Histologic and immunohistochemical characterization of thymic epithelial tumours in the dog

Prognostic factors and outcome in cats with thymic epithelial tumours: 64 cases (1999-2021)

OBJECTIVES

To describe the clinical presentation, treatment and outcomes of cats diagnosed with thymic epithelial tumours and to determine prognostic factors for survival and recurrence.

MATERIALS AND METHODS

Clinical records of cats diagnosed with a thymic epithelial tumour between 1999 and 2021 at three referral institutions were retrospectively reviewed.

RESULTS

Sixty-four cats were included. Paraneoplastic syndromes were present in nine cats and metastatic disease was seen in two cats, one at diagnosis and one at the time of recurrence. Median tumour diameter was 6 cm (range, 2 to 15) and a cystic appearance was described on imaging in 25 cats. Surgical excision was attempted in 54 cats with a perioperative mortality rate of 11%. Median survival time for cats surviving to hospital discharge was 897 days (range, 21 to 3322). The 1-, 2- and 5-year survival rates for surgically treated thymic epithelial tumour were 86%, 70% and 66%, respectively. Survival was longer for cats with Masaoka-Koga stage I and II tumours compared to stages III and IV (1366 days versus 454 days; P=0.002). Masaoka-Koga stage was the only significant prognostic factor detected on multi-variable analysis, with stage III and IV tumours associated with increased risk of death (hazard ratio: 5.67, 95% confidence interval: 1.29 to 24.91, P=.021). Tumour recurrence occurred in 11 cats at a median of 564 days (range, 93 to 1095); no significant prognostic factors for recurrence were identified.

CLINICAL SIGNIFICANCE

Cats with thymic epithelial tumours had a good long-term prognosis following surgery. Tumour recurrence can occur late in the disease course and ongoing monitoring should therefore be considered. Masaoka-Koga stage may influence survival time and could be used to predict outcome.

Spindle-cell thymoma colliding with a bronchogenic cyst in a Yorkshire terrier

An 8-year-old male Yorkshire terrier was presented to the Tufts Veterinary Hospital for evaluation of increased respiratory effort. A mediastinal mass composed of a spindle-cell thymoma within a bronchogenic cyst was diagnosed with computed tomography thoracic imaging, ultrasound-guided fine-needle aspirate biopsy, and histopathologic evaluation after surgical removal. Histologic evaluation showed a multilocular cyst structure as well as a mass characterized by spindle to polygonal thymic epithelial cells. The cyst was characterized by a lining of ciliated pseudostratified respiratory epithelium. To the authors' knowledge, this is the first report of a spindle-cell thymoma being associated with a mediastinal bronchogenic cyst in a dog.

Canine Epithelial Thymic Tumors: Outcome in 28 Dogs Treated by Surgery

Simple Summary

Thymoma is a tumor of the cranial mediastinum rarely reported in dogs and should be differentiated from mediastinal lymphoma. CT and fine-needle aspirates or core biopsies are helpful in differential diagnosis, but flow cytometry may improve the pre-operative diagnostic ability. In thymomas, paraneoplastic syndromes such as myasthenia gravis and hypercalcemia may develop concurrently with the tumor. Their role as prognostic factors is not well determined. Surgical excision is the treatment of choice, but adjuvant radiotherapy and/or chemotherapy may prolong survival in cases of incomplete excision or if a thymic carcinoma is diagnosed. Local recurrence and metastasis are infrequently reported; therefore, a long survival is expected if the tumor is completely excised or if adjuvant therapy is undertaken. This article reports the authors’ experience with 28 dogs affected by 18 thymomas and 10 thymic carcinomas surgically treated from January 2000 to August 2021. The median overall survival time of the entire population was 1137 days; the median disease-free time was 903 days. Dogs with thymic carcinomas had significantly shorter disease-free intervals and shorter, although not statistically significant, survival times. Dogs with Masaoka Stage III tumors had worse outcomes.

Abstract

Thymoma is a tumor rarely reported in dogs and should be differentiated from mediastinal lymphoma. Clinical signs may have a late onset, and thymoma is often diagnosed when symptoms related to the space-occupying effect or paraneoplastic syndromes occur. CT and fine-needle aspirates or core biopsies are helpful in differential diagnosis, but flow cytometry may improve the pre-operative diagnostic ability. Concurrent paraneoplastic syndromes such as myasthenia gravis and hypercalcemia have been reported; however, their role as prognostic factors is not well determined. Surgical excision is the treatment of choice; adjuvant radiotherapy and/or chemotherapy may prolong survival in cases of incomplete excision or when a thymic carcinoma is diagnosed. Local recurrence and metastasis are infrequently reported; therefore, a long survival time is expected if the tumor is completely excised or if adjuvant therapy is undertaken. This article reports the authors’ experience with 28 dogs affected by 18 thymomas and 10 thymic carcinomas. The median overall survival in this series was 1173 days, and the median disease-free interval was 903 days. Dogs with thymic carcinoma had significantly shorter disease-free intervals and shorter, although not statistically significant, survival times. Dogs with Masaoka Stage III tumors had worse outcomes.

Thymic epithelial tumours in 51 dogs: Histopathologic and clinicopathologic findings

Canine thymic epithelial tumours (TET) are uncommon and little is known about their behaviour. Previous attempts at histologic classification have varied, and as such reliable prognostic information is unavailable. The aim of this retrospective multi-institutional study was to evaluate cases of canine TETs, irrespective of subtype, in order to identify useful histopathologic and clinicopathologic prognostic factors. Cases were included if the tumour arose from the cranial mediastinum and a diagnosis of TET was made on the basis of histopathology. Fifty-one dogs were included. In addition to clinicopathologic data, histology samples were reviewed for the following features: mitotic count, percentage of necrosis, presence of Hassall's corpuscles, lymphocytic infiltrate, cellular pleomorphism and vascular or capsular invasion. The median survival time for all dogs was 449 days. The 1- and 2-year survival rate was 52.6% and 26.3% respectively. On multivariable analysis surgical excision of the thymic tumour was associated with significantly prolonged survival; the presence of metastasis, myasthenia gravis and moderate or marked cellular pleomorphism were associated with significantly reduced survival. Additional studies are needed to further evaluate prognostic factors to aid treatment recommendations.

Immunophenotypic Characterization of Canine Nodal T-Zone Lymphoma

T-zone lymphoma (TZL) is an indolent nodal T-cell lymphoma most commonly observed in submandibular lymph nodes in dogs. The diagnosis is based on its distinct morphology and expression of CD3. TZL has been reported to have a low Ki67 index and to lack expression of CD45. The latter feature has been used to diagnose this type of lymphoma via fine needle aspirate and flow cytometry without confirmation of the characteristic tissue architecture. The goal of this study was to characterize the immunophenotype of canine nodal TZL in greater detail. Twenty-seven TZLs were selected based on their characteristic morphology. A tissue microarray was generated, and immunohistochemical expression of CD3, CD5, CD20, CD21, CD25, CD45, Bcl-6, and Ki67 was evaluated. Neoplastic T cells in all cases were positive for CD3, CD5, and CD25, and negative for CD20, CD21, and Bcl-6. Positive labelling for CD45 was detected in 2 of the 27 cases with the remaining 25 cases being negative. All cases had a low Ki67 index with an average index of 19.56%. For the CD45-positive TZLs, clonality of the T-cell antigen receptor gamma gene was confirmed in only one of these cases. The observed immunophenotype of canine TZL is similar to previous publications with the exception that 2 cases expressed CD45. Expression of CD45 in TZLs in this study emphasizes the importance of interpreting immunophenotypic findings in conjunction with histopathology to reach an accurate diagnosis and not to use lack of expression of a particular antigen as the sole diagnostic criterion.

Cystic thymic degeneration with pseudoepitheliomatous hyperplasia in a Beagle puppy: an idiopathic lesion?

A 6-mo-old female Beagle dog was inappetent and depressed. The radiographic, ultrasonographic, and computed tomographic examination of the chest revealed a 10 × 7 cm multicystic mediastinal structure interpreted as altered thymus, in association with moderate pleural effusion that laboratory tests confirmed as hemothorax. No history of trauma or anticoagulant drug intoxication was reported, and no coagulation disorders were detected. Afterward, medial cranial sternotomy was performed to remove the altered tissue. Histologically, this tissue was compatible with a thymic remnant, characterized by numerous cystic lesions, mostly blood filled and lined by flattened-to-cuboidal epithelial cells, occasionally projecting into the surrounding stroma, and forming cytokeratin-positive ribbons, trabeculae, and papillae. Lymphocytes were scant, and numerous areas of congestion and hemorrhage were present throughout the samples. This case of idiopathic thymic hemorrhage with cystic degeneration of the thymus and pseudoepitheliomatous hyperplasia was an incidental finding; the dog recovered from surgery uneventfully.

Mediastinal lymphoma in dogs is homogeneous compared to thymic epithelial neoplasia and is more likely to envelop the cranial vena cava in CT images

In order to identify CT signs that could be used to distinguish cranial mediastinal lymphoma and thymic epithelial neoplasia, a retrospective case-control study was done. Associations between CT signs and diagnosis were tested using binary logistic regression and results expressed as odds ratio and 95% confidence interval. Sixty-two dogs that had thoracic CT and confirmed diagnosis of lymphoma (n = 33) or thymic neoplasia (n = 29) were sampled. Thymic neoplasms included 24 thymomas and five thymic carcinomas. Dogs with thymic epithelial neoplasia were significantly older than dogs with lymphoma (median age 8.6 years versus 6.0 years, P = .007), but there were no significant differences in prevalence of clinical signs. Diagnosis of thymic epithelial neoplasia was associated with heterogeneous attenuation in pre- (odds ratio 23.3, 95% confidence interval, 4.5-121.1) and post-contrast (odds ratio 30.7, 95% confidence interval, 3.6-265.0) images. Conversely, envelopment of the cranial vena cava by the mass was less likely with thymic epithelial neoplasia than lymphoma (odds ratio 0.07, 95% confidence interval, 0.007-0.66). Greater standard deviation of Hounsfield unit values in post-contrast images was associated with thymic epithelial neoplasia (P = .005). Based on ROC analysis, SD > 17HU of the mass in post-contrast images had a sensitivity of 72% and specificity of 79% for thymic epithelial neoplasia. There were no significant differences in morphology, prevalence of calcification, mediastinal lymphadenopathy, cranial vena cava invasion, collateral vessels, or pleural fluid associated with these tumors. Thymic epithelial neoplasms tended to occur in older dogs and were heterogeneous in CT images, whereas mediastinal lymphoma was more homogeneous and more likely to envelop the cranial vena cava.

CT imaging features of canine thymomas

Canine thymomas have been evaluated based on clinical features, treatment options, surgical excision, and outcomes with limited information on specific CT features. The objective of this retrospective, descriptive, cross-sectional study was to describe the CT characteristics of confirmed thymomas and to compare these imaging features to outcome. A total of 22 dogs met the inclusion criteria of histologically confirmed thymomas with concurrent CT imaging. Tumor size varied widely ranging from small and well-circumscribed to large and invasive. Delayed-phase, contrast-enhanced CT studies were best for determining the degree of contrast enhancement in tumors. Of these, 19 of 22 masses had heterogeneous enhancement and three of 22 masses had homogeneous enhancement. Vascular invasion was present in seven of 22 cases. Larger tumors were associated with vascular invasion (height: P = .04; width and volume: P = .02). On precontrast CT, larger tumors (16/21) were heterogeneous and cystic, with smaller tumors (5/21) being more homogeneous (all values P < .05). A larger size was associated with recurrence in fully resected masses (height: P = .03), but not a shorter outcome (P > .3 for all size dimensions). Postoperative complications and incomplete tumor resection were associated with shorter outcome (both values P < .01). Metastasis was confirmed in four cases. There were six cases with lymphadenopathy noted on CT; five of the six cases did not have evidence of metastasis. Larger tumors were more likely to be cystic and associated with vascular invasion.

Radiographic Differentiation of Cranial Mediastinal Lymphomas from Thymic Epithelial Tumors in Dogs and Cats

In both dogs and cats, the most common cranial mediastinal masses (CMMs) are lymphoma and thymic epithelial tumors (TETs). Pretreatment differentiation of these tumors using fine needle aspiration or biopsy is essential because lymphomas are treated medically, whereas TETs are treated surgically. The purpose of this retrospective study was to determine whether thoracic radiographic findings can be used to aid clinicians in preliminarily differentiating the two tumor types before cytology or histopathology results become available. Medical records, available cytologic or histologic samples, and thoracic radiographs were evaluated for 62 dogs and 28 cats. Seventeen radiographic criteria were assessed by two examiners, and regression modeling was performed to test for significant predictors of tumor type. In dogs, CMMs with at least two well-defined radiographic margins on a lateral view and CMMs causing a rightward shift of the cardiac silhouette on a ventrodorsal or dorsoventral view were significantly more likely to be TETs than lymphomas (P < .001 and P < .01, respectively). No significant predictive variables were identified in cats. Radiographic findings do not eliminate the need for invasive sampling, but in dogs, they may guide the clinician in providing preliminary information to owners regarding the staging and therapeutic measures that may eventually be recommended.

A previously undescribed cutaneous paraneoplastic syndrome in a cat with thymoma

BACKGROUND

Exfoliative dermatitis is a well-recognized cutaneous paraneoplastic syndrome (PNS) associated with thymoma in cats, of which the clinical and histopathological presentation has been well-characterized.

OBJECTIVES

To describe a novel clinical skin manifestation associated with thymoma in a cat.

ANIMAL

A 14-year-old neutered female domestic short hair cat.

METHODS AND MATERIALS

Physical, abdominal ultrasonographic, thoracic radiographic, ultrasonographic and computed tomographic examinations, histopathological assessment of the skin and mediastinal mass.

RESULTS

The cat was presented with noninflammatory alopecia, with a dorsal multifocal distribution. Examination of the alopecic areas using a dermascope indicated an apparent lack of follicular ostia. Histopathological assessment of alopecic areas confirmed follicular and epidermal atrophy, trichilemmal keratinization and mild orthokeratotic hyperkeratosis. Diagnostic imaging revealed a mediastinal mass, which was surgically removed. Histopathological and immunohistopathological examination of the mass was consistent with a thymoma, associated with multiloculated cyst formation and multifocal cholesterol granulomas. Following surgery, hair re-growth was noted in the previously alopecic areas. The cat was euthanized 3.5 months later because of recurrent chylothorax suspected to be a postoperative complication. The alopecic lesions had improved markedly.

CONCLUSIONS AND CLINICAL IMPORTANCE

Thymoma-associated PNS might not always manifest as an exfoliative dermatitis and should be considered in the differential diagnosis of multifocal noninflammatory alopecia.

Immunophenotypic Characterization of Canine Splenic Follicular-Derived B-Cell Lymphoma

Marginal zone lymphoma (MZL) and mantle cell lymphoma (MCL) belong to a subgroup of indolent B-cell lymphomas most commonly reported in the canine spleen. The goal of this study was to characterize the immunophenotype of splenic MZL and MCL in comparison to their human counterparts. Ten MCLs and 28 MZLs were selected based on morphology. A tissue microarray was generated, and expression of CD3, CD5, CD10, CD45, CD20, CD79a, Pax-5, Bcl-2, Bcl-6, cyclin D1, cyclin D3, MCL-1, MUM-1, and Sox-11 was evaluated. Neoplastic cells in all MCLs and MZLs were positive for CD5, CD20, CD45, CD79a, and BCL2 and negative for CD3, CD10, Bcl-6, cyclin D1, and cyclin D3. Positive labeling for Pax-5 was detected in 8 of 10 MCLs and 26 of 28 MZLs. Positive labeling for MUM-1 was detected in 3 of 10 MCLs, and 27 of 28 MZLs were positive for MUM-1. No MCLs but 8 of 24 MZLs were positive for MCL-1. Canine splenic MZL and MCL have a similar immunophenotype as their human counterparts. However, human splenic MCL overexpresses cyclin D1 due to a translocation. A similar genetic alteration has not been reported in dogs. In addition, in contrast to human MZL, canine splenic MZL generally expresses CD5. Following identification of B vs T cells with CD20 and CD3, a panel composed of BCL-2, Bcl-6, MUM-1, and MCL-1 combined with the histomorphological pattern can be used to accurately diagnose MZL and MCL in dogs. Expression of Bcl-2 and lack of MCL-1 expression in MCL may suggest a therapeutic benefit of BCL-2 inhibitors in canine MCL.

Paraneoplastic immune-mediated neutropenia in a dog following thymoma excision with later development of metastatic thymic carcinoma treated with toceranib phosphate

A thymoma was an incidental finding in a seven-year-old, female neutered, crossbred dog referred for an acute hepatopathy. Excision of the thymoma was performed via median sternotomy, and 11 weeks later the dog presented with severe neutropenia. The bone marrow was hypercellular, and paraneoplastic immune-mediated neutropenia was diagnosed following exclusion of other causes. The neutrophil count increased to normal within seven days of treatment with prednisolone, which was subsequently tapered. A caudal mediastinal mass was documented 17 months following initial thymoma excision, and pleural and mediastinal metastases were evident on CT. Cytology was consistent with carcinoma. Following the prescription of toceranib phosphate, there was stable disease on CT at eight weeks and six months. At 12 months there was severe locoregional progression and the dog died nine days thereafter. Postmortem examination showed diffuse metastatic carcinoma of the caudal mediastinum, parietal pleura and diaphragm, and histopathology/immunohistochemistry was most consistent with thymic carcinoma.

Perioperative Mortality and Long-Term Survival in 80 Dogs and 32 Cats Undergoing Excision of Thymic Epithelial Tumors

OBJECTIVE

To examine perioperative mortality, long-term survival, causes of death, and prognostic factors for dogs and cats undergoing surgical excision of thymic epithelial tumors (TETs).

STUDY DESIGN

Multi-institutional case series.

ANIMALS

Eighty dogs and 32 cats.

METHODS

Follow-up information was obtained for dogs and cats that underwent surgical excision of a TET between 2001 and 2012.

RESULTS

Perioperative mortality was 20% in dogs and 22% in cats. No independent risk factors for perioperative mortality were identified. The estimated median survival time for all dogs was 1.69 years (95% CI 0.56-4.32) and the 1- and 4-year survival rates were 55% (95% CI 44-67) and 44% (95% CI 32-56). The estimated median survival time for all cats was 3.71 years (95% CI 0.56-unestimatable) and the 1- and 4-year survival rates were 70% (95% CI 53-87) and 47% (95% CI 0-100). Of animals that survived to discharge, 42% of dogs and 20% of cats eventually died of TET-related causes. The presence of paraneoplastic syndromes (hazard ratio [HR] 5.78, 95% CI 1.64-20.45, P = .007) or incomplete histologic margins (HR 6.09, 95% CI 1.50-24.72, P = .01) were independently associated with decreased survival in dogs. No significant predictors of survival were identified in cats. Conclusions regarding the effect of chemotherapy or radiation therapy could not be made.

CONCLUSIONS

While there is substantial risk of perioperative death in dogs and cats undergoing surgery for TETs, many animals that survive to discharge have prolonged survival. Survival is significantly decreased in dogs with paraneoplastic syndromes or incomplete histologic margins.