Pitfalls of FNA diagnosis of thymic tumors

Abstract

BACKGROUND: Fine-needle aspiration (FNA), a minimally invasive and cost-effective procedure, often is used in the initial diagnosis of thymic lesions. However, the diagnosis can be challenging. Knowledge of the diagnostic pitfalls is important to improve diagnostic accuracy. METHODS: The authors retrospectively searched the pathology database of The University of Texas MD Anderson Cancer Center for FNA cases using the keywords "thymoma" or "thymic" in cytologic diagnoses or in corresponding final histologic diagnoses rendered from January 2002 to June 2018. The authors reviewed the FNA diagnostic accuracy and pitfalls in comparison with the final histologic diagnoses. RESULTS: A total of 118 FNA cases were identified from 115 patients. The FNA diagnoses were concordant with the final pathologic diagnoses in 110 cases (93.2%), including thymoma (97 cases), atypical thymoma (5 cases), and thymic carcinoma (8 cases). Discrepant FNA and final diagnoses were noted in 8 tumors (6.8%): thymoma versus atypical thymoma/thymic carcinoma (3 tumors), thymoma versus lymphoma (2 tumors), suspicious for lymphoma versus thymoma (1 tumor), and T-lymphoblastic lymphoma versus thymoma (2 tumors). Factors contributing to misinterpretation included intrinsic limitations of the FNA sample (sampling error and a lack of histologic architecture information) and similarities of the cytologic and immunophenotypic features of lymphocyte-rich thymoma and T-lymphoblastic lymphoma. CONCLUSIONS: An accurate FNA diagnosis of thymic tumors can be rendered in the majority of cases. Diagnostic pitfalls can be encountered in rare cases. It is important to handle each case carefully to avoid erroneous diagnoses that may lead to inappropriate treatment.

CT and MRI features in 22 cases of primary malignant thymic germ cell tumours

“…Decisions regarding the CT findings were reached in consensus. CT imaging features of thymic GCTs were evaluated for the following variables 1 : size: the mean diameter was calculated by (a þ b þ c)/3, and the maximal cross-section was used to measure the long (a) and short (b) diameters, the longest diameter of the tumour measured as c in sagittal or coronal plane 2 ; location: midline or lateral 3 ; shape: oval or irregular 4 ; boundary: smooth, lobulate, or ill-defined 5 ; capsule integrity: almost complete (smooth or shallow lobulation), incomplete (deep lobulation or spiculate protuberance) 6 ; homogeneity: on a mediastinal window setting, differences in CT attenuation values within tumours less than or greater than 10 HU is evaluated as almost homogeneous or heterogeneous, respectively. The presences of calcification, necrosis, or cystic change were also evaluated 7 ; involvement of adjacent tissues: the mediastinal fat layer of peritumour, pleura and pericardium 8 ; involvement of great vessels: no, invasion, or compression 9 ; lymph node enlargement in mediastinum: a lymph node with a short diameter >1 cm is defined as lymphadenopathy 10 ; pattern of contrast enhancement: based on a maximal difference of CT attenuation values between plain scan and the contrastenhancement phase in the solid component of tumour.…”

“…4 In current practice, the diagnosis of malignant thymic GCTs is usually based on fine-needle aspiration biopsy or mediastinoscopic biopsy, serum tumour markers, and imaging studies 4,5 ; however, a percutaneous needle biopsy may cause postoperative complications, and the results are easily affected by sampling errors. 6 The a-fetoprotein (AFP) and b-subunit of human chorionic gonadotropin (b-hCG) display limited sensitivity and specificity, being indicative of yolk sac tumour (AFP) and choriocarcinoma or syncytiotrophoblast (b-hCG) subtypes. 7,8 The radiological features of mediastinal mature teratoma have been well described, while those of malignant GCTs, especially non-seminomas have been only sporadically reported.…”

Diagnostic Challenges in Fine-Needle Aspiration Cytology of Mediastinal Tumors and Lesions

“…Differentiating lymphocyte-rich type B1 thymomas from lymphoma is another challenging scenario in cytology diagnosis. 22 In such cases, demonstration of immature Tlymphocyte component (TdT þ , CD3 þ , CD99 þ , CD1a þ ) on flow cytometry and a neoplastic epithelial component (smears, hematoxylin-eosin stain [H&E], and immunohistochemistry with cytokeratins) will help make a diagnosis of thymoma. Most acute lymphoblastic lymphoma/leukemia (ALL) cells are T-cell-derived with an immunophenotype of T lymphocytes (TdT þ , CD3 þ , and CD99 þ ), and are larger than mature lymphocytes with enlarged nuclei containing evenly dispersed chromatin, but ALL usually occurs in children.…”

“…It is important to note that expression of CD3, CD45, CD4, and CD8 in thymocytes shows a smearing pattern ranging from high density to negative, reflecting all stages of T-cell maturation, whereas T-ALL cells express these antigens in a tight clustering pattern. 22 In addition, loss of T-cell antigens, and expression of CD10 and CD34, are in favor of T-ALL.…”

“…Misdiagnosis of primary thymic Hodgkin lymphoma as thymoma has been reported in the literature, especially in cases when Hodgkin lymphoma developed in the background of residual thymic parenchymal cells. 22 Reed-Sternberg cells are believed to be of germinal center B-cell origin, yet they have lost most of their B-cell-specific antigen expression. Reed-Sternberg cells are positive for CD15, CD30, PAX-5 (weak), and MUM1, negative for CD45, and show decreased or negative expression of B-cell markers, including CD20, CD79a, OCT2, and BOB1.…”