Ultrasonography of the small intestine in small animals - Part 2
Lymphomas, adenocarcinomas, and mast cell tumors are the most common intestinal tumors in cats, whereas adenocarcinomas and leiomyomas are more common in dogs. Intestinal fibrosarcomas, hemangiosarcomas, carcinoids, and plasma cell tumors are rare. Clinical signs usually include weight loss and anorexia. Diarrhea, melena, vomiting, abdominal discomfort, abdominal effusion and anemia may also occur. Intussusception and intestinal perforation may occur as a consequence of intestinal tumors (20).
The most common ultrasonographic features of intestinal neoplasia are thickening of the bowel wall, loss of its normal layered appearance, and alterations in the contour of the mucosal and/or serosal surfaces (17). Changes associated with intestinal neoplasia are most often focal as a mass-like lesion but can also be diffuse, especially in the case of canine gastrointestinal lymphoma. The mass may be quite variable in appearance. Focal, concentric thickening of the bowel may be present, or the thickening may be eccentric in location. Larger lesions are usually complex, with mixed echogenicity. While it is not difficult to identify large mass lesions, it may be more of a challenge to associate the mass with the bowel. Metastasis to regional lymph nodes and occasionally to the liver or other organs can occur (17).
Alimentary lymphoma (involving the gastrointestinal tract and/or the mesenteric lymph nodes) is one of the most common sites of occurrence for feline lymphoma (20).
In dogs and cats, the most common ultrasonographic findings are a transmural thickening associated with the diffuse loss of normal wall layering, reduced wall echogenicity, decreased localized motility and regional lymph node enlargement (Figures 11A-11C) (21). In cats, alimentary lymphoma can affect the intestinal tract without fully disrupting the wall layering (2).
Adenocarcinomas are considered the most common gastrointestinal tumor in dogs. The most common ultrasonographic findings are transmural thickening with complete loss of layering, and often with associated lymphadenopathy (Figures 12A-12C). In the majority of these cases, there was evidence of fluid accumulation proximal to the intestinal thickening associated with localized ileus (22).
Intestinal carcinoma has similar ultrasonographic appearance to intestinal lymphoma, (20) but the length of the lesion tends to be shorter in carcinoma than lymphoma, and mechanical ileus is more common in carcinoma than in lymphoma. So, these lesions must be biopsied with ultrasonic guidance to provide a definitive diagnosis (2).
Smooth muscle tumors
Smooth muscle tumors include leiomyomas, which are the most common benign tumors of the canine gastrointestinal tract, and leiomyosarcomas, which are the most common sarcoma of the gastrointestinal tract (2).
Leiomyoma are typically found within the stomach of older dogs, although they can occasionally be found in the bowel (20). This tumor is small and has a uniform echogenicity (Figure 13).
Leiomyosarcomas are usually seen as large, complex masses. These tumors are originated intramurally and grow out of the serosa as large eccentric, extraluminal masses, or less commonly grow into the bowel lumen. Due to their distribution and large size, it is difficult to assess the anatomic origin of the mass (23). Internally, these masses may have anechoic and hypoechoic foci which may correlate with the areas of necrosis and hemorrhage, accounting for their complex ultrasound appearance (23). Anemia is the most frequent hematological abnormality.
Percutaneous ultrasound-guided aspiration or tissue-core biopsy can be performed to confirm the mesenchymal nature of these lesions. However, careful choice of the biopsy path should be made to avoid anechoic cavities and subsequent leakage or hemorrhage (20).
Fibrosarcomas, mast cell tumors, hemangiosarcoma, adenomatous polyps and non-functional carcinoid tumors tend to be focally invasive as poorly echogenic masses or as focal thickening with loss of layering. No specific ultrasonographic appearance helps to differentiate the tumors (2), so the ultimate diagnosis of the tumor type must be confirmed by histopathology using an endoscopic, surgical or aspiration/biopsy obtained with ultrasound guidance (20).
Fine-needle aspiration or tissue-core biopsies of bowel masses under ultrasound guidance are safe alternative procedures to use instead of endoscopic or surgical biopsy. The main rule is to carefully locate and avoid the lumen, since intestinal content leakage can be a serious complication (2).
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