Micronodular Thymic Carcinoma with Lymphoid Stroma: A Case Report

Background. Micronodular thymic carcinoma (MTC) with lymphoid hyperplasia is believed to be the malignant counterpart of micronodular thymoma (MT) with lymphoid hyperplasia. Since MT and MTC share a similar morphology, MTC is considered a malignant form of MT; there have been a few malignant transformations from MT to MTC. We report a case of MTC with lymphoid hyperplasia. Case presentation. A 53-year-old woman presented with an incidental tumor on a chest X-ray. The resected tumor consisted of nodular epithelial nests and lymphoid tissue within a surrounding germinal center. Some epithelial nests showed apparent malignant morphology. Atypical epithelial cells with large vesicular nuclei formed nests, some of which showed comedo necrosis. These cells showed transition continuously to low-grade type B thymoma-like cells, demonstrating cord-like arrangements. Carcinomatous elements, expressed GLUT1, CD5, KIT, and BCL2; conversely, low-grade nests displayed attenuated expression of these markers. GTF2I point mutation and Langerhans/dendritic cells, which are indicators of favorable thymoma prognosis, were not detected. Due to pleural metastasis, the patient was treated with lenvatinib 27 months postoperatively. Conclusions. This is the first report of a partially low-grade, GTF2I-negative MTC. Histological and genetic findings might be predictive of tumor prognosis.


Background
Micronodular thymic carcinoma (MTC) with lymphoid hyperplasia is believed to be the malignant counterpart of micronodular thymoma (MT) with lymphoid hyperplasia. [1][2][3][4] MT, which was first described in 1999 by Suster et al, is a rare subtype of thymoma, accounting for 5% of thymomas, and the World Health Organization recognized it in 2021 as a new subtype MTC. 1 It comprises neoplastic epithelial nests and surrounding hyperplastic lymphoid tissue containing a germinal center. 2 Since MT and MTC share a similar morphology, MTC is considered a malignant form of MT; there have been a few malignant transformations from MT to MTC. [2][3][4][5][6][7][8][9] Herein, we describe a case of MTC accompanied by a low-grade histological component of thymoma with a literature review.

Case Presentation
Our patient was a 53-year-old woman who presented with a left pulmonary hilar region mass, discovered incidentally on routine chest radiography 3 months prior to presentation. This mass did not exist on the chest radiograph taken one year previously. Computed tomography (CT) revealed a tumor measuring approximately 3 × 2 cm in the anterior mediastinum, in contact with the main trunk of the pulmonary artery ( Figure 1A). The lesion displayed slight contrast on contrast-enhanced CT ( Figure 1B). As the tumor showed a tendency for rapid growth, it was surgically removed.
Macroscopically, the tumor was not encapsulated but was relatively well-demarcated, 33 mm × 23 mm in size, and with a brownish-yellow cut surface ( Figure 1C). Histologically, the tumor was composed of epithelial cell nests or micronodules as well as stromal lymphoid tissue with germinal centers (Figure 2A, B). Some epithelial cell nests showed comedo necrosis ( Figure 2C). In most parts of the tumor, neoplastic cell nests showed apparent malignant features, such as vesicular, prominent nucleoli, and numerous mitotic figures ( Figure 2C). We also found type B thymoma-like elements within the tumor, which consisted of polygonal epithelial cells and scattered stromal lymphocytes ( Figure 2D). These findings continued to transform along with the geography of the tumor, showing a shift from a malignant appearance into a low-grade, thymoma-like region ( Figure 2B). In addition, there was neoplastic epithelium arranged in a cord-like pattern, which appeared to be regressed lymphoid cell infiltration.
Immunohistochemically, lymphocytes around the tumor nest were negative for TDT ( Figure 2E, F). The malignant tumor cells were positive for KRT (AE1/AE3), CEA, EMA, GLUT1, KIT (CD117), CD5, p63, p40, KRT5/6, TDT (DNTT), and BCL2 ( Figure 2E, G, I, K). In the low-grade thymoma-like region, tumor cells were negative for GLUT1 and a few were positive for KIT, CD5, and TDT ( Figure 2F, H, J, L). Chromogranin A, synaptophysin, and NCAM were negative in both regions. The Ki67 labeling index was 42% in the malignant region and lower in the thymoma-like region (manual examination). Stromal lymphoid cells were predominantly composed of CD20-and CD79a-positive B-cells, with scattered CD3-and TDT-positive T cells. The germinal centers were positive for CD10 (MME) and negative for BCL2. In some malignant areas, tumor cells were positive for PD-L1 ( Figure 2M). The PD-L1 expression is 3% in the lowgrade part and 35% in the malignant part, respectively (Figure 2m, n). S-100 protein, CD1A, and Langerin-positive cells were absent in the tumor region. Therefore, we regarded this case as MTC. The tumor was found to have invaded the adjacent adipose tissues but had not reached the phrenic nerve (pT2, Masaoka stage IIb). The patient displayed pleural metastases 10 months after tumor resection and was treated with a gamma knife. The targeted treatment against VEGF kinases (Lenvatinib) was employed 27 months after the surgery.
We performed Sanger sequencing to detect mutations in GTF2I, which is found in low-grade thymomas, such as type A and type AB thymomas. 10 The sequences of the primer set are as follows: Primer-F, ATCCCGTACCC TCTTTTCC; Primer-R, AGACAAGAGTTCAAACAGG. The mutations were examined separately in low-grade and malignant areas using microdissection; however, no mutations were detected. The TP53 gene was not searched for mutations.

Discussion
MTC is a rare subtype of thymic carcinoma, and the histological characteristic of MTC or MT is the formation of tumor nodules accompanied by lymphoid stroma, unlike other common thymomas. 1 The absence of TDT-positive immature T cells in the background is a distinctive feature of MTC compared to MT. To date, 23 patients with MTC have been reported (Table 1). [2][3][4][5][6][7][8][9] The mean age of the patients was 62.9 years (range 42-82 years), and the mean tumor size was 3.7 cm (range 1.1-10 cm). There was a relative male dominance (male: female = 15:8). MTC shows prominent B-cell infiltration and lymphoid follicles, suggesting a good prognosis. However, one patient died due to this disease, and our patient experienced recurrence 10 months after surgery.
The histological differential diagnosis of MTC should include MT. The tumor cells of MT show oval or spindle morphology, similar to that of type A thymoma, and occasionally, atypical cells, such as those of type B thymoma. In addition to atypia, mitotic counts can distinguish between low-grade and malignant tumors. The number of mitoses was less than 5/10 high-power fields (HPF) in MT and more than 5/10 HPF in MTC. 11 Our case corresponded to MTC.
Immunostaining is useful in the diagnosis of thymic tumor malignancy. 12 Thymic carcinoma cells, including MTC, are positive for CD5, KIT, and GLUT1, for which thymoma and MT cells are negative. Although Langerhans and dendritic cells were absent in the MT section, neither was detected in our patient. 13 Therefore, CD1a and CD21, which are markers for Langerhans and dendritic cells, respectively, are possible markers for differentiation from MTC to MT.
Lymphocytes infiltrating the thymoma are usually T cells; however, in MTC, B-cell infiltration is more commonly found. [1][2][3][4] The formation of lymphoid stroma occurs due to the host immune response to the tumor. [2][3][4] In our case, tumor cells in the cord-like structure region with dense lymphocytic infiltration were smaller than those surrounding micronodules or nests. This suggests that the host immune response to the tumor was effective. The PD-L1 expression by tumor cells supported this hypothesis.
In some cases, a tumor with the transition between MT and MTC has been observed, suggesting a continuous transformation from MT to MTC. 2,4,5 Usually, MT shows type A or AB-like characteristics. 1 Our patient had a conventional type B thymoma-like component which showed partial continuity with the MTC component. Tumor cells with type B thymoma-like elements were negative for Glut1, which is a positive marker of thymic carcinoma. However, they were only partially positive for CD5, KIT, and BCL2, which are markers of thymic carcinoma. These findings suggest that these cells are potentially malignant; therefore, we considered that type B thymomalike cells transformed to form MTC in our case. Recently, a point mutation in GTF2I was reported in thymic epithelial tumors. 10 Approximately 80% of type A and AB epithelial Immunohistochemically, lymphocytes around the tumor nest both the malignant and the low-grade parts were negative for TDT (Fig. E, F). The tumor cells are almost positive for GLUT1 (G), KIT (I), and CD5 (K) in the malignant region. In thymoma-like low-grade regions, tumor cells are negative for GLUT1 (H), KIT (J), and CD5 (l). The PD-L1 expression is 3% in the low-grade part and 35% in the malignant part, respectively (Fig. M, N). TDT; Terminal Deoxynucleotidyl Transferase.  thymomas carry GTF2I mutation, whereas most type B thymomas and thymic carcinomas do not. 10 In other words, thymic tumors with GTF2I mutation have a favorable prognosis; however, this mutation was not detected in our patient in either the type B thymoma-like low-grade or the carcinoma section. Owing to the small number of reported cases and we did not search for any TP53 mutations, the prognosis cannot be accurately evaluated, but most cases were benign, and the patients are alive. However, MTC with malignant to low-grade transition might provide evidence of good progress because the lowgrade portion of the tumor showed either a type A/AB thymoma-like lesion or the presence of GTF2I mutation. The transition from low-grade MT to MTC supports the possibility of de-differentiation or transformation process, rather than de-novo.

Conclusions
We report a case of MTC arising from MT. To the best of our knowledge, this is the first case of genetic analysis of malignant and low-grade parts present simultaneously, and neither component showed GTF2I mutation. The malignant-to-low-grade transition in MTC might be associated with tumor progression since the tumor displayed findings characteristic of histology or GTF2I mutation.

Declaration of Conflicting Interests
The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.