*Corresponding author: Hojung Choi College of Veterinary Medicine, Chungnam National University, 99 Daehak-ro, Yuseong-gu, Daejeon 34134, Korea Cardiac tumors are rare in dogs. Previous studies have reported an incidence ranging from 0.12% to 4.33%, with one retrospective study estimating it at 0.19% [1]. Conflicting reports exist regarding whether primary or metastatic tumors are more common, with secondary cardiac tumors reported to occur in 16% to 86% of all cardiac neoplasms [1,2].
Cardiac tumors typically affect middle-aged to older dogs, but lymphoma may also occur in younger patients. Hemangiosarcoma is the most frequently diagnosed cardiac tumor in dogs, followed by aortic body tumors (such as chemodectoma and paraganglioma), lymphoma, and ectopic thyroid carcinoma.
The anatomic location and imaging characteristics of a mass can provide important diagnostic clues, because the fine-needle aspiration or biopsy of intracardiac masses has traditionally been regarded as a technically difficult and potentially high-risk procedure. Hemangiosarcoma is predominantly reported in the right atrium and right atrial appendage, whereas chemodectomas, which are classified as neuroendocrine carcinomas, occur primarily at the heart base [3]. Cardiac metastases have been reported in the left ventricular free wall or the interventricular septum, or both [2].
This case report describes the imaging characteristics of a primary neuroendocrine carcinoma located in an atypical region of the heart in a dog.
A nine-year-old castrated male Chihuahua weighing 1.95 kg presented with dyspnea and anorexia. The patient had a history of thyroid gland follicular carcinoma diagnosed three years earlier following the resection of a right thyroid tumor. Eighteen months earlier, a mass was palpated in the right neck, prompting a computed tomography (CT) scan because of a suspected recurrence. A second surgical resection was performed, but no histopathological examination was performed at that time.
Thoracic radiography showed a poorly defined cardiac silhouette caused by a marked pleural effusion. Thoracocentesis was performed before further imaging studies, yielding 92 mL of fluid. The fluid was serous, slightly turbid, and consistent with a modified transudate. The cytology suggested an epithelial origin, with evidence of anisokaryosis and multinucleated cells displaying malignant characteristics.
Echocardiography showed that the coronary sinus region was severely dilated and filled with an echogenic mass. This mass protruded into the right atrium through the coronary ostium, forming a large oval-shaped mass that crosses the tricuspid valve and moves back and forth between the right ventricle and right atrium with each heartbeat (Fig. 1A-C). The other side of this mass was related to the right ventricular wall mass, which appeared to be thickened and heterogeneous. A small amount of anechoic pericardial effusion was observed around the ventricular wall (Fig. 1D). The E peak velocity of the transtricuspid inflow was increased relative to transmitral inflow, because of an obstruction by the right atrial mass. The abdominal ultrasound revealed dilation of the caudal vena cava and hepatic veins, along with a small amount of ascites, indicative of right heart failure.
Subsequently, a thoracic CT examination (Alexion; Canon Medical Systems, Japan) was performed to evaluate the cardiac mass. The patient was positioned in sternal recumbency under general anesthesia. Anesthesia was induced with propofol at a dose of 6 mg/kg administered intravenously and maintained with isoflurane in oxygen. The CT parameters were as follows: 120 kV peak, 150 mAs, 1.0 mm slice thickness, 1.0 mm interval, 0.75 s rotation time, and 0.938 collimation beam pitch. Postcontrast CT images were acquired at 7, 30, and 90 s after an intravenous injection of 600 mg iodine/kg iohexol (Omnipaque; GE Healthcare, Ireland). Similar to the echocardiographic findings, the CT images revealed a heterogeneous, low-attenuating mass within the right atrium that was related to the mass in the dilated coronary sinus and along the right ventricular wall (Fig. 2). A right ventricular wall mass was bulging outward, containing multiple hypodense areas (Fig. 3). In addition, the CT images showed cranial mediastinal lymph node enlargement and pleural masses indicative of a metastasis.
Considering the size of the cardiac mass, it was suspected to have originated from the right ventricle or the coronary sinus. Metastatic thyroid carcinoma and primary hemangiosarcoma were the foremost considerations given the patient’s history, particularly because hemangiosarcoma most commonly arises from the right atrial/auricular region. Other differentials for a right ventricular wall mass included ectopic thyroid carcinoma and, less commonly, intracardiac lymphoma. Although chemodectoma typically occurs at the base of the heart, its overall incidence warrants inclusion in a differential diagnosis.
The patient underwent a thoracotomy, during which a mass was found on the right ventricular wall (Fig. 3A), and a biopsy was performed. The dog died during recovery from anesthesia. A histopathological examination revealed a malignant tumor of uncertain cellular origin, leading to differential diagnoses of ectopic thyroid carcinoma and neuroendocrine carcinoma (Fig. 3B). Immunohistochemical testing was performed to characterize the tumor. Thyroglobulin staining was negative on the tumor cell population (with positive staining in control tissue), excluding ectopic thyroid carcinoma. Mild to moderate chromogranin A positivity supported a neuroendocrine origin, and strong diffuse staining for cytokeratin AE1/AE3 helped exclude other spindle or round cell neoplasms, such as lymphoma, histiocytic sarcoma, and hemangiosarcoma. Based on these results, the tumor was definitively diagnosed as a primary neuroendocrine carcinoma.
In dogs, cardiac neuroendocrine carcinomas most commonly arise at the heart base and are classified as either endodermal (epithelial) or neuroectodermal in origin [1]. Tumors of an endodermal origin arise from epithelial endocrine tissues such as the pituitary and parathyroid glands, thyroid C cells, pancreatic islets, and diffuse neuroendocrine cells distributed throughout luminal organs such as the gastrointestinal tract (so-called carcinoid tumors). In contrast, neuroectodermal tumors derive from chromaffin cells of the adrenal medulla or from extra-adrenal paraganglia [4]. Paraganglia are broadly classified as sympathetic or parasympathetic structures. The adrenal medulla represents a sympathetic paraganglion, whereas parasympathetic paraganglia include chemoreceptive organs such as the aortic and carotid bodies, the vagal body, and smaller collections of cells located within the thoracic, abdominal, or retroperitoneal regions [4]. Chemodectomas and paragangliomas are two major tumor types originating from these chemoreceptive tissues in dogs. Although the two terms are sometimes used interchangeably in the veterinary literature, they denote distinct entities. Chemodectomas are typically nonfunctional and lack catecholamine secretion, whereas paragangliomas often exhibit catecholamine-producing activity [4,5].
In dogs, chemodectoma is the most common type of cardiac tumor and is typically located at the heart base. The term “heart base tumor” is used to describe any mass lesion located at the base of the heart, which is the craniodorsal portion of the heart that receives the great veins and emits the great arteries [6]. According to a study analyzing CT images of heart-base tumors in dogs and cats, most cases were found between the aortic arch and the cranial vena cava, namely in the cranial mediastinum, or dorsally at the pulmonary trunk bifurcation [6].
In contrast to veterinary medicine, neuroendocrine carcinomas of the heart in humans are predominantly metastatic [7]. Upon reviewing the sites of occurrence in the reported cases, the tumor can arise in all four chambers of the heart [7-9].
Tumors involving the coronary sinus are extremely rare. Ten cases of coronary sinus tumors have been reported in the human literature, comprising six metastatic and four primary tumors [10-19]. Among the metastatic cases, the primary sites included thymoma, renal cell carcinoma, and uterine squamous cell carcinoma, all of which invaded the right atrial cavity via the coronary sinus ostium [12,14,16-19]. Of the primary tumors, histopathological diagnoses included hemangioma, lymphoma, myxoma, and a presumptive angiosarcoma [10,11,13,15]. Specifically, two of these cases showed an extension into the right atrium via the coronary sinus ostium [13,15]. In veterinary medicine, however, there have been no reports of a primary tumor located in the coronary sinus, even though one case of coronary sinus invasion by an aortic body tumor has been documented [11].
The limitation of this case is that biopsy and histopathological examination were not performed on the mass within the right atrium or the coronary sinus, preventing a definitive determination of its exact origin. Nevertheless, immunohistochemical analysis of the right ventricular wall mass supported the diagnosis of primary cardiac neuroendocrine carcinoma with suspected extension into the right atrium via the coronary sinus.
In conclusion, this case expands the recognized anatomic spectrum of canine primary cardiac neuroendocrine carcinoma by describing a tumor arising from the right ventricle, as demonstrated by echocardiography and CT.
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