Intrapelvic extraluminal leiomyoma in a dog with severe acquired megacolon

Leiomioma extraluminal intrapélvico em um cão com megacolon adquirido grave

ABSTRACT:

Leiomyomas are benign smooth muscle tumors that, within the pelvic canal, most frequently arise in the female genital or gastrointestinal systems, but are rarely reported as extraluminal intrapelvic masses in dogs. In this location, neoplasia usually leads to constipation and, seldom, to acquired megacolon. This report described a case of an 11-year-old intact male mixed-breed dog with an extraluminal, intrapelvic leiomyoma leading to a severe case of megacolon. Clinically, the dog presented progressive weight loss, anorexia, obstipation, and a markedly distended abdomen. Hematological and biochemical analyses revealed severe anemia, leukocytosis, hypoglycemia, and hypoalbuminemia. The ultrasonographic and radiographic findings were consistent with megacolon. The patient died after one day of hospitalization. Necropsy revealed a 9 x 4 x 2 cm, white, firm intrapelvic, extraintestinal mass compressing the rectum and severe colonic dilation. Histopathology revealed that the mass consisted of a neoplastic proliferation of spindle cells arranged in bundles with mild nuclear and cellular pleomorphism. Immunohistochemistry confirmed multifocal immunolabeling for vimentin, desmin, and smooth muscle actin, thereby establishing the diagnosis of leiomyoma. The megacolon was classified as acquired and secondary to outlet obstruction by the intrapelvic tumor. This case highlighted a rare presentation of extraintestinal intrapelvic leiomyoma in a dog, emphasising the importance of considering intrapelvic neoplasia as a differential diagnosis in patients with chronic constipation, progressive weight loss, and megacolon.

Key words:
pelvic canal neoplasia; smooth muscle tumors; smooth muscle actin; extrinsic intestinal obstruction

RESUMO:

Leiomiomas são tumores benignos de músculo liso que, no canal pélvico, mais frequentemente surgem nos sistemas genital ou gastrointestinal de fêmeas e raramente são relatados como massas intrapélvicas extraluminais em cães. Nessa localização, a neoplasia geralmente leva à constipação e, raramente, ao megacólon adquirido. Este relato tem como objetivo descrever o caso de um cão macho, não castrado, de 11 anos de idade, sem raça definida, com um leiomioma intrapélvico extraluminal que levou a um caso grave de megacólon. Clinicamente, o cão apresentou perda de peso progressiva, anorexia, constipação e abdômen marcadamente distendido. Além disso, as análises hematológicas e bioquímicas revelaram anemia grave, leucocitose, hipoglicemia e hipoalbuminemia. Os achados ultrassonográficos e radiográficos foram compatíveis com megacólon. O paciente veio ao óbito após um dia de internação. A necropsia revelou uma massa intrapélvica extraintestinal, branca e firme, de 9 x 4 x 2 cm, comprimindo o reto e causando dilatação colônica grave. O exame histopatológico revelou que a massa consistia em uma proliferação neoplásica de células fusiformes dispostas em feixes com pleomorfismo nuclear e celular leve. A imuno-histoquímica confirmou a imunomarcação multifocal para vimentina, desmina e actina de músculo liso, estabelecendo assim o diagnóstico de leiomioma. O megacólon foi classificado como adquirido e secundário à obstrução do trato digestivo inferior pelo tumor intrapélvico. Este caso destaca uma apresentação rara de leiomioma intrapélvico extraintestinal em um cão, enfatizando a importância de considerar neoplasias intrapélvicas como diagnóstico diferencial em pacientes com constipação crônica, perda de peso progressiva e megacólon.

Palavras-chave:
neoplasias de canal pélvico; tumores de músculo liso; actina de músculo liso; obstrução intestinal extrínseca

Intrapelvic soft tissue neoplasia is rare in both dogs and cats (SPECTOR et al., 2011). In this space, these neoplasms may arise from the gastrointestinal tract, urogenital tract, regional lymph nodes, nerves, adipose tissue, blood, and lymphatic vessels (SPECTOR et al., 2011; PIROTH et al., 2021). Leiomyomas are benign tumors that originate from smooth muscle cells (COOPER et al., 2017), mainly from the female genital and gastrointestinal systems and, less commonly, in the lower urinary tract, soft tissue, and spleen (COOPER et al., 2017; AVALLONE et al., 2022). In dogs, less than 4% of smooth muscle tumors arise from the pelvic canal, outside of the intestinal and genital tracts (AVALLONE et al., 2022). These intrapelvic, extraintestinal leiomyomas have rarely been reported in dogs, and the only described cases of intrapelvic leiomyomas were associated with intestinal extrinsic obstruction (KATAMOTO et al., 2003; PIROTH et al., 2021; TSIMPOU et al., 2025).

Chronic obstruction of the pelvic canal - such as that caused by intrapelvic extraluminal tumors - can lead to acquired megacolon, characterized by a distended, typically feces-filled, hypomotile colon secondary to functional outlet obstruction (NEMETH et al., 2008; GELBERG et al., 2017; RYAN et al., 2023). However, literature on acquired causes of megacolon in dogs remains limited. This report described the clinical, gross, histopathological, and immunohistochemical features of an intrapelvic extraintestinal leiomyoma causing rectal compression and subsequent severe acquired megacolon in a dog.

In March 2022, an 11-year-old male, intact, mixed-breed dog was brought to the emergency care at the veterinary teaching hospital of the Universidade Federal do Rio Grande do Sul, Porto Alegre, Rio Grande do Sul, Brazil, with a history of progressive weight loss for 2 years and a 7-day history of anorexia and obstipation. At the time of the consultation, the dog was severely cachectic, comatose, with 8% dehydration, severe hypoglycemia (20 mg/dl), bradycardia (30 bpm), and hypotension (40 mmHg). At abdominal palpation, the patient had severe pain and markedly dilated intestines. For stabilization, a hypertonic solution was administered intravenously, along with bolus administration of glucose, calcium, and ephedrine (0.1 mg/kg). Dipyrone (25 mg/kg) and metadone (0.3 mg/kg) were administered for analgesia. Additionally, a nasogastric catheter was placed for food administration, and an enema was performed. The animal was hospitalized, and ultrasonographic, radiographic, hematologic, and biochemical exams were conducted. At the complete blood count, there was severe anemia, with 1.66x10⁶ erythrocytes/µL (reference interval [RI] 5.5-8.5x10⁶/µL), 4.0 g/dL hemoglobin (RI 12-18 g/dL), and 13% hematocrit (RI 37-55%), with mild hypochromasia (1+). At the leukogram, the dog had marked neutrophilic leukocytosis (50.600 leukocytes/µl [RI 6.000-17.000/µL]) by neutrophilia (49.082 segmented neutrophils/µL [RI 3.000-11.500/µl]), with mild toxic neutrophils (1+) and reactive lymphocytes. At the biochemical examinations, there was a total plasma protein of 29 g/dl (RI 60-80 g/dl), hypoalbuminemia (6 g/L, RI 26-33 g/L), and the urea was 85 mg/dL (RI 21-60 mg/dL). No other biochemical alterations were present.

At the abdominal ultrasonographic exam, the gastrointestinal tract was filled with feces and gas, with impaired visualisation of other abdominal organs, and there was a moderate quantity of anechoic fluid within the peritoneum. An abdominocentesis was performed for fluid collection - about 3 mL of protein-rich (3.1 g/dL), translucent fluid, with 1.600 cells/µL and 1.008 specific gravity. Cytological analysis of the fluid revealed a hypocellular sample composed mainly of intact neutrophils (93%), reactive mesothelial cells (5%), and rare small lymphocytes and macrophages (1%), with rare erythrocytes in the background. Therefore, the physicochemical and cytological findings of the effusion were consistent with a modified transudate. Due to the difficulty in properly assessing the organs of the abdominal cavity at the ultrasonographic exam, a radiographic exam was performed, which revealed that the descending colon was markedly dilated, measuring up to 7.4 cm, and filled with dense fecal matter (consistent with megacolon). Other structures were not visualised due to the colonic distention. The animal died after a day of hospitalization, and the cadaver was sent to the Veterinary Pathology Section of the Universidade Federal do Rio Grande do Sul for post-mortem examination.

At necropsy, the dog had severe sarcopenia, a diffusely distended abdomen, and pale mucous membranes. In the abdominal cavity, there was approximately 20 mL of translucent yellow fluid, and the large intestine was markedly dilated, measuring about 8 cm in diameter at the level of the descending colon, extending to the cecum (Figure 1A). When opened, the colon had markedly thinned walls and was filled with mushy, fetid fecal matter. At the level of the transition of the descending colon to the rectum, on the ventral surface, there was a 9 x 4 x 2 cm white, firm, irregular-surfaced mass adhered to the colon serosa. At the cut surface, the mass was firm to fibrous, slightly multilobulated, and homogeneously white (Figure 1B). Besides its adherence to the intestinal serosa, intestinal wall involvement was not observed. Fragments of the mass, large and small intestines, stomach, pancreas, liver, kidneys, urinary bladder, spleen, adrenal glands, mesenteric lymph nodes, bone marrow, heart, lungs, thyroid glands, telencephalon, cerebellum, and brainstem were collected and fixed in 10% neutral-buffered formalin.

Figure 1
Intrapelvic extraluminal leiomyoma with severe acquired megacolon, dog. A. The colon was severely and diffusely dilated, occupying almost all the abdominal cavity space. Bar, 5 cm. B. A 9 x 4 x 2 cm extraluminal mass located within the pelvic canal compressed the whole extension of the rectum. At the cut surface, the mass was firm to fibrous, slightly multilobulated, and homogeneously white. Bar, 2 cm.

After fixation, the fragments were routinely processed for histopathology, embedded in paraffin, cut into 3-µm-thick sections, and stained with hematoxylin and eosin (H&E). At histopathology, the mass was composed of a neoplastic proliferation of mesenchymal cells arranged in bundles and sustained by scant fibrovascular stroma (Figure 2A). The neoplastic cells were fusiform and had a moderate, strongly eosinophilic cytoplasm with indistinct cell borders, oval to elongated nuclei, finely stippled chromatin, and an inconspicuous nucleolus. There were mild anisocytosis and anisokaryosis, and absent mitotic figures in 2.37 mm² (10 FN22/40x). In the liver, centrilobular necrosis of hepatocytes was observed, along with granular, golden-yellow pigment deposition. No microscopic lesions were observed in the large intestine or in other organs.

Figure 2
Intrapelvic extraluminal leiomyoma, dog. A. Histologically, the mass was composed of multidirectional bundles of mesenchymal cells sustained by scant fibrovascular stroma. The neoplastic cells were fusiform, with moderate, strongly eosinophilic cytoplasm with indistinct cell borders, oval to elongate nuclei, finely stippled chromatin, and an inconspicuous nucleolus, resembling normal smooth muscle tissue. Hematoxylin and eosin. Bar, 200 µm. B. The neoplastic cells exhibited multifocal cytoplasmic vimentin immunolabeling. IHC. Bar, 150 µm. C. Diffuse, intense cytoplasmic immunolabeling for desmin within the neoplastic leiomyocytes. IHC. Bar, 150 µm D. Diffuse, intense cytoplasmic immunolabeling for smooth muscle actin within the neoplastic cells. IHC. Bar, 150 µm.

Selected sections of the neoplasm were cut into 2-µm-thick sections and submitted to anti-vimentin, anti-pancytokeratin, anti-desmin, and anti-smooth muscle actin (SMA) immunohistochemistry. Details of the antibodies, dilutions, antigen retrieval methods, and positive controls are listed in table 1. The primary antibodies were diluted in commercial antibody diluent (Dako, <www.agilent.com>) and incubated overnight at room temperature (20 ºC) for 12 hours. As the detection system, NovoLink Max Polymer Detection System (Leica Biosystems, <www.leicabiosystems.com>) was used, and the reaction was revealed with Romulin AEC Kit (Biocare Medical, <www.biocare.net>). The slides were counterstained with Harris’ hematoxylin. Negative Control Serum (Biocare Medical) was used instead of the primary antibodies as a negative control. The neoplastic cells exhibited multifocal to diffuse, marked cytoplasmic immunolabelling for vimentin, desmin, and SMA (Figure 2 B-D). No pancytokeratin immunoreaction was observed.

Table 1
Antibodies and immunohistochemical protocols used in a case of intrapelvic leiomyoma in a dog.

Antibody	Dilution	Antigen retrieval	Positive control
Anti-pancytokeratin, monoclonal (clone AE1/AE2)a	Ready-to-use	40 min at 96 ºC, digital pressure cooker, citrate buffer (pH 6.0)	Normal feline skin
Anti-vimentin, monoclonal (clone V9)b	1:200	40 min at 96 ºC, digital pressure cooker, citrate buffer (pH 6.0)	Normal feline skin
Anti-desmin, monoclonal (clone D33)a	1:300	Two cycles of 5 min in a microwave, Tris-EDTA buffer (pH 9.0)	Normal feline small intestine
Anti-human smooth muscle actin, monoclonal (clone 1A4)a	1:50	20 min at 120 ºC, digital pressure cooker, Tris-EDTA buffer (pH 9.0)	Normal feline small intestine

• Abbreviations: AEC, 3-Amino-9-Ethylcarbazole; EDTA, Ethylenediaminetetraacetic Acid.
• ^aDako, Carpinteria, California, USA.
• ^bZymed, Thermo Fischer Scientific, Massachusetts, USA.

In this case, a diagnosis of extraintestinal intrapelvic leiomyoma with secondary megacolon was established based on gross, histopathological, and immunohistochemical findings. Intrapelvic extraluminal soft tissue neoplasms are rare in dogs, with previously reported tumors including leiomyomas (KATAMOTO et al., 2003; PIROTH et al., 2021; AVALLONE et al., 2022), hemangiosarcomas (SPECTOR et al., 2011; YOO et al., 2017), a fibrosarcoma (VANHAESEBROUCK et al., 2012), a peripheral nerve sheath tumor (SPECTOR et al., 2011), a hemangiopericytoma (CHO & PARK, 2006), and a chondrolipoma (MUTINELLI et al., 2007). Classically, leiomyomas are well-differentiated neoplasms that closely resemble normal smooth muscle and are easily diagnosed on H&E staining (COOPER et al., 2017); however, in more pleomorphic variants, immunohistochemistry may be necessary to distinguish them from fibromas or peripheral nerve sheath tumors (COOPER et al., 2017). In our case, the strong histological similarity to normal smooth muscle, along with cytoplasmic immunolabeling for vimentin, desmin, and SMA, confirmed the diagnosis of leiomyoma (ANDREASEN et al., 1987; AVALLONE et al., 2022). The absence of cellular pleomorphism, nuclear atypia, mitotic figures, and metastatic behaviour excluded a diagnosis of leiomyosarcoma (AVALLONE et al., 2022). The tumor’s location outside the intestinal wall, confirmed grossly and histologically, ruled out intestinal leiomyoma, as these typically arise within the muscularis propria of the intestine (UZAL et al., 2016). Additionally, the extraintestinal location also ruled out a diagnosis of gastrointestinal stromal tumor (GIST) (COOPER et al., 2017).

Leiomyomas are slow-growing tumors and are typically diagnosed only after reaching a substantial size or incidentally during necropsy (PIROTH et al., 2021). In dogs, when they develop within the pelvic cavity, chronic constipation and urinary disturbances are the most common clinical signs (KATAMOTO et al., 2003; FERRARIS et al., 2021; PIROTH et al., 2021). Within the pelvic canal, leiomyomas most often originate from the uterine wall in females (AVALLONE et al., 2022), and extraluminal intrapelvic leiomyomas are rarely reported in veterinary medicine (KATAMOTO et al., 2003; PIROTH et al., 2021). In contrast, similar tumors are described in humans, mostly in women, where bowel compression and chronic constipation are also frequently reported clinical manifestations (PAAL et al., 2001; LIN et al., 2010). At this location, complete surgical excision via a dorsal rectal approach is typically the treatment of choice (KATAMOTO et al., 2003; TSIMPOU et al., 2025).

In this case, the tumor was only diagnosed at the post-mortem examination, which is expected since intrapelvic neoplasms are typically not visualised at conventional abdominal radiographic and ultrasonographic exams (SPECTOR et al., 2011; FERRARIS et al., 2021). Although most intrapelvic masses are detected through digital rectal examination (FERRARIS et al., 2021), this procedure was not performed. Still, the clinical presentation of progressive weight loss, anorexia, obstipation, and a distended abdomen, suggestive of megacolon (NEMETH et al., 2008; RYAN et al., 2023), was attributable to the outlet obstruction caused by the intrapelvic mass. Additionally, in a few cases of severe colonic impactation, depression and coma, as observed in our case, are reported clinical signs (TZIMTZIMIS et al., 2019).

Megacolon is an infrequently reported condition in dogs and, based on its pathophysiology, may be classified as primary (idiopathic) or secondary, and as congenital or acquired (BRIGHT et al., 1986; NEMETH et al., 2008; PROKIC et al., 2010; RYAN et al., 2023). Functionally, it is categorised into colonic inertia or outlet obstruction, with the latter including intrapelvic neoplasia (RYAN et al., 2023). In this case, secondary/acquired colonic dilation resulted from a pelvic canal mass, and the dilation was so pronounced that the colon occupied nearly the entire abdominal cavity, indicating a chronic disease course. Notably, despite the marked gross distension observed at necropsy, histopathological examination of the large intestine revealed no abnormalities, which is an expected finding in small animals with megacolon (UZAL et al., 2016). Before death, severe colonic dilation and substantial fecal retention were detected on ultrasonographic and radiographic evaluation, supporting the clinical diagnosis of megacolon, as these imaging modalities are essential for confirming the condition in affected animals (PROKIC et al., 2010). The primary treatment for acquired megacolon is the elimination of the underlying cause (PROKIC et al., 2010); however, in this case, the animal died before the intrapelvic mass could be identified and removed.

Finally, regarding the clinicopathological alterations, as observed in our case, anemia and leukocytosis are the main findings in dogs with megacolon of any cause (NEMETH et al., 2008). Additionally, in this case, the patient presented severe hypoglycemia, which is reported as a paraneoplastic syndrome in dogs with smooth muscle tumors (BEAUDRY et al., 1995; BAGLEY et al., 1996; COOPER et al., 2017). However, the 7-day history of anorexia can not be ruled out as a primary contributor, or maybe it may have exacerbated a potential paraneoplastic hypoglycemic state. Paraneoplastic hypoglycemia is typically confirmed by the normalisation of blood glucose levels following tumor removal (COOPER et al., 2017), but, in this case, the animal died before the intrapelvic leiomyoma was discovered. Consequently, this finding could not be confirmed as paraneoplastic.

This report described a rare case of intrapelvic extraluminal leiomyoma leading to severe acquired megacolon in an 11-year-old mixed-breed dog. Although rare, intrapelvic neoplasia, especially leiomyomas, should be considered as a differential diagnosis in dogs with progressive weight loss, chronic constipation, and subsequently megacolon.

ACKNOWLEDGMENTS

This research was supported by the Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq) and Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES) - Finance code 001.

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• CR-2026-0091.R1
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  The authors of the article entitled “Intrapelvic extraluminal leiomyoma in a dog with severe acquired megacolon” declare, for all due purposes, that the project that gave rise to the present data of the same has not been submitted for evaluation by the Ethics Committee of the Universidade Federal do Rio Grande do Sul. However, we are aware of the content of the Brazilian resolutions of the Conselho Nacional de Controle de Experimentação Animal (CONCEA) if it involves animals. Thus, the authors assume full responsibility for the presented data and are available for possible questions, should they be required by the competent authorities.
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  Additional information is available from the corresponding author upon reasonable request.
• DECLARATION OF USE OF ARTIFICIAL INTELLIGENCE
  During the preparation of this article, the author used ChatGPT AI technology to assist with grammar, spelling, and stylistic revisions. The tool was used solely for language editing and did not generate any original scientific content for this review. Following its use, the author critically reviewed and edited the manuscript as needed and assumes full responsibility for the publication’s final content.

Edited by

• ASSOCIATE EDITOR:
  Rudi Weiblen (0000-0002-1737-9817)

Data availability

Additional information is available from the corresponding author upon reasonable request.

Publication Dates

• Publication in this collection
  17 Aug 2026
• Date of issue
  2026

History

• Received
  23 Feb 2026
• Accepted
  13 Apr 2026
• Reviewed
  04 June 2026