Case report
A 16 year old female was incidentally found to have mediastinal widening in plain chest X-ray took at medical check-up and referred to thoracic surgery department of Seoul National University Hospital (SNUH) at 2019-04-10. She denied any underlying diseases or family history of malignancy, and she had never experienced smoking or alcohol. She had dysphagia, weight loss and exertional dyspnea at the time of the first visit, but her symptom was not severe and neglected until plain chest X-ray was taken. Her initial height was 160cm and body weight was 42.5kg.
Chest CT taken at outside hospital showed about 9.5x6.1cm sized fat containing mass lesion without calcification in the lower neck to posterior upper mediastinum, and tracheal compression was observed. The differential diagnosis made by radiologist was well differentiated liposarcoma or immature teratoma. F-18 FDG PET/CT was taken and it showed mild uptake (~1.9) at aforementioned mass, with metabolic defect in fat portion (Figure 1). The core needle biopsy of the mass was done, and the pathologic report confirmed lipogenic tumor with some brown fat cells, suggestive of hibernoma.
Mediastinal mass excision via full sternotomy under V-V ECMO support was done by thoracic surgery team. Anesthesiologist recommended V-V ECMO support because there was mid tracheal compression more than 50%, with diameter lower than 5mm, through which plain E-tube ID 3.0mm could barely pass. Before the anesthesia induction, fiberoptic, rigid bronchoscope was prepared and high flow nasal cannula was applied for the possibility of emergent airway collapse. Under MAC anesthesia, the V-V ECMO was inserted with 17Fr catheter at right and left femoral vein, then the general anesthesia with endotracheal intubation with plain ID 6.5mm tube was followed. There was no difficulty during the intubation process, and the intubation depth was 22cm. Airway patency was checked with fiberoptic bronschoscope.
During the main procedure, approach through full median sternotomy was made, and large mass through superior aspect of anterior mediastinum to anterior neck was identified. The mass was well circumscribed and lobulated, and the adhesion to nearby tissues was minimal. There was no evidence of vessel, nerve, pleural invasion, and delicate dissection with complete removal was performed. Bilateral recurrent laryngeal nerves were identified and preserved. The patient was transferred to pediatric intensive care unit after the surgery, and was discharged at postoperative day 5 without any complication.
The size of the mediastinal mass was 9.6 x 7.9 x 3.8cm, and the final pathology confirmed mediastinal hibernoma. On gross examination, resected mediastinal lesion contained fatty tissue-like yellowish mass measuring 5.3 cm. The mass was composed of polygonal cells resembling brown fat with multivacuolated cytoplasm, admixed with mature adipocytes. The lesion was also characterized by fibrous septae and myxoid stroma, suggestive of myxoid variant. Small, delicate, branching capillaries were noted. Nuclei were small with no significant atypia, and necrosis was not observed. Immunohistochemistry for CD31 showed staining in hibernoma cells as well as capillary endothelial cells (Figure 2).
Regular follow up at outpatient clinic was done with chest PA, and there was no evidence of recurrence. Initial symptoms such as dysphagia and dyspnea were also disappeared, and she had no complaint. She also gained weight, and her body weight was 47.4kg at 2022-01-10. However, the chest CT taken at 2020-08-26 showed small attenuating lesion at left supraclavicular area. The size of the mass was increased to about 2.2cm, which was suspicious for residual mass or recurrence of hibernoma. Due to its location, the patient was referred to Otolaryngology department at SNUH.
2022-01-10 neck CT was taken, and the size of the left level VI mass was increased (2.2 -> 2.6cm) (Figure 3). In addition, 1.6cm mass was incidentally identified at right submandibular gland posterior aspect, and followed ultrasound gun biopsy confirmed pleomorphic adenoma.
Eventually, left level VI neck dissection with right submandibular gland mass excision was done at 2022-01-25. Electromyography tube was intubated for intraoperative recurrent laryngeal nerve monitoring. About 5cm midline horizontal incision was made along the skin crease at the level of thyroid gland. During the removal process, the mass was well circumscribed with capsule and pinpoint capsule violation was observed, but there was no definite spillage of tumor content. Delicate dissection was performed to save nearby structures (Figure 4). Eventually, complete removal was achieved, and left vagus nerve, carotid vessels, thyroid gland were saved. Left recurrent laryngeal nerve was not identified due to severe adhesion, but the patient’s vocal fold movement was intact postoperatively. Suspicious fat tissue nearby main mass was also removed. The right submandibular gland mass was also removed in a routine manner.
The recurred tumor at left level VI was also confirmed as hibernoma, which had similar histologic features with more pronounced myxoid stroma compared to mediastinal mass (Figure 2). Removed fat tissue adjacent to main mass was diagnosed as mature adipose tissue. The right submandibular gland was confirmed as pleomorphic adenoma with clear resection margin.
The patient was discharged at postoperative day 2 without any complications, including vocal fold palsy, hematoma, wound infection, etc. There was no evidence of recurrence at 6 month follow up neck CT.