Leiomyosarcoma is a malignant mesenchymal neoplasm of smooth muscle origin that commonly involves the intestinal tract, spleen, and liver in older dogs (1,7). Common symptoms of canine intestinal leiomyosarcoma are non-specific but tend to be associated with the location of the tumor. These symptoms may include vomiting, diarrhea, anorexia, polyuria, polydipsia, lethargy, melena, and acute collapse. In human medicine, approximately 5-10% of all soft tissue sarcomas are leiomyosarcomas. Anorectal or anal leiomyosarcomas are rarely reported, with an incidence of less than 0.1% among all anorectal tumors (12). Until 2019, only 51 cases were confirmed and reported as anal leiomyosarcoma. Anal or rectal tumors have minimal or nonspecific symptoms, making diagnosis even more challenging in dogs, just as in humans. Canine rectal leiomyosarcomas have scarcely been reported: one with immunohistochemical confirmation and the others without immunohistochemical results (4,14). In veterinary reports, one case of perineal leiomyosarcoma have been reported in a heifer (11). Based on our review of the literature, this is the first report describing primary anorectal or anal leiomyosarcoma in veterinary medicine.

A 16-year-old neutered male poodle weighing 4.31 kg was admitted with a large perianal mass that had been growing gradually for 4-5 years and had recently begun bleeding. There was no history of illness nor associated symptoms such as fever, diarrhea, or abdominal pain. Physical examination revealed perianal pain and a large, reddish, and hard mass with bleeding. Complete blood count and serum biochemistry evaluation results were unremarkable, except for a mild increase in the alkaline phosphatase level (122 U/L; reference range, 13-83 U/L).

Abdominal radiographs (60 kVp, 4 mAs; Vetter-DR9; Median International, Anyang, Korea) revealed a 7.6 cm x 9.6 cm, large, internally calcified, soft tissue mass in the perineal area, dorsally displacing the caudal vertebrae (Fig. 1).

Figure 1. Right lateral (A) and ventrodorsal (B) radiographs illustrating a massive, ovoid, soft tissue opacity mass in the perineal area, with intralesional calcification.

Computed tomography (CT) was performed to identify the origin and metastatic status of the perineal mass and inform the surgical planning. Pre- and postcontrast images were acquired using a 16-slice helical CT scanner (Brivo CT385; GE Healthcare, Chicago, IL, USA; 120 kVp, 200 mA, 1.25 mm slice thickness). The postcontrast images were obtained 40 s after the injection of iodinated contrast medium, 600 mg iodine/kg of iohexol (Omnipaque, 300 mg/mL; GE Healthcare, Shanghai, China) via the cephalic vein using a power injector.

Abdominal CT scans with contrast revealed a well-defined, oval, soft tissue attenuated mass (approximately 7.5 cm x 7.0 cm x 8.0 cm) containing multiple calcifications in the dorsal perineal region (Fig. 2). Moderate contrast enhancement and non-enhanced hypoattenuation inside the mass were observed. The rectum (approximately 10 cm in length) was flat, severely compressed, and displaced ventrally by the mass. The anal sphincter had a diameter of approximately 2 cm, was included in the mass, and appeared to have been pushed backward owing to its massive size. In the area adjacent to the coccyx and dorsal to the mass, the border between the coccyx and mass was difficult to distinguish. The left anal sac was severely compressed, flat, and visible on the left side of the mass. In contrast, the right anal sac was not observed. Thoracic CT findings were unremarkable, with no evidence of tumor metastasis. Based on CT findings, the differential diagnoses included perianal sac adenoma, adenocarcinoma, and distal rectal tumors. Since no clear evidence of tumor metastasis was found, it was decided to perform a histopathological examination after surgical removal.

Figure 2. Postcontrast sagittal (A), dorsal (B), and transverse (C) images showing a well-defined, oval, soft tissue attenuated mass containing multiple intratumoral mineralization in the dorsal perineal area are shown. The mass is located dorsal to the rectum with severe compression, includes anal canal, and displaces the coccyx dorsally. The left anal sac (arrow) was observed to the left to the anal mass. Volume rendering images of the perianal mass in the oblique sagittal (D) and dorsal (E) planes reveal a large soft tissue mass located caudodorsal to the pelvis.

Surgery was performed under general anesthesia (Fig. 3). When the skin around the anus was incised, many new blood vessels leading to the mass were identified. Adhesion to the tail was insignificant; therefore, the mass could be easily separated. The area of adhesion between the mass and the rectum was not large, allowing for complete resection that included 3 cm of the distal rectum. Histopathology revealed a mass composed of spindle cells with indistinct cell borders and a mild-to-moderate amount of wispy, vacuolated, and eosinophilic to amphophilic cytoplasm. Anisocytosis (variation in the size of cells) and anisokaryotics (variation in the size and shape of cell nuclei) were mild to moderate. The mitotic count (the number of cells undergoing mitosis or cell division) was 1 per 10 high-magnification fields (2.37 mm²). Scattered areas of necrosis comprised approximately 25% of the mass. Immunohistochemical studies reported negative labeling for c-kit and positive labeling for smooth muscle actin, differentiating the cells from those of gastrointestinal stromal tumors (GIST). Overall, both pathological and immunohistochemical studies revealed a well-differentiated leiomyosarcoma originating from or adjacent to the anal sphincter with negative margins.

Figure 3. Preoperative (A) and perioperative (B, C) images revealing a large, well-separated anal tumor are shown. Histopathological examination (D, E) revealed that the mass is composed entirely of a thinly pseudo-encapsulated, multinodular, poorly demarcated, moderately cellular neoplasm composed of spindle cells with indistinct cell borders.

The patient began passing normal stools on a prescribed diet (Hill’s i/d) from the 5th day post-surgery and was discharged after 7 days of intensive postoperative inpatient care. Thereafter, outpatient treatment was performed every 7-10 days for approximately 1 month. One year of follow-up was conducted via telephone contact with the animal’s owner. Although this patient required permanent management of fecal incontinence due to anal sphincter removal, the patient has been doing well without any postoperative complications or recurrence while undergoing regular hip baths, walking exercises, and a normal diet.

An anal sphincter consists of the internal and external anal sphincter (3). The internal anal sphincter, the caudal portion of the circular coat of the anal canal, is composed of smooth muscle. The external anal sphincter is composed of a circular band of striated muscle. One possible reason for the rare occurrence of anal leiomyosarcomas is the paucity of smooth muscle on the anal side. Most anal leiomyosarcomas in human medicine are reclassified as anorectal tumors (92.2%); however, upon re-examination (12), pure anal tumors are likely to be rare (only 7.8%). Although the large size of the tumor made determining its origin difficult in this case, the mass was likely an anorectal leiomyosarcoma.

Both histological and immunohistochemical analyses are required to diagnose leiomyosarcomas. Because GISTs and leiomyosarcomas have similar histological characteristics, immunohistochemical markers are useful for differentiating the two tumor types. In dogs, approximately 66.7 to 84.7% of masses previously categorized as smooth muscle tumors such as leiomyosarcomas and leiomyomas were reclassified as GISTs based on immunohistochemical staining (10,15). GISTs express c-kit (CD-117), likely because their origin is thought to be the interstitial cells of Cajal. In contrast, leiomyosarcoma are negative for the c-kit protein. Intestinal leiomyosarcoma frequently develops in the stomach and small intestine, whereas GISTs are frequently found in the cecum and large intestine (15). Owing to the differing biological behaviors and treatments of GISTs and leiomyosarcomas, differentiation using immunohistochemical assays is recommended. In the present case, anorectal leiomyosarcoma was confirmed through both histological and immunohistochemical findings, which revealed that the tumor was c-kit-negative and smooth muscle actin-positive results.

In human medicine, different treatment options for anorectal leiomyosarcomas include radical resection, sphincter-preserving surgery, adjuvant therapy, and medical therapy (4). Among these, surgical excision with negative margins is preferred. Radical resection has a lower rate of local recurrence than wide local excision (20% vs. 30%, respectively); however, the overall rate of total metastasis is quite high (51.62%) and the prognosis for human anorectal leiomyosarcoma is poor. Sphincter-preserving surgery followed by brachytherapy has been introduced as an alternative to abdominoperineal resection and resulted in a local recurrence rate of approximately 25% (6). However, the optimal treatment for anorectal leiomyosarcomas remains controversial since the tumors are rare. In veterinary medicine, surgery is the treatment of choice for gastrointestinal leiomyosarcomas. The treatment is associated with a favorable prognosis for cases of R0 resection with no distant metastasis (1,7). The reported median survival time for canine leiomyosarcoma in other areas is 10 months, with a range from 1 month to 7 years, and metastasis was not found in 79% of cases at the time of surgery (7). Re-resection should be considered in cases of incomplete excision because positive margins are considered an independent indicator of local recurrence in human leiomyosarcoma (5).

In the present case, total resection of the anal sphincter was required to secure adequate surgical margins to prevent local recurrence because the tumor was massive. Because surgery excising an anal sphincter circumference of more than 180° causes partial to complete fecal incontinence (9), foods known to generate soft stools were provided as postoperative care for 1 week, with regular walking exercises and baths recommended. Sphincter incontinence resulting from surgical intervention in the anal canal does not appear to be a major problem in the daily life of animals. In our case, sphincter incontinence appeared to be well managed. No remarkable findings were reported at the 1-year follow-up, with low-grade leiomyosarcoma with benign behavior observed. Although additional cases will need to be studied to better understand malignancy, canine anorectal leiomyosarcomas do not appear to exhibit more malignant behavior than leiomyosarcomas at other locations.

Diagnostic imaging is always recommended for tumor staging and presurgical planning in dogs with massive masses. Previous studies have reported that MRI is superior to CT in evaluating the margins of leiomyosarcoma masses (16). However, due to its longer imaging time, higher costs, and limitations in assessing systemic tumor metastasis, its application in real-world clinical settings, where cost is a significant concern, remains limited. Ultrasound examination is also limited in accurately assessing the extent of the mass, adjacent invasions, and metastasis due to its inability to effectively visualize large masses. CT is a fast, noninvasive, and useful modality for evaluating the size, location, and extent of the mass and its relation to the surrounding organs, as well as for delineating metastatic potential (4). The mass in this patient was large, focal, and well-marginated with soft-tissue attenuation on CT, similar to previous gastrointestinal leiomyosarcomas. Intralesional calcification, lobulated growth, and internal cystic portions resulting in a heterogeneous contrast pattern, identified in this patient, are also common findings in other spindle cell sarcomas (2,8,17). These heterogeneous contrast patterns overlap with those of other colorectal tumors (13), so histological examination should always be performed.

Despite the large tumor size in this case, it took approximately 4-5 years for the patient to be admitted to the hospital. This is partly due to the slow-growing nature of the tumor but is also thought to be because, unlike tumors in other locations, symptoms caused by a mass around the anus were mild. In this case, the dog’s anus was torn and bleeding occurred. The dog experienced pain around the anal mass upon palpation; however, vitality and appetite remained good. In human anorectal leiomyosarcoma, the most common symptom is rectal bleeding, as was observed in this case, whereas other symptoms, if present, include pain, weight loss, and constipation (12).

In conclusion, we present the first case of primary canine anorectal leiomyosarcoma confirmed by histology and immunohistochemistry. Despite the low prevalence of anorectal leiomyosarcoma in veterinary medicine, the tumor type should be included in the differential diagnosis for animals with large bleeding perineal masses. Considering the good prognosis of the animal described here, surgical resection with no positive margins appears to be the most appropriate treatment for primary canine anorectal leiomyosarcomas in cases in which neighboring invasion or distant metastases are absent. Advanced imaging, such as CT, plays a crucial role in diagnosing and planning treatment for complex conditions by providing detailed visualization of tumors and surrounding tissues, helping to stage the disease, and guiding surgical interventions. Additionally, follow-up care is essential for managing long-term outcomes, as it allows for the early detection of recurrence or metastasis, ensuring timely intervention and optimizing patient prognosis. This is particularly important, as local and distant tumor recurrence is common in human anorectal leiomyosarcomas.

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

The authors have no conflicting interests.