Bladder tumors are usually malignant; according to a previous study, only 3% of bladder tumors in dogs were benign (2).Epithelial tumors, such as invasive transitional cell carcinoma (TCC) and squamous cell carcinoma, are the most frequently recognized malignant neoplasms of the canine urinary bladder (4,13,21). However, mesenchymal tumors such as leiomyoma and leiomyosarcoma are rare, and leiomyoma is rarer than leiomyosarcoma (13).

In humans, leiomyoma is the most common benign bladder neoplasm; however, it is rare and accounts for < 0.5% of all bladder tumors (11). Based on the location of the tumor, bladder leiomyoma can be classified as endovesical, extravesical, or intramural. The endovesical location is the most common, followed by the extravesical and intramural locations (5,7,10). Cystoscopy, ultrasonography, computed tomography (CT), or magnetic resonance imaging (MRI) can be used for diagnosis; however, the definitive diagnosis is made based on histopathological findings.

In veterinary medicine, knowledge regarding the characteristics of mesenchymal tumors, such as leiomyoma and leiomyosarcoma, is limited. Both leiomyoma and leiomyosarcoma present as well-defined, single, rounded intraluminal masses with smooth contours, serosal extensions (19), and mixed echogenicity on ultrasonography (6). Only a few studies on the CT characteristics of intramural leiomyosarcoma and extramural leiomyoma in dogs have been reported (1,3,12,16).

Calcified bladder masses are commonly observed in epithelial tumors such as carcinoma in both dogs and humans. In humans, mesenchymal masses accompanied by calcification are rare, and few cases of calcified leiomyosarcoma have been reported (8,17). However, leiomyoma or leiomyosarcoma with calcification has not been reported in dogs. Herein, we report a rare case of calcified intramural leiomyoma of the bladder in a dog diagnosed using ultrasonography and CT.

A 16-year-old neutered male Shih Tzu dog was referred to the Western Referral Animal Medical Center with a chief complaint of corneal perforation. No clinical symptoms related to the urinary system were reported, and no major abnormalities were identified on hematological examination (complete blood count, serum chemistry, and electrolytes). Thoracic radiography and echocardiography revealed no remarkable findings.

Abdominal ultrasonography performed as part of the preoperative screening revealed a 6.3 × 8.8-mm, solitary, rounded homogenous mass on the ventral aspect of the bladder neck (Fig. 1A) on day 0. The mass was separate from the ureteral opening, and no evidence of ureteral dilation was observed. The mass originated from the muscular layer and protruded outward from the wall without an associated stalk, suggestive of an intramural mass. No distinct blood flow was observed within the mass, including in the hyperechoic foci in the center of the mass (Fig. 1B). Numerous small calculi were identified in both kidneys and within the urinary bladder. The urinary bladder wall showed irregular changes indicative of cystitis near the apex.

Figure 1. Abdominal ultrasonographic images showing the bladder leiomyoma on day 0. (A, B) A round, hypoechoic intramural mass is observed at the ventral aspect of the bladder neck. A hyperechoic area with shadowing is observed in the center of the mass revealing shadowing (yellow arrows). The hypoechoic muscular layer of the bladder wall is continuous with the bladder mass (white arrow). (B) No blood flow is observed within the mass.

CT performed to evaluate the origin and precise location of the mass revealed a homogenously attenuated rounded mass excluding the hyperattenuated areas in the center with the 204-242 Hounsfield Unit (HU), measuring approximately 9 mm in diameter (Fig. 2A, B) before contrast enhancement. The mass was located in the ventral wall near the bladder neck and showed uniform and mild parenchymal contrast enhancement continuous with the bladder wall, suggesting an intramural origin (Fig. 2C, D). The attenuation values of mass were 40 and 62 HU on precontrast and postcontrast, respectively. However, due to its protruding outward appearance, an extravesical mass with a tiny stalk could not be ruled out. Additionally, small calculi were observed in the bladder lumen. No evidence of abdominal or thoracic metastasis from the bladder mass was noted, including to the iliac lymph nodes. Based on ultrasound and CT examination, the tentative diagnosis was a leiomyoma, which can originate from the muscular layer of the bladder. The tumor exhibited a pattern of protruding outward from the bladder wall, suggesting it originated from intramural or extravesical rather than endovesical. During surgery, a careful examination was planned to assess the involvement of the bladder mucosa.

Figure 2. CT images of the bladder leiomyoma. Transverse view (A, B, C) and sagittal view (D) of the urinary bladder. (A) Unenhanced CT shows a 9-mm-diameter exophytic mass with heterogeneous attenuation near the ventral wall of the bladder neck (dotted circle). (B) Unenhanced CT in bone window demonstrates a hyperattenuating center of the mass (yellow arrow). (C, D) Contrast-enhanced CT reveals mild enhancement of the mass excluding the central calcified area (dotted circles). CT, computed tomography.

Surgery was performed with a low midline incision on day 1. The mass was located on ventral side of the bladder neck and bulged externally without a stalk-like structure connecting it to the urinary bladder wall (Fig. 3A). The mass was easily excised surgically without exposing the mucosal layer of the bladder (Fig. 3B). Histopathologic examination revealed a benign bladder leiomyoma with complete excision on day 8 (Fig. 4). The neoplastic spindle cells had a moderate amount of eosinophilic cytoplasm with indistinct cell borders. Their nuclei were cigar-shaped with finely stippled chromatin and occasionally one prominent nucleolus. These cells exhibited mild anisocytosis and anisokaryosis without overt mitotic activity. Two weeks postoperatively, neither the previously identified bladder mass nor a recurrent tumor was identified.

Figure 3. Intraoperative view of the bladder leiomyoma on day 1. (A) Urinary bladder and the palpable margin of the mass (dotted circle). (B) Ventral aspect of the urinary bladder after surgical removal of the mass.

Figure 4. Histologic findings of the bladder mass on day 8. Microscopically, the submucosa is expanded and effaced by a moderately cellular well-defined and encapsulated neoplasm composed of interweaving fascicles of well-differentiated smooth-muscle cells with a small amount of fibrovascular stroma between some fascicles. Hematoxylin and eosin staining, scale bar 12.5 μm, magnification 400×.

We reported a rare case of histologically confirmed leiomyoma without clinical symptoms related to the urinary system in a dog. In humans, cystoscopy is useful to distinguish an intramural tumor from an endovesical tumor, and intramural leiomyoma can be easily diagnosed using ultrasonography (5,7,10). Endovesical bladder tumors are usually polypoid or pedunculated, whereas intramural bladder leiomyomas are well-encapsulated lesions surrounded by the bladder-wall muscle (9). Therefore, endovesical tumors cause clinical signs such as hematuria; however, clinical symptoms are minimal in small intramural or extravesical. MRI and CT are useful for evaluating the anatomical location, origin, extension, and metastasis of tumors. MRI is superior to CT for distinguishing the mesenchymal components of leiomyomas and preserving the muscle layer (20). On MRI, bladder leiomyomas are visualized at intermediate signal intensity on T1-weighted images and at low signal intensity on T2-weighted images. Although MRI is generally used for extravesical tumors, differentiating between extravesical and intramural leiomyomas based on imaging findings alone may be difficult, especially in cases of large masses. Therefore, a comprehensive evaluation incorporating various imaging modalities, clinical symptoms, and histopathological examination is required for diagnosis.

In our case, no urinary symptoms associated with the small-sized intramural leiomyoma of urinary bladder were reported. Although cystoscopy was not performed, ultrasound revealed a protruding tumor originating from the muscular layer of the bladder wall and not within the bladder itself. Similarly, CT showed a tumor continuous with the bladder wall, with uniform contrast enhancement and outward protrusion. During surgery, the bladder tumor was completely excised, and extension to the mucosal layer was not observed. Based on the imaging findings, clinical symptoms, and surgical findings, we determined that the tumor originated within the intramural layer of bladder.

In veterinary medicine, leiomyoma is defined as a benign mesenchymal tumor arising from smooth-muscle cells (14). Given the rarity of leiomyoma in dogs, a classification of masses based on their location, similar to that in humans, has not been established. In our case, although not confirmed using cystoscopy, the lesion appeared as an intramural mass on ultrasonography, and CT revealed a mass originating from the wall and protruding outward. Additionally, intraoperatively, the mass was visually observed to be separated from the bladder mucosal layer, leading us to conclude that it originated intramurally. In humans, intramural leiomyoma of the bladder is characterized by a smooth-walled homogeneous hypoechoic intraluminal solid mass on ultrasonography (15). Similarly, in our case, except for the areas with internal calcifications, which were observed as hyperechoic regions, the mass appeared as a homogeneous, hypoechoic, smooth, rounded intramural mass on ultrasonography.

Although reports of bladder leiomyoma in veterinary medicine are rare, few studies have described the location of the tumors. In most cases, the tumors were located at the bladder apex, and single rounded masses with smooth margins were observed on ultrasonography and CT. Among the three reported cases of leiomyoma in dogs, two were intraluminal, and in the remaining one case, an extraluminal pedunculated mass attached to the bladder wall was observed (3,6). Although the bladder tumor in our case was a rounded intramural mass, unlike the previously mentioned leiomyomas that were positioned at the bladder apex, it was located on the ventral aspect of the bladder neck. Thus, differentiating it from other tumors that are observed around the bladder neck, such as TCC or leiomyosarcoma, was necessary.

In veterinary medicine, bladder leiomyosarcomas are rare and do not have a site predilection. However, they are reported to occur around the cranial apex and cranial dorsal areas of the bladder (1,6,12,16). These tumors can vary in shape from rounded to irregular, and in some cases, they can grow to a very large size, causing obstruction. However, metastasis has not been reported. In this case, a rounded mass was observed near the ventral aspect of the bladder without evidence of metastasis. Considering the non-specific location of the mass, exclusion of leiomyosarcoma was necessary.

Among bladder tumors, calcification is commonly associated with TCC, which typically occurs at the caudal dorsal aspect of the bladder, specifically at the trigone level (4,19). Although the bladder mass in this case exhibited calcified foci, it was located on the ventral aspect of the bladder neck and showed a rounded benign-like morphology; these characteristics were different from those of TCC. Till date, calcified bladder leiomyoma has not been reported in veterinary medicine, and is rarely reported in humans (18).

We reported a rare case of the bladder leiomyoma in a dog that was incidentally discovered on ultrasonography. If the mass is located in the muscle layer revealing mild contrast enhancement with no evidence of invasion or metastasis to other organ, although the presence of calcification, the urinary leiomyoma should be considered as a differential diagnosis.

The authors thank the owner of the dog included in this study.

The authors received no financial support for the research, authorship, and/or publication of this article.

The authors have no conflicting interests.