RETRACTED ARTICLE: Cervical type AB thymoma (Mixed) tumour diagnosis in a mynah as a model to study human: clinicohistological, immunohistochemical and cytohistopathological study

Thymomas are rare malignancies both in human and veterinary medicine. They have been described in a wide range of domestic and laboratory animals, such as rats and mice [1, 2], and occur more frequently in older animals. In humans, thymomas usually present in the fourth to fifth decade but can occur at all ages without sex predisposition [3]. In dogs and cats, thymomas are also found in older animals [4], and they are rarely reported in avian species and have only been documented in an African grey parrot (Psittacus erithacus),[5] a finch (unspecified species), [6] domestic chickens,[7, 8] a budgerigar (Melopsittacus undulatus), [9] and a Java sparrow (Padda oryzivora) [10].

The etiology of thymomas is not yet understood. In contrast to lymphomas, there is no evidence of a etiology of thymomas in humans and domestic animals. A single report exists that describes a family with high incidence of thymomas and might point to a genetic predisposition [11].

Thymomas are derived from thymic epithelium with variable benign lymphocytic infiltration and thus are classified as lymphocyte-predominant, epithelial-predominant, or mixed [12, 13]. Type AB thymoma (also known as mixed thymoma) accounts for approximately 28% to 34% of all thymomas [14, 15]. Approximately 16% of this type may be associated with myasthenia gravis [14]. Morphologically, type AB thymoma is a thymic tumor in which foci having the features of type A thymoma are admixed with foci rich in nonneoplastic lymphocytes.[16]. Histologically, these tumors are composed of neoplastic thymic epithelial cells and a variable number of lymphocytes leading to a myriad of histologic subtypes depending on the proportion of these elements [17]. This report describes the clinical signs, cytopathological and immunohistichemical characteristics of a thymoma in a Mynah bird and because there is no established classification of thymic neoplasms in veterinary medicine and the avian thymomas bore a striking resemblance to their human counterparts, an attempt to classify this neoplasm according to the new World Health Organization (WHO) classification of human thymic epithelial tumors was made.

Thymomas were tumours arising from or exhibiting differentiation toward thymic epithelial cells. It has been reported that different subtypes of thymoma have multifarious genetic characteristics, recent studies indicated that chromosomal 1 gain plays an significant role in molecular genetic mechanism of thymic epithelium tumors [3, 12, 23]. In a study, Yuqing et al., 2012 suggested that different genes on chromosome 1 might employ different functions in the generation and development of thymic epithelium tumors [12].

Thymic epithelial tumors are rare both in human and veterinary medicine. To our knowledge, this study describes the largest serial of mixed thymoma observed in Mynah bird. In addition to, a description of the morphologic findings, immunohistochemical and cytohistopathological investigations were performed on this tumor with their human counterparts. Because of the high resemblance of the avian thymomas to their human counterparts, the current human WHO classification for thymic epithelial tumors was used.

The avian thymus gland consists of 7 flattened lobes of tissue that are located bilaterally in the subcutis of the neck adjacent to the trachea [24]. Thus, thymomas in birds occur cranial and ventrolateral to the thoracic inlet. In mammals, the thymus gland is located within the mediastinum in the thoracic cavity [25]. The biological behaviors of thymomas are benign and the neoplasia arises from the epithelial portion of the thymus [26].

In all avian species, the differential diagnosis for a cervical swelling is fairly extensive and includes foreign body reaction, trauma, fungal granuloma, abscess, and neoplasia [5]. In this mynah, cervical swelling was observed 4 weeks before presentation and had progressively increased in size over that period. Radiographs performed at that time demonstrated mineral opacities in the lateral cervical region, and degenerative changes in right tarsal joint and inflammation of its adjacent soft tissues accompanying mineralization of its external soft tissue were observed. In addition, hepatomegaly was recorded as well.

In all previously reported avian thymomas, limited presurgical diagnostics were performed before attempted mass resection. Ideally, a complete diagnostic work-up consisting of complete blood count, plasma biochemical analysis, radiographs, and computed tomography would be performed before surgical exploration.Fine-needle aspirate (FNA) with cytologic evaluation or biopsy are useful diagnostic modalities that can be used for an accurate preoperative diagnosis.

FNA biopsy has gained increasing acceptance as a rapid, noninvasive, and effective diagnostic procedure in the investigation of cervical masses [27, 28]. Because thymomas are uncommon neoplasms, experience with the cytologic diagnosis of these tumors is limited. To our knowledge, the correlation between cytologic findings of thymomas and various histologic classification has not been well studied previously. Ali and Erozan [26] found that it was possible to correlate the FNA findings with histologic subtypes determined on resection with adequate well preserved material. In the current study, aspirates with a high L:E ratio had a tendency to belong to predominantly mixed.

Tao et al. [28] suggested a classification based on the size, shape, and pleomorphism of the epithelial component, which has been shown to have prognostic value. Riazmontazer et al. [29] also reported a case of invasive thymoma with atypical cytologic features in the aspirate. They described invasive malignant thymomas with cytologic atypia. Our data generally are compatible with these observations. The more atypical the neoplastic cells are, the more likely the tumor will display aggressive behavior.The cytologic features of thymomas include dual lymphoid and epithelial cellular populations and unique neoplastic tissue fragments that reflect histology and allow their accurate identification.

The cytologic diagnosis of thymoma can be extremely challenging. In part, this is because a technically proficient interventional radiologist is needed, epithelial cells may be difficult to recognize in lymphoid rich aspirate smears, and there is inherent sampling error in a tumor that frequently displays heterogeneous histopathology [16, 30].

Immunohistochemistry for cytokeratin is helpful in this case, since the presence of rare epithelial cells in serial sections is suggestive of thymoma [31, 32]. Cytokeratin profiles have been established in human medicine for the thymus and thymomas, and have been shown to be clinically useful in determining the invasive potential of these neoplasms [16]. Since a cytomorphologic and histologic classification of thymomas seems not to be a useful prognosticator in animals, the use of a pan-specific cocktail of antibodies for cytokeratins is sufficient for the diagnosis of these tumors in veterinary medicine.

Thymoma should be differentiated from other anterior mediastinal neoplasms with epithelial and/or lymphoid differentiation, including Non-Hodgkin (NHL) and Hodgkin lymphomas, thymic carcinomas, and germ cell malignancies. NHL and Hodgkin lymphoma can be separated from thymoma by their dispersed cell population, distinctive cytologic features, and positive staining for CD45, CD20, CD15, and CD30, and negative staining for CK14,CK18, CK20 respectively. Helpful cytologic and immunocytochemical features in making the diagnosis of thymic carcinoma are clear-cut cytological atypia, absence of immature lymphocytes (CD3 + .CD1a+, CD99+), and expression of CD5 and CD70 by neoplastic epithelial cells [12].

Immunohistochemical markers can be used to differentiate the epithelial cells and lymphocytes, aside from the proportion of both cells type [31].In this case, the round cells of the tumor were all positive for CK 14, CK18 and negative for CK10. Based on the WHO and veterinary classification of thymomas [32], this particular case is categorized as mixed type. Overall, the present study confirms previous observations [29, 30] that FNA of anterior mediastinal thymic lesions generally yields adequate cellular tissue with distinct cytologic and immunophenotypic features that enables thymoma diagnosis.

In this case, lymphocytes were present individually or in small clump as lobules of small polygonal cells with small round or oval vesicular nuclei and indistinct nucleoli. The proportion of neoplastic cells and nonneoplastic lymphocytes varies widely between tumors and between different lobules of the same tumor [33‐35]. Based on this study, thymomas may be categorized in veterinary medicine as lymphocyte predominant, epithelial predominant, or mixed [32].When lymphocytes predominate, the neoplasm must be differentiated from thymic lymphoma.

Thymomas have been described in different sites of the body. The anterior mediastinum or thoracic inlet is their usual site of occurrence, but these neoplasms can also be seen elsewhere, including the cervical region and posterior mediastinum, with variable compression of adjacent structures such as trachea, esophagus, and mediastinal vessels [32, 36]. The majority of the thymomas are benign. Local invasion and metastasis are considered by most authors to be uncommon, with metastases being reported in the pulmonary and pericardial pleura, [37] lung, [32, 36] mediastinal lymph node, [35] cervical portion of the thymus, [38] kidney, [32] and uterus. Despite, Marx and Mueller-Hermelink considered human type A and AB thymoma as clinically benign tumours [20].

The thymomas examined in our study showed massive local growth with compression, albeit not invasion, of adjacent organs. Furthermore, lymphatic congestion was seen in the cervical lymph nodes and based on their microscopic features of malignancy tumor (such as areas of high cellularity, cellular pleomorphism, high mitotic index, necrotic foci accompanied by pyknosis and karyorrhexis and high number of undifferentiated neoplastic cells), in this case, the tumor was considered to be malignant in nature.

Limitations of the cytological method include an unproven ability to definitively separate thymoma into specific WHO subtypes using cytology alone, and to determine capsular invasion [16]. Altogether, the present report confirms previous observations [26, 27] that fine needle aspiration of cervical thymic lesions generally yields adequate cellular tissue with distinct cytologic, histopathologic and immunophenotypic features that enables thymoma diagnosis.

Based on this report, we recommend that thymomas be included in the differential diagnosis of neck masses in avian. Because it was highly similar to human thymomas, the new WHO classification for human epithelial thymic tumors was used. The tumor was diagnosed as AB type thymomas, and the WHO nomenclature was found to be highly applicable. The incidence of thymoma in avian is unknown; we hope this will become clearer. To our knowledge, this is the first report of cervical thymoma type AB in a mynah, suggesting that this tumour should be included as a differential diagnosis for neck round and spindle cell tumours. Finally, for understanding of these complex neoplasms and the development of the effective differential diagnosis, further investigation will be needed into the clinical features and the basic science.