History
A 10-year-old 4.3-kg spayed female domestic shorthair cat presented to the primary veterinarian for a 1-month history of intermittent dyschezia. Physical examination findings at that time were unremarkable, except for palpation of a moderate amount of hard fecal material in an otherwise soft, apparently nonpainful abdomen. The primary veterinarian (not available) performed 3-view abdominal radiography, which reportedly showed constipation. An enema was performed, removing 5 to 6 inches of firm stool. The patient was conservatively managed with stool softeners and appetite stimulants. Four days later, the cat was referred to the University of Florida Small Animal Hospital for evaluation of continued dyschezia and inappetence. A CBC was clinically unremarkable. A serum biochemistry panel showed mild hyperglycemia (212 mg/dL; reference range, 72.1 to 156.1 mg/dL), slight hypermagnesemia (2.9 mg/dL; reference range, 1.9 to 2.6 mg/dL), and moderately elevated creatine kinase (893 IU/L; reference range, 20 to 440 IU/L). Three-view abdominal radiographs were obtained (Figure 1).
Right lateral (A), left lateral (B), and ventrodorsal (C) abdominal radiographic images of a 10-year-old 4.3-kg spayed female domestic shorthair cat presenting for a 1-month history of dyschezia.
Citation: Journal of the American Veterinary Medical Association 261, 6; 10.2460/javma.22.11.0502
Diagnostic Imaging Findings and Interpretation
Abdominal radiography revealed a smoothly marginated, locally extensive descending colonic wall thickening, extending from the level of L6-L7 into the pelvic canal to the level of the coxofemoral joints. Orad to the descending colonic thickening, an abnormal, sharp triangular truncation of luminal gas was noted, along with a moderate volume of soft tissue to mineral opaque fecal material, which filled the remainder of the colon. Dorsal and craniodorsal to the descending colon were several well-defined, smoothly marginated and ovoid soft tissue opaque nodules compatible with enlarged caudal mesenteric lymph nodes (Figure 2). A descending colonic mural mass with partial colonic mechanical obstruction and concurrent caudal mesenteric lymphadenopathy was suspected. The primary differential diagnosis was malignant neoplasia, such as round cell neoplasms (eg, lymphoma, mast cell tumor) or carcinoma, with regional metastatic lymphadenopathy. Three-view thoracic radiography was performed and was unremarkable.
Same radiographic images as Figure 1. A smoothly marginated, homogenously soft tissue opaque mass (arrowheads) associated with the aborad descending colon and rectum is seen extending into the pelvic canal. There is an abrupt, persistent, triangular tapering of descending colonic gas orad to the lesion and faintly mineral opaque fecal material and gas in the rest of the colon. Several well-defined, ovoid, homogeneous soft tissue structures are noted craniodorsal (pound signs) to the mass, compatible with caudal mesenteric lymphadenomegaly.
Citation: Journal of the American Veterinary Medical Association 261, 6; 10.2460/javma.22.11.0502
For further evaluation, an abdominal ultrasound was performed, which revealed a marked, locally extensive, irregularly concentric descending colonic wall thickening (up to 1 cm) with heterogeneously mildly hypoechoic thickening of mostly the submucosa and multifocal decreased wall layering distinction (Figure 3). The mucosal layer was multifocally effaced, and the muscularis and serosal layers remained distinct. Within the muscularis layer was a thin, well-defined hyperechoic parallel line, which likely corresponded to fibrous connective tissue. The mass extended caudally into the pelvic canal beyond the field of view, preventing assessment of its caudal extent. The mass caused severe colonic luminal stenosis, with accumulation of clean shadowing fecal material and gas orad to the mass. In addition, there was marked caudal mesenteric lymphadenomegaly (up to 1.6 X 1.5 cm), with multiple lymph nodes containing central hypoechoic to anechoic cavitations and surrounding hyperechoic fat. A scant volume of peritoneal effusion was also present. The majority of the mass was centered on the submucosa; thus mast cell tumor was prioritized as a primary differential diagnosis. Ultrasound-guided fine-needle aspirates of the mass and a caudal mesenteric lymph node were performed without complications.
Transverse ultrasonographic images of the caudal abdomen of the cat described in Figure 1. There is severe asymmetric thickening of the descending colonic wall and especially of the submucosal layer with a heterogeneously hypoechoic submucosa (A; between asterisks). The junction between normal (hyperechoic) and abnormal (hypoechoic) submucosa can be visualized (A; arrow). The mucosal layer (A; pound sign) is well-defined in some regions but effaced in others. The muscularis layer (A and B; arrowhead) remains well-defined. In the muscularis, a central hyperechoic line oriented between and parallel to the hypoechoic interior and exterior muscular layers is present, which likely corresponds to fibrous connective tissue. A centralized luminal accumulation of formed fecal material creates a strong distal acoustic shadow. There is scant regional anechoic fluid (A; dotted arrow) and surrounding hyperechoic fat. A moderately enlarged caudal mesenteric lymph node (C; star) contains a central hypo- to anechoic cavitation and is surrounded by hyperechoic fat and a scant amount of anechoic free fluid. DC = Descending colon. UB = Urinary bladder.
Citation: Journal of the American Veterinary Medical Association 261, 6; 10.2460/javma.22.11.0502
An abdominal CT scan was performed the following day (Figure 4) for surgical planning and visualization of the mass extension in the pelvic canal. The descending colonic mass was smoothly margined, well-defined, and isoattenuating, with moderate heterogeneous contrast enhancement. The mass extended from the level of L4-L5 to the level of S2 and measured approximately 6.2 X 2.7 X 2.6 cm. The caudal mesenteric lymph nodes were severely enlarged with rounded margins, the largest measuring 1.9 X 1.4 X 1.3 cm and 2.6 X 1.5 X 1.3 cm, homogenously soft tissue attenuating, and moderately homogeneously contrast enhancing, and contained multiple non–contrast-enhancing fluid-attenuating cavities.
Sagittal (A) and dorsal (B) contrast-enhanced CT images displayed in a soft tissue window of the abdomen of the cat described in Figure 1. There is severe locally extensive irregular thickening of the wall of the descending colon and adjacent rectum (arrowheads) and accumulation of mineral-attenuating fecal material orad of the mass. Moderately enlarged caudal mesenteric lymph nodes (B; pound signs) are noted, with the cranial node containing an irregular fluid-attenuating cavitation. The images are displayed in a postcontrast soft tissue window (window width, 400 HU; window level, 40 HU) with a 3-mm slice thickness. The cat was scanned in sternal recumbency. In the dorsal image, the cat’s right is to the left side of the image.
Citation: Journal of the American Veterinary Medical Association 261, 6; 10.2460/javma.22.11.0502
Treatment and Outcome
Cytology of the colonic mass confirmed the suspected mast cell neoplasia, the caudal mesenteric lymph node aspirate revealed metastatic mast cell infiltration, and subsequent liver and splenic aspirates found no cytologic evidence of metastatic disease. The patient underwent exploratory laparotomy with marginal resection and anastomosis of the colonic mass and lymphadenectomy. Histopathology of the mass showed a large population of mast cells expanding the submucosa and compressing the intestinal lumen, with multifocal invasion into the muscularis and serosal layers, diagnostic for mast cell tumor. Evaluation of the caudal mesenteric lymph nodes was compatible with metastatic mast cell tumor. The patient was medically managed with lactulose and prednisolone, returning 3 weeks postoperatively for chemotherapeutic infusion of 1.5 mg/m2 of vinblastine once weekly for 4 weeks, then once every other week, totaling 7 doses. A 3-month recheck abdominal ultrasound showed no evidence of mass recurrence, lymphadenopathy, or distant metastasis. At last follow-up, 19 months following the diagnosis, the patient was doing well with no clinical signs of the disease.
Comments
Mast cell tumors account for up to 20% of all tumors in cats.1 The most common form is cutaneous, with visceral presentations being infrequent. Mast cell tumors of intestinal origin in cats most commonly involve the small intestine, with < 15% of cases originating from the colon.1,2 Reported ultrasound features of mast cell tumors are variable, with most cases presenting as one or multiple eccentric, hypoechoic thickenings or masses of the submucosal or muscularis layers, usually with retained wall layer distinction.2 In this case, the primary mass originated from the colonic submucosa, and despite the normal sonographic appearance of the muscularis layer, there was histologic confirmation of invasion into the deeper muscularis and serosa. Secondary intestinal obstruction, as seen in this case, is uncommon. Metastasis to the local lymph nodes, spleen, and liver is common.1,2
Radiography proved useful in this case to identify the mass lesion, secondary partial colonic obstruction, and local lymphadenopathy. While radiographs are traditionally inaccurate for determining gastrointestinal wall thickening due to the inability to discriminate intraluminal soft tissue or fluid material border effacing the wall from pathologic mural thickening, in this case a colonic mass was still strongly suspected due to the soft tissue opaque thickening being persistent on multiple views, abrupt triangular truncation of gas, and evidence of constipation orad to the lesion (Figure 2).3 To further improve the diagnostic accuracy of radiographs for assessment of the gastrointestinal wall, a barium enema or a pneumocolonogram could have been considered. In this case, ultrasound was a useful modality in discriminating neoplastic from nonneoplastic causes of intestinal wall thickening. It also allowed for identification of a severely expanded colonic submucosal layer, which is supportive of mast cell tumor and helped to narrow the differential diagnosis list.2 Ultrasound also assisted in better characterizing the caudal abdominal lymphadenopathy and in assessing the architecture of the spleen and liver. Whole-body CT provided the additional benefit of determining the caudal extent of the lesion into the pelvic canal, as well as allowing a more global assessment for distant metastatic disease or other comorbidities. The clinical decision to aspirate both the spleen and liver over the course of diagnostic testing regardless of their imaging appearance has been supported by previous studies4,5 in dogs, which have shown poor sensitivity of ultrasound and CT for detecting mast cell metastasis to the spleen and liver.
Ultimately, this case demonstrated the utility of a multimodality approach to diagnosis, staging, and treatment of intestinal mast cell neoplasia in cats; showed that intestinal obstruction may occur; and suggested that ultrasound is poorly sensitive for the detection of mast cell invasion into the deeper layers of the intestine.
Acknowledgments
No external funding was used in this study. The authors declare that there were no conflicts of interest.
References
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Vail DM, Thamm D, Liptak J. Withrow and MacEwen’s Small Animal Clinical Oncology. 6th ed. Saunders Elsevier; 2007:382–403.
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Laurenson MP, Skorupski KA, Moore PF, Zwingenberger AL. Ultrasonography of intestinal mast cell tumors in the cat. Vet Radiol Ultrasound. 2011;52(3):330–334.
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Book AP, Fidel J, Wills T, Bryan J, Sellon R, Mattoon J. Correlation of ultrasound findings, liver and spleen cytology, and prognosis in the clinical staging of high metastatic risk canine mast cell tumors. Vet Radiol Ultrasound. 2011;52(5):548–554.
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Hughes JR, Szladovits B, Drees R. Abdominal CT evaluation of the liver and spleen for staging mast cell tumors in dogs yields nonspecific results. Vet Radiol Ultrasound. 2019;60(3):306–315.
