A 34-year-old man experienced intermittent diplopia for 6 months. Serum test was negative for acetylcholine receptor antibody in Cheng Ching Hospital on 03/2021, where the patient was diagnosed with an unknown virus infection. He had diplopia again on 08/2021 and reported to Shin Kong Wu Ho-Su Memorial Hospital for further evaluation. Serum test was negative for muscle-specific receptor tyrosine kinase test; however, single fiber electromyography revealed ocular myasthenia gravis. Double-seronegative myasthenia gravis was suspected, and chest computed tomography was performed, which showed a 1.5 cm nodule in the upper anterior mediastinum, suspected as thymoma (Fig. 1). The patient reported to Chung Shan Medical University Hospital for a second opinion.
Physical examination revealed a male in apparent good health and no distress. His temperature was 36.4 ℃, pulse rate was 66 per minute and regular, and blood pressure was 128/78 mmHg. The findings were within normal limits. There was no tracheal shift, and the thyroid gland was normal to palpation. Both lungs were clear to auscultation.
Laboratory data revealed a white blood cell count of 8,050 per microliter with 67% polymorphonuclear leukocytes; hemoglobin, 14.5 grams per deciliter; hematocrit, 43.1%. Serum electrolytes were normal. A pre-operative survey, including ultrasonography of the abdomen and whole-body bone scan, showed no liver or bone metastasis. Ultrasonography of the abdomen revealed mild fatty changes in the liver with bright echogenicity. A whole-body bone scan revealed no significant abnormal uptake.
The patient underwent video-assisted thoracoscopic thymectomy. Diplopia resolved after surgery, and the patient was discharged in stable condition on postoperative day 4.
In 12/2021, follow-up physical examination showed no epididymal or testicular lesions. Follow-up positron emission tomography showed no active lesion. Follow-up tumor markers, such as lactate dehydrogenase, alpha-fetoprotein and beta-human chorionic gonadotropin, were within normal range.
Macroscopically, the thymus (with tumor) measured 20.0 × 7.0 × 2.0 cm and weighed 124.6 grams. A soft to solid, mild irregular and well-defined tumor (about 1.5 × 1 × 1 cm) in the high body of thymus was noted (Fig. 2A). The tumor measuring 1.3 × 1.0 × 0.8 cm showed a homogeneous yellow-white to flesh cut surface with a rubbery consistency (Fig. 2B).
Histologically, the lesion shows the characteristic appearance of nests of large, uniform polygonal cells with clear or eosinophilic cytoplasm and distinct cell membranes. Marked thymic follicular hyperplasia with numerous germinal centers is seen (Fig. 3A). High-power field shows squared-off nuclei and prominent central nucleoli (Fig. 3B).
Immunohistochemical staining shows positive CD117 with strong and diffused circumferential cytoplasmic membrane reactivity (Fig. 3C). Positive SALL4 with strong, uniform nuclear staining (Fig. 3D). Negative for CK, P63, CD20, CD3, CD68, CD79a and Tdt stain.
Overall, the appearance is consistent with primary mediastinal seminoma with thymic follicular hyperplasia. The surgical margins are free.