High grade follicular cell-derived non-anaplastic thyroid carcinoma is a new classification of WHO 5th. It has been reported to occur in less than 1% to 6.7% of all thyroid cancers.

[Case] 83-year-old female [Present Illness] She had noticed a mass in her anterior neck. It had been growing, so she visited her primary care physician. CT scans suggested a malignant thyroid tumor, leading to her referral to our hospital. Her subjective symptoms included swelling under her chin, which initially seemed to be growing downward, but was not painful. Physical examination revealed a large, firm but mobile mass in the anterior neck, measuring approximately 55 mm in length and skin infiltration was noted. CT scans revealed a 78 mm mass primarily in the left lobe of the thyroid gland, extending beyond the capsule. There was no evidence of tracheal invasion, but skin infiltration was suspected. Clinically, the tumor was thyroid cancer, left lobe to isthmus, cT4aN0M0, stage III. Approximately one year after her initial consultation, she underwent a total thyroidectomy, including skin resection, and anterior tracheal dissection.

[Pathology] The specimen measured 9.4 x 8.0 x 7.2 cm, including a 10 x 4.5 cm skin resection. The cut surface revealed a milky white to light brown, solid, slightly blurred-margin tumor measuring 85 x 45 x 35 mm, accompanied by bleeding. No exposed tumor was observed on the resection surface. Histologically, the tumor was primarily well-differentiated papillary carcinoma, with some tall cell components and hobnail-like and micropapillary atypical cells. Relatively extensive necrosis and bleeding were also present, along with hemorrhagic granulation tissue surrounding the tumor. More than five mitotic figures per 10 HPF were observed. Venous invasion was present. The tumor had infiltrated the subcutaneous fat. A small portion of the resection margin was positive. The pathological diagnosis was papillary carcinoma, corresponding to high-grade follicular-derived thyroid carcinoma according to the new WHO classification (5th edition).

[Discussion] This case presents with papillary thyroid carcinoma measuring 8.5cm in diameter and infiltrating into the subcutaneous fat tissue. There were relatively extensive necrosis and hemorrhage within the tumor, and mitotic figures were prominent, with more than five per 10 HPH. This was a case of well-differentiated papillary carcinoma, with no clear components of undifferentiated carcinoma. Based on these findings, the case was diagnosed as a high-grade follicular-derived thyroid carcinoma according to the new WHO (5th edition).