Ectopic Thymic Carcinoma Located on The Dorsal Side of The Thyroid Gland

Abstract

Migration of the embryonic thymus is thought to be the basis for the formation of ectopic thymic tumors. Thymic ectopy may be a result of the abnormal migration of the third or fourth branchial cleft to the anterior mediastinum during weeks 5–6 of embryonic development. However, ectopic thymic carcinoma has highly nonspecific histologic features and occurs in variable and unpredictable locations making it difficult to diagnose. However, the clinical diagnosis and treatment should not overlook the possibility of ectopic thymic tumors. Here, we report a case of ectopic thymic carcinoma diagnosed as thyroid cancer before surgery occurring in a location consistent with current assumptions. Furthermore, we briefly review the literature on ectopic thymic carcinoma and discuss current diagnostic and treatment approaches.

Background

Thymic ectopia may be a result of the abnormal migration of the third or fourth branchial cleft to the anterior mediastinum during the 5th–6th weeks of embryonic development. Ectopic thymus tissue can occur in the parotid gland, ¹ mandible, lateral cervical area, ² thyroid area, ³ paratracheal region, pericardium, ⁴ thorax, ⁵ and pleura. ⁶ The cervical region is the most common site for ectopic thymic neoplasms. Neoplastic and non-neoplastic lesions can occur in the ectopic thymic tissue. Adenomas are the most predominant tumor lesions, while adenocarcinoma is rare.

Here, we report a case of ectopic thymic carcinoma diagnosed as thyroid carcinoma before surgery.

Patient summary

On examination, a mass approximately 2 cm in diameter was found on the dorsal side of the left lobe of the thyroid gland in a 66-year-old man. The lump was hard, had poor mobility, and caused no discomfort for the patient. The patient's family and personal history were unremarkable. A thyroid ultrasound showed that the nodule was adjacent to the left parathyroid gland (Figure 1A), and the lymph nodes in zone II and III of the neck were imaged. A computed tomographic scan of the neck revealed an approximately 2.4 × 1.7 cm tumor/nodule on the dorsal side of the left lobe of the thyroid (Figure 1B). Thyroid function and corresponding antibodies were within normal reference ranges. Cytology of a fine-needle aspiration showed atypical hyperplasia of thyroid epithelial cells and a malignancy could not be excluded. The presumptive diagnosis was a thyroid tumor or lymphoma.

Figure 1.

The results of preoperative imaging examination. (A). Neck ultrasound showed a 2.9 × 1.8 × 2.1 cm hypoechoic nodule on the dorsal side of the left lobe of the thyroid gland that was irregular in shape, lobulated, and close to the left common carotid artery and trachea. There are abundant blood flow signals within the nodules. (B). An enhanced computed tomographic scan of the neck revealed a tumor/nodule approximately 2.4 × 1.7 cm in size with a moderately enhanced soft tissue shadow on the dorsal side of the left lobe of the thyroid (in the yellow circle).

During the operation, gray and yellow irregular tissue was found behind the left thyroid gland adhered to the thyroid lobe and was separate from the mediastinal thymus. The left lobe of the thyroid gland and mass were removed and the sentinel lymph node was negative (Figure 2A). The solid mass measured approximately 2.5 × 1.8 × 1.5 cm, and a section of it was gray and brittle (Figures 2B and 2C). Intraoperative frozen sections showed benign thyroid lesions and a solid mass consistent with squamous cell carcinoma. Postoperative pathological sections verified the nodular goiter (Figure 2D), and no metastasis were found in the central lymph nodes (Figure 2E). The mass was diagnosed as thymic squamous cell carcinoma and was positive for CK, CK19, p63, CD5, CD20, CD117, p53 (60%), and Ki-67 (20%, Figures 2F and 2G).

Figure 2.

Intraoperative and postoperative pathological findings of the cervical nodules. (A). Location diagram of the cervical nodules. (B). The nodule located on the dorsal side of the left lobe of the thyroid was separated from the thyroid tissue. The section of the nodule consisted of a tough, gray, fish-like tissue. (C). The thyroid follicles varied in size. Interfollicular tissue edema and inflammatory cell infiltration were observed. The pathological diagnosis was a nodular goiter. (Hematoxylin and eosin, ×40) (D). Lymphofollicular hyperplasia was found within the lymph nodes of the central neck area and no malignant lesions were found. (Hematoxylin and eosin, ×100) (E). Immunohistochemical CD117 staining in the epithelial cells of the nodules were positive. (Immunoperoxidase,×200) (F). Immunohistochemical staining of p63 in epithelial cells of the nodules were positive. (Immunoperoxidase,×200).

Discussion

Our case study describes an asymptomatic neck mass in an older man. Due to its location and cytological results of a fine-needle aspiration, the preliminary diagnosis was a thyroid tumor or lymphoma. However, the postoperative pathological report confirmed that it was a thymic carcinoma. Pathologic and immunohistochemical evidence supported that this thymic carcinoma was caused by abnormal thymic migration during embryonic development.

Thymic cancer can occur at any age but is most commonly seen in adults between the ages of 30–60. Thymic carcinoma has highly nonspecific histological characteristics and is a diagnosis of exclusion that requires a thorough investigation to rule out other malignancies. Because its location is variable and unpredictable, the preoperative diagnosis of ectopic thymic carcinoma is extremely challenging. Ectopic thymic carcinoma of the neck is usually characterized by a painless mass that can easily be confused with thyroid nodules or enlarged lymph nodes due to its location and nonspecific cytology results on fine-needle aspiration. A literature search revealed only a few cases^2,3,7-9of cervical ectopic thymic carcinoma (Table 1). The limited experience in diagnosing and treating such cases makes it crucial to summarize previous cases. From the existing limited number of case reports, the prognosis is relatively good due to postoperative adjuvant therapy. Currently, there are no standardized diagnostic or treatment recommendations for ectopic thymic carcinoma. The current literature shows that specific immunohistochemical markers, especially CD117 and CD5, are helpful in the diagnosis of primary thymic cancer.^7,10 The inclusion of ectopic thymic carcinoma in the differential diagnosis of a thyroid region mass and the use of immunohistochemistry after an incisional biopsy, when feasible, are the best ways to obtain the correct preoperative diagnosis of this rare neoplasm. The main treatment approaches for cervical ectopic thymic cancer include surgical resection, neck lymph node dissection, postoperative adjuvant radiotherapy, and chemotherapy. ³ Some researchers have suggested that the thymus be removed simultaneously with the ectopic thymic cancer to reduce recurrence. ⁸ Because this disease is rare, there is a lack of sufficient evidence-based guidance to establish common diagnostic and treatment standards. However, the clinical diagnosis and treatment should not disregard the possibility of an ectopic thymic carcinoma.

Table 1.

Previously reported cases of ectopic thymic carcinoma of the neck.

Reference	Age/Sex	Treatment mode	Diagnostic basis
H. Nomori et al.	59/female	Complete mass resection + total thymectomy + neck and mediastinal lymph node dissection postoperative radiotherapy	The neck mass was not associated with mediastinal thymus, and histopathological diagnosis was consistent with thymic carcinoma
Hsu et al.	49/male	Complete mass resection + thymectomy + neck lymph node dissection postoperative radiotherapy and chemotherapy	Immunohistochemical staining was CD5 positive
Jung et al.	65/male	Complete mass resection + neck lymph node dissection postoperative radiotherapy	Recurrence followed by tumor at the bottom of the tongue, which was consistent with the histology of metastatic thymic carcinoma
Yao et al.	24/female	Complete mass resection postoperative radiotherapy and chemotherapy	Immunoexpression of CD5 in tumor cells; thymus tissue near the tumor
Giap et al.	17/male	Complete mass resection + neck lymph node dissection postoperative radiotherapy and chemotherapy	Immunoexpression of CD5 and CD117 in tumor cells