Case Report: Diffuse Large B-Cell Lymphoma in the Wall of a Thymic Cyst

Thymic cysts are rare lesions (1–5% of all mediastinal masses) and, most of the times, are incidental findings. The coexistence of a lymphoma and a thymic cyst is rare. In the case reported herein, microscopic foci of a diffuse large B-cell lymphoma were identified in the wall of a resected thymic cyst. This case report adds to the current knowledge of this rare entity and highlights the necessity of early surgical resection of mediastinal cysts over watchful waiting.


INTRODUCTION
Thymic cysts are rare lesions (1-5% of all mediastinal masses) and, most of the times, are incidental findings (1)(2)(3). The coexistence of a lymphoma and a thymic cyst is rare (4). In the case reported herein, microscopic foci of a diffuse large B-cell lymphoma were identified in the wall of a resected thymic cyst. The aim of this case is to add to the current knowledge of this rare entity and to underline the importance of surgical treatment of cystic lesions of the anterior mediastinum over watchful waiting.

CASE REPORT
A 32-year-old female patient with unremarkable medical history had a fortuitous diagnosis of an anterior mediastinal mass. A chest CT scan showed a voluminous mass (82 mm on long axis) with cystic features situated in the anterior mediastinum. There were no signs of invasion of the adjacent structures (Figure 1). An 18 fluorodeoxyglucose PET-CT scan showed hypermetabolic activity in the periphery of the mass (Standard uptake value max 5.1) with no other local or distant hyperfixation (Figure 2). The lesion was deemed resectable, and upfront surgery was scheduled. Because of the volume of the mass, access was gained through a full median sternotomy. A total thymectomy with resection of the mass was performed with no intraoperative complications (Figure 3). The postoperative course was uneventful, and the patient was discharged on postoperative day 4.
The histopathological analysis of the surgical specimen was compatible with a thymic cyst. However, in its wall, there were foci of neoplastic cells, which were bigger than adjacent lymphocytes. Some of the cells were even bigger; they had bigger and multiloculated nuclei that were similar to those of Hodgkin cells. Immunohistochemical analysis was negative for cytoceratin AE1/AE3, placental alkaline phosphatase, and Octamer-binding transcription factor 3/4, and positive for CD45, CD20, CD79a, and CD30. Many neoplastic  cells were partially positive for p63 (Figure 4). In conclusion, the analysis was compatible with an early-stage lymphoma suggesting a primary mediastinal diffuse large B-cell lymphoma.
Taking into account the aggressive potential of the disease, the multidisciplinary tumor board decided to administrate adjuvant treatment, even though resection margins were free from tumor invasion and thymic cyst involvement by the neoplastic tissue was microscopic. Consequently, 6 cycles of an R-CHOP regimen (rituximab, cyclophosphamide, doxorubicin, vincristine, and prednisone) were administered.
However, in case of significant growth or complications, they can become symptomatic. Their predominant localization is the anterior mediastinum. Chest CT scan can safely establish the diagnosis (5). There is no consensus about their treatment, but surgical resection is the privileged strategy, since it can provide definitive histological diagnosis and avoid potential complications such as infection, spontaneous rupture, or malignant transformation (3).
The coexistence of a lymphoma and a thymic cyst is a rare entity, with few cases reported in the literature (7)(8)(9). The exact etiopathology is largely unknown. There are scarce reports on different types of lymphomas with the predominance of Hodgkin's disease. In 1983, Smith et al. described a case, similar to the case reported herein, in a 14-year-old boy (8). There were foci of Hodgkin's disease (nodular sclerotic morphology) in the form of nodules in the cyst wall. The authors concluded that Hodgkin's disease should be part of the differential diagnosis of any mediastinal cyst especially in younger individuals (8).  Neoplastic cells can be organized in nodules, and they are bigger than normal lymphocytes, as in the case reported by Haque et al. (4). The cyst can contain hyperplastic thymic epithelium. Calcification is the result of chronic inflammatory process. In the case reported by Tabata et al. a mediastinal gray zone lymphoma originated from CD20+CD30+ Hodgkin celllike cells and was associated with a previously resected thymic cyst. This type of cells proliferated together with thymic epithelial cells (7).

CONCLUSIONS
The coexistence of a thymic cyst and a lymphoma is rare and, most of the time, is discovered during histopathological examination of the resected specimen. The exact pathogenetic mechanism is unknown. This case report adds to the current knowledge of this rare entity and highlights the necessity of early surgical resection of mediastinal cysts over watchful waiting.

DATA AVAILABILITY STATEMENT
The original contributions presented in the study are included in the article/supplementary material, further inquiries can be directed to the corresponding author/s.

ETHICS STATEMENT
Written informed consent was obtained from the patient for the publication of any potentially identifiable images or data included in this article.