sex predilection of SMN of the urinary bladder in dogs, but leiomyoma has been shown to affect women more than men.^14,15 Urinary bladder SMN in cats is rare, and the ultrasonographic descriptions of an intraluminal mass that is poorly echogenic with mild hyperechoic striations or speckles are reported in two cats, one with leiomyosarcoma and the other with leiomyoma.^16,17 In humans, dogs, and cats, SMN of the urinary bladder tends to be a single mass, although multiple leiomyoma in the urinary bladder has been previously reported in a woman.¹⁸ Histopathologically, the bladder SMN arises from the submucosa with only leiomyosarcoma infiltrating into the muscularis propria.^4,19 In people, leiomyomas of the urinary bladder are categorized into endovesicle (pedunculated or polyloid), intramural (encapsulated by bladder wall muscles), and extravesicle based on its location.²⁰ However, these categories have not been consistently used in the veterinary literature. In dogs, SMN of the urinary bladder irrespective of leiomyoma or leiomyosarcoma are frequently described as either intraluminal, extraluminal, or sometimes a combination of both, based on the imaging or surgical findings at the time of diagnosis. In contrast to humans, in which intraluminal and/or endovesicle mass is the most common type of bladder SMN accounting for up to 86% of the cases,¹⁴ seven (7/14) cases in the literature include descriptions resembling an endovesicle SMN in the urinary bladder in dogs.^4,7 On ultrasound, leiomyomas of the urinary bladder in both humans and dogs tend to have a rather smooth and round contour. Leiomyomas in humans are most often homogeneous, whereas those in dogs may be either homogeneous or heterogeneous.^6–8,21 On CT, bladder leiomyosarcomas have been reported to be more heterogeneously contrast-enhancing in people and dogs,^4,11,22 in contrast to the very mildly, heterogeneously contrast enhancement noted in our dog.

Conclusion

We presented a rare case of canine leiomyosarcoma of the urinary bladder that had been growing without causing lower urinary tract signs due to its extraluminal growth. The slightly irregular shape, heterogeneity on ultrasound, and nearly absence of signals on Doppler examination were consistent with previous reports of canine leiomyosarcoma of the bladder. On CT, we reported that canine bladder leiomyosarcoma could resemble the reported features of leiomyoma mildly heterogeneous precontrast and postcontrast and with minimal contrast uptake. The characteristics of different imaging modalities could help include SMN as a differential diagnosis.