Diagnostic imaging and pathological findings of an abdominal mesenteric granular cell tumour in a dog

Abstract A 12‐year‐old mixed‐breed dog was presented for a follow‐up examination after ablation of an auricular mast cell tumour. An abdominal ultrasound and computed tomography were performed and an irregular, ill‐defined and partially mineralised lesion was observed around the caudal duodenal flexure without evidence of metastasis. The cytologic examination was highly suggestive of a granular cell tumour. Partial surgical ablation with histological and immunohistochemical examination of the lesion confirmed the diagnosis. According to our review of the literature, this is the first report documenting an abdominal granular cell tumour in a dog.


| INTRODUC TI ON
Granular cell tumours, once called granular cell myoblastomas, are uncommon and usually benign lesions in both human and veterinary medicine. The oral cavity (notably the tongue) is the most common location in dogs, cats and people, but a wide range of other locations have been described (Higgins et al., 2017;Levitin et al., 2019;Mobarki et al., 2020;Patnaik, 1993). In dogs, documented locations for this type of tumour include the brain, meninges, spinal nerve root, heart, lung, eye, vocal cord and trachea (Levitin et al., 2019;Patnaik, 1993).
According to our review of the literature, there is not a previous publication of an abdominal granular cell tumour in the dog.

| C A S E HIS TORY
A 12-year-old neutered female mixed-breed dog was presented for a follow-up examination 8 months after ablation of an auricular grade II mast cell tumour. At presentation, the patient was bright, alert and responsive. All vital parameters were within normal limits. The owner did not report any gastrointestinal signs.
A complete blood cell count, serum biochemistry and abdominal ultrasound were performed as part of the oncologic examination.
On the abdominal ultrasound an irregular and elongated lesion with ill-defined margins was observed in the mesentery adjacent to the serosa of a small intestinal loop in the right caudal abdomen ( Figure 1). The lesion measured 3.5 cm in length and 0.5 cm in thickness. It was markedly hyperechoic with several pin-point hy-

| D ISCUSS I ON
According to our review of the literature, this is the first report describing an abdominal granular cell tumour in a dog. In this case, the tumour was in the mesentery and deeply attached to the serosal layer of the caudal duodenal flexure. Unfortunately, it was not possible to determine if the lesion originated from the mesentery or the intestinal serosa, as enterectomy or full-thickness biopsy was not performed. In human medicine, up to 8%-11% of the granular cell tumours involve the gastrointestinal tract, most commonly the oesophagus, colon and stomach. Small intestine involvement is extremely rare (<1%), with only few cases of this tumour affecting the duodenum and ileum (Barakat et al., 2018;Nakachi et al., 2000;Radaelli & Minoli, 2009). Most gastrointestinal granular cell tumours in human medicine arise from the mucosal or submucosal layer (Mobarki et al., 2020;Nakachi et al., 2000). In the present case, the lesion was in close contact with the serosal layer and mesenteric adipose tissue, being the other intestinal layers normal in ultrasound and computed tomography. Based on these considerations, the mesentery is a more likely organ of origin compared to the intestine.
The histogenesis of the granular cell tumour remains controversial since its original description in 1926 (Patnaik, 1993). Initially, based on light microscopic features of the neoplastic cells, the tumour was thought to be of skeletal muscle in origin. Neurons, fibroblasts, histiocytes and myoepithelial cells are other potential origins that have been proposed in human medicine (Suzuki et al., 2015). Lately it is suspected that they arise from the neural crest, specifically the Schwann cells.
Evidence that the canine and human granular cell tumours shares a similar origin has been recently published (Patnaik, 1993;Suzuki et al., 2015;Wilson, 2017). In our case, granular cells reacted markedly to PAS and ubiquitin, as it has been previously described (Higgins et al., 2017), but no positivity was observed under Luxol fast blue and S-100 protein IHC. According to this results a Schwann cell origin was not confirmed. S-100 negative granular cell tumours have been previously reported in veterinary medicine (Reifinger et al., 2020). A definitive explanation of this fact is beyond the aim of this study.
Some publications in human medicine have described findings suggestive of a previous foreign body in the histopathology of granular cell tumours (Meyer et al., 2010;Rocanti et al., 2013). In our case, the histological evaluation of the biopsy revealed (in addition to the granular cell population) some necrotic foci associated with reactive inflammatory cells. As no definitive foreign body was identified, those changes are considered secondary to the granular cell tumour.
Granular cell tumour are usually benign. Malignant transformation can happen, but this is rare in both human and veterinary medicine (Bouayyard et al., 2020;Higgins et al., 2017;Reifinger et al., 2020).
In a recent human article, a 1%-2% of malignant conversion has been described (Bouayard et al., 2020). Few reports including malignant granular cell tumours have been published in veterinary patients (Patnaik, 1993). The present case has imaging, surgical and histopathological characteristics suggestive of local neoplastic infiltration, but no other indications of malignancy such as lymphadenopathy or evident metastatic disease were detected in the imaging studies.
Our patient presented for a follow-up examination after ablation of an auricular mast cell tumour. To the authors' knowledge, no correlation has been described between granular cell tumour and mast cell tumour in neither veterinary nor human literature. In absence of any evidence the authors assume that the two pathologies in this case are unrelated.
Ultrasound descriptions of granular cell tumours are lacking in veterinary medicine. Recently, ultrasound appearance of a celomic granular cell tumour has been described in a California kingsnake as a large mass, not associated with any organ, with cysts, and a wall of moderate echogenicity (Reifinger et al., 2020). In human literature, granular cell tumours affecting the gastrointestinal tract have been described as hypoechoic, homogenous, smooth-edged solitary lesions that appear to originate from the submucosal layer (Barakat et al., 2018). Differently, in our case, the lesion was hyperechoic, heterogeneous and it appeared to arise from the mesentery or the intestinal serosal layer.

F I G U R E 3
Round and large cell proliferation with eccentric nuclei; occasional multinucleated cells are present. Cytoplasmic granules are stained purple for periodic acid-Schiff (PAS) (a) and show marked immune reaction against ubiquitin (b). Note the infiltrative pattern in mesenteric adipose tissue