Thymomas are tumors derived from thymic epithelial cells and contain various proportions of benign lymphoid proliferating cells. Located in the ventral cervical region or cranial mediastinum, these tumors are usually well circumscribed and rarely metastasize.1 Thymomas have been reported in several species, including humans, dogs, cats, horses, pigs, cows, and goats.2–7 Considered a rare tumor in most species, thymomas appear to be more common in goats. In a large study8 involving goats submitted for necropsy, thymomas were identified in 9 of 100 (9%) goats with tumors, representing the third most common tumor type. Likewise, in another study,9 thymomas were incidental findings in 17 of 67 (25%) dairy goats > 2 years of age submitted for necropsy.
The clinical importance of thymomas in goats is largely unknown. In dogs, thymomas are typically found in the cranial mediastinum.10 Tumors in this location are often associated with clinical signs and effects, such as pleural effusion, respiratory distress, and sudden death. Myasthenia gravis and megaesophagus are also common in dogs with cranial mediastinal thymomas.1,11 Conversely, in the few reports8,9 of thymomas in goats, these tumors are described as incidental findings at necropsy. This gives the impression that thymomas rarely cause clinical disease even if located in the mediastinum8,9; however, individual case reports12–14 exist of clinical disease in goats attributable to thymomas.
Treatment options for thymomas include tumor excision, radiotherapy, and chemotherapy.7,15,16 Excision is the treatment of choice for humans, with adjunctive radiotherapy used to treat certain subtypes.16 In 20 dogs and cats with thymomas, excision alone resulted in a favorable long-term outcome, with 1-year survival rates of 64% for dogs and 89% for cats.6 The purpose of the study reported here was to characterize the anatomic location and clinical signs of thymomas in goats and obtain long-term follow-up information on a subset of goats treated by tumor excision. We hypothesized that in goats, thymomas would be located in the ventral cervical region or mediastinum and that complete excision as the sole treatment would result in long-term (> 1 year) survival without the need for additional treatment.
Materials and Methods
Case selection criteria
Medical records of all goats that received a diagnosis of thymoma between January 1, 1990, and December 31, 2014, at the Cornell University Hospital for Animals were reviewed. Goats were included in the study if they had been admitted to the hospital for treatment and a histologic diagnosis of thymoma had been made following tumor biopsy or excision or at necropsy. Goats submitted directly to the necropsy service during this period were not included.
Medical records review
Data extracted from medical records included signalment, history, physical examination findings, diagnostic imaging results, thymoma size and location, treatment, and outcome. Follow-up information was obtained by telephone and email contact with the owners and by review of medical records at subsequent patient visits when available.
Results
Animals
Medical records of 17 goats with a clinical diagnosis of thymoma during the study period were reviewed. Four of these goats were excluded because a diagnosis of thymoma was not histologically confirmed (2 goats) or histologic examination led to an alternative diagnosis (thymic cyst in one goat and thymic carcinoma in another goat). Consequently, 13 goats were included in the study.
Signalment
Eight of the 13 included goats were females, 4 were castrated males, and 1 was intersex. Goats were classified as mixed breed (n = 5), Pygmy (4), Alpine (2), Nigerian Dwarf (1), and LaMancha (1). Median age at diagnosis of thymoma was 9.5 years (range, 3 to 12 years); the age of 1 castrated male goat was unknown. Median body weight at diagnosis was 53 kg (116.6 lb; range, 31 to 112 kg [68.2 to 246.4 lb]).
History
Clinical signs that prompted owners to seek veterinary attention could broadly be categorized into respiratory signs or the appearance of a mass in the ventral cervical region. Nine goats were evaluated primarily because of respiratory signs that included tachypnea (n = 6) or acute respiratory distress (3). Other historical findings for these goats included a decrease in lung sounds on auscultation (n = 3), muffled heart sounds (2), coughing and weight loss (1), and signs of lethargy (1). One goat with tachypnea had received a tentative diagnosis of a mass in the thorax by the referring veterinarian. Another goat with tachypnea and signs of discomfort had been referred for suspected urinary tract obstruction. The remaining 4 goats were initially evaluated because of a visible and palpable mass in the ventral cervical region near the thoracic inlet that had grown in size. Owner-reported duration of clinical signs for these 4 goats prior to evaluation ranged from acute onset to 8 months. Goats with respiratory signs were typically referred for evaluation sooner after clinical signs were first noticed (mean, 24 days) than were goats with a mass in the ventral cervical region (mean, 120 days).
Physical examination
Physical examination findings for the 9 goats evaluated because of respiratory signs included tachypnea (n = 9), a decrease in lung sounds on auscultation (6), tachycardia (3), and high rectal temperature (2). In addition to tachycardia and tachypnea, 1 goat had a grade 4/5 systolic heart murmur, with the point of maximal intensity on the left side. Examination findings for the 4 goats without respiratory signs were unremarkable, except for a conspicuous soft tissue mass palpable in the ventral cervical region, just cranial to the thoracic inlet and ventral to the trachea. One of the 9 goats initially evaluated because of respiratory signs (goat A) was also noted to have an external mass just cranial to the thoracic inlet.
Mass location
Nine goats had a mediastinal mass, and 5 had an externally visible ventral cervical mass (Figure 1). These counts included 1 goat with a mass in both locations (goat A).
Diagrammatic summary of treatments and outcomes for 14 thymomas in 13 goats on the basis of initial owner complaint and mass location. Numbers in parentheses represent the number of goats with a given characteristic. The 1 goat with a mediastinal mass identified as discharged from the hospital was lost to follow-up. *One goat (goat A) had both a ventral cervical and mediastinal mass and is represented twice.
Citation: Journal of the American Veterinary Medical Association 251, 7; 10.2460/javma.251.7.829
Diagnostic imaging
Ultrasonography was performed for 12 of the 13 goats. Eight goats underwent a complete thoracic ultrasonographic examination owing to respiratory signs. Clinically important abnormal findings in these goats included pleural effusion (n = 5), a visible mass in the cranioventral region of the thorax (5), and apparent caudal displacement of the heart (2). Five goats had only 1 ultrasonographic abnormality, 2 had 2 abnormalities, and only 1 goat had all 3 of the listed abnormalities.
In the remaining 4 goats, ultrasonographic examinations were focused on the external cervical mass. In this group, ultrasonography consistently revealed a discrete, well-encapsulated mass with mixed echogenicity often containing distinct anechoic fluid pockets (Figure 2). The masses were consistently located just cranial to the thoracic inlet, close to the carotid arteries and jugular veins. The goat with both a ventral cervical mass and respiratory signs (goat A) underwent both thoracic ultrasonography and an ultrasonographic examination of the cervical mass.
Lateral thoracic radiography was performed for 7 goats (6 initially evaluated because of respiratory signs and 1 because of a mass in the ventral cervical region). Six goats with respiratory signs (including goat A) underwent radiographic examination because of these signs, which revealed a soft tissue mass in the cranioventral region of the thorax that displaced the trachea dorsally (Figure 2). The other goat underwent thoracic radiography to rule out thoracic involvement, and no intrathoracic abnormalities were identified, although the external soft tissue mass was visible just cranial to the thoracic inlet. None of the goats had radiographic evidence of megaesophagus, which may have suggested concurrent myasthenia gravis as a complication.
Ultrasonographic (A), radiographic (B), and CT (C) images of the thorax of a 10-year-old, mixed-breed intersex goat with 2 thymomas (goat A). A—Ultrasonography reveals a well-defined ventral cervical thymoma (asterisk) with mixed echogenicity that is close to the carotid artery (arrow). B—A lateral radiographic image reveals a thymoma in the cranial mediastinum (asterisk). The trachea is pushed dorsally, and the cranioventral lung field is obscured by a soft tissue mass. C—A sagittal CT image reveals the cranial mediastinal thymoma (asterisk) and ventral cervical thymoma (carets). Both masses are well-defined, with mixed soft tissue and fluid attenuation.
Citation: Journal of the American Veterinary Medical Association 251, 7; 10.2460/javma.251.7.829
Two goats underwent contrast-enhanced CT examination of the ventral cervical region and thorax to further determine the extent of the mass. One goat had a ventral cervical mass, and the CT scans were used for surgical planning and to check for a concurrent intrathoracic mass. For the other goat (goat A), the CT scans were used to determine a treatment plan. All 3 masses were similar in appearance, with soft tissue attenuation and large fluid-filled regions. The soft tissue components of the masses were heterogeneous in appearance and moderately contrast enhanced.
Treatment
Complete excision of the thymomas was attempted in 6 goats. Ventral cervical masses were successfully removed from 4 of 5 goats, including goat A. In goat A, the ventral cervical mass was surgically removed without complication and the intrathoracic mass was treated by external beam radiation (6-MV photons and 4 Gy/fraction, every other day for 8 treatments). In the fifth goat, the cervical mass could not be completely removed because of its proximity to the carotid arteries. This mass contained a large fluid-filled cavity and was surgically opened, its lining was sutured to the skin to allow for continuous drainage, and the resulting wound was left to heal by second intention.
The sixth goat in which the thymoma was surgically removed underwent thoracotomy between the fourth and fifth ribs on the right side to treat a mediastinal mass (goat B). This large mass (20 × 15 × 10 cm) was successfully removed, but multiple smaller masses (2 × 1 × 1 cm) were also identified within the thoracic cavity during surgery. Attempted excision of several of these masses resulted in considerable hemorrhage, so surgery was aborted.
Five goats with mediastinal thymomas were stabilized while mass biopsy results remained pending. Stabilization and supportive care included antimicrobial administration (n = 3), thoracocentesis to drain pleural fluid (2), IV fluid administration (1), intranasal oxygen supplementation (1), and a tapering course of dexamethasone (1). Median duration of hospitalization for the 5 goats with mediastinal thymomas that were treated with supportive care was 3 days (range, 2 to 10 days). Two goats with mediastinal thymomas were euthanized shortly after hospital admission without treatment.
Histologic examination
Results of histologic examination of all masses were consistent with a diagnosis of thymoma. Tumors generally consisted of encapsulated, densely cellular, round masses composed of cords of lymphocytes separated by bundles and sheets of spindloid neoplastic cells (Figure 3). Mild to moderate anisocytosis and anisokaryosis were observed, and mitotic activity was absent to rare in all histologic specimens. Large cystic spaces lined by the neoplastic cells were also common.
Photomicrograph of a tissue specimen obtained from the ventral cervical mass in the goat in Figure 2. The tissue is composed of thymic lymphocytes (arrow) and spindle-shaped neoplastic cells (caret), both of which are typical of thymomas. H&E stain; bar = 50 μm.
Citation: Journal of the American Veterinary Medical Association 251, 7; 10.2460/javma.251.7.829
Short-term outcome
Five of the 6 surgically treated goats (including all 5 with a ventral cervical mass [one of which was goat A]) survived until hospital discharge (Figure 1). The nonsurvivor was the goat with a mediastinal mass for which thoracotomy was performed (goat B). Considerable hemorrhage had been observed in this goat during surgery, and it subsequently died in the recovery period.
Of the 5 goats with a mediastinal mass for which stabilization and supportive care was pursued, 4 were euthanized once biopsy results confirmed that the mass was neoplastic. For most owners, the decision to euthanize was based largely on concerns about quality of life. The 1 goat receiving supportive care that was not euthanized was discharged from the hospital before biopsy results were available and was discharged from the hospital with a 2-week course of antimicrobials (procaine penicillin G) and a 3-week, tapering course of dexamethasone. This goat was lost to follow-up.
Necropsy
With the exception of the goat that died following thoracotomy (goat B), necropsy findings were available for all 6 goats with mediastinal masses that were euthanized in the hospital. All 6 goats had serosanguinous pleural effusion (total volume ranging from 50 mL to 4 L), a large space-occupying mass in the cranioventral mediastinum (size range, 20 × 12 × 5 cm to 25 × 15 × 15 cm), and various degrees of adhesions located between the mass and cranioventral lung lobes and the mass and pleural surface. One goat had additional, previously undetected, smaller masses along the pleural surface of the 8th through 13th ribs on the right that were histologically consistent with a thymoma.
Long-term outcome
Long-term follow-up information was available for 5 of 6 goats that were discharged from the hospital (including goat A). Three goats (including goat A) were still alive at the time of final follow-up (2 still alive 1.5 years after surgery and 1 still alive 1 year after surgery). Goat A underwent a second CT examination after the sixth radiotherapy treatment, which revealed that the mediastinal mass had slightly decreased in size (0.5-cm decrease in all dimensions) and there was no evidence of recurrence of the ventral cervical mass. This goat returned for regular recheck examinations, and no recurrence of the cervical mass was observed. It continued to intermittently cough after eating but had negative results of myasthenia gravis (antibody against acetylcholine receptors) testing, and no change in the radiographic and ultrasonographic appearance of the lungs was observed over the 1.5-year follow-up period. The other 2 surviving goats eventually died but lived for at least 1 year following surgery; the exact duration of this survival period was unclear. Both goats were reported to have died of causes unrelated to thymoma.
Discussion
Outcomes in the present study suggested that the anatomic location of thymomas was an important factor influencing survival time in affected goats. All 5 goats with ventral cervical thymomas underwent complete tumor excision. All survived until hospital discharge, and none had a recurrence of the thymoma. In contrast, only 2 of 9 goats with mediastinal thymomas survived to hospital discharge. Although successful outcomes have been reported for excision of mediastinal thymomas in dogs, cats,6 and humans,17 the same have not been reported for goats. This could be because of the severity of clinical signs in affected goats, the potential for complications associated with tumor excision, or an unwillingness of goat owners to pursue costly, invasive surgery. In most goats with mediastinal thymomas in the present study, tumor excision was not even attempted. Therefore, it remains unknown whether excision of thymomas in this anatomic location would be as successful as it is in other species.
One goat (goat A) in the present study was treated with external beam radiotherapy after the ventral cervical mass was excised. This decision, as opposed to surgery for the mediastinal mass, was made because of the size of the mediastinal mass (9 × 9 × 7 cm), its proximity to major blood vessels, and the hazards associated with intrathoracic surgery. To the authors’ knowledge, no reports exist of the use of external beam radiotherapy to treat thymomas in farm animals, but findings in human and small animal studies15,16 suggest that thymomas are sensitive to radiotherapy. The treatment goal for goat A was palliation as opposed to complete resolution of the mediastinal thymoma. Although the decrease achieved in the size of the mediastinal mass was minimal after treatment, the goat improved clinically throughout the course of radiotherapy. This clinical improvement may have been confounded by the fact that this goat had fairly mild clinical signs (tachypnea and intermittent coughing) and had also been treated for bronchopneumonia prior to radiotherapy. None of the reported complications associated with radiotherapy (eg, pulmonary fibrosis, thrombosis of thoracic blood vessels, or damage to heart, lungs, trachea, or esophagus) occurred in this goat.
Diagnostic imaging was used for all goats in the study reported here to facilitate surgical planning and determine prognosis. In humans and now dogs, CT examination is considered the gold standard for imaging the mediastinum.7,16 Only 2 goats in the present study underwent CT examination. One of these goats had a ventral cervical mass, and CT was used for surgical planning because of the proximity of the mass to major blood vessels. The other goat had an intrathoracic mass, and CT was used for both diagnostic purposes and to guide targeted radiotherapy.
When CT examination was not economically feasible, the combination of ultrasonography and thoracic radiography was the most useful diagnostic procedure in the study reported here. Ultrasonographically, all imaged thymomas had a characteristic appearance that aided the tentative diagnosis of thymoma. In goats with external masses, the mass and surrounding area were easy to see and major blood vessels could be identified for presurgical planning. Radiographically, thymomas had characteristics similar to those reported for cranial mediastinal masses.18 We therefore recommend that thoracic radiography be included in the diagnostic workup for goats with a suspected ventral cervical thymoma to rule out the presence of a concurrent mediastinal mass.
Clinical signs of goats in the present study were largely dependent on thymoma location. Goats with mediastinal masses had more severe clinical signs, such as respiratory distress and pleural effusion, than those with ventral cervical masses. This finding was consistent with findings in small animals with mediastinal masses but contradicts the limited published information for goats, in which thymomas were incidental findings at necropsy.9 In the present study, most goats with a mediastinal thymoma underwent necropsy once the owners elected to euthanize. Necropsy revealed substantial thymoma-associated pathological changes, including pleural effusion, extensive adhesions, and regions of atelectasis. Most thymomas were larger than those previously reported,9 and differences in thymoma size at the time of necropsy could explain the nature of the associated clinical signs.
Findings of the present study highlighted that in goats, thymomas can form in the ventral cervical region, mediastinum, or both locations. In dogs and cats, thymomas have been reported only in the mediastinum.6,7 The difference in location is likely due to developmental differences among species. In ruminants, the thymus starts as a large, lobulated gland that extends from the larynx to the pericardium. As ruminants age, the gland regresses and the cervical portion ultimately disappears within the first year after birth.19 In dogs, on the other hand, the thymus is completely confined to the thorax and there is no cervical portion.20
The low number of goats in the study reported here precluded drawing of conclusions regarding age or breed predilections. Nevertheless, several interesting patterns were identified. Two goats with a ventral cervical mass were only 3 years old. In small animals, thymomas are most likely to develop in older, large-breed dogs and the median age at initial evaluation for thymomas is reportedly 9.5 years (range, 5 to 15 years or 1.5 to 14 years).6,7,21 Because the only previous reports of thymomas in goats involved single cases12–14 or necropsy studies,8,9 it is unknown whether ventral cervical masses are more likely to develop at an earlier age in goats than in other species. Second, previous studies8,9 of thymomas in goats have revealed possible breed predilections, with Saanen and dwarf breeds (Pygmy and Pygora) overrepresented. The present study also included a high proportion of Pygmies (4/13), but a larger study would be needed to confirm any breed predilection given that Pygmies are also a common breed of pet goat, and this may have influenced these results.
Thymomas in goats are still considered rare tumors, as highlighted by the small number of cases in the present study and the lack of any recent literature on the subject. The present study could have been biased given that only goats with severe clinical signs or goats with a palpable ventral cervical mass large enough to be noticed by the owner were evaluated at our referral institution. The benign nature of thymomas and the low apparent incidence in goats raise the question as to whether these tumors are actually rare or whether most thymomas go undiagnosed or untreated. Results of the present study suggested that goats with ventral cervical thymomas had a favorable prognosis when treated by excision, with all goats affected surviving ≥ 1 year after surgery. Conversely, goats with thymomas in the cranial mediastinum were more likely to have more severe clinical signs and a guarded prognosis.
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