Differential diagnosis, investigations and treatment
The patient underwent stereotactic posterior fossa craniotomy and resection of the right cerebellar tumour, which showed fragments of lesional tumour tissue and normal cerebellum. The tumour cells were arranged in papilliform clusters, with the tumour cells conspicuously showing nuclear clearing and overlapping. Some of the nuclei also showed nuclear grooves. On further immunoperoxidase staining the tumour cells were positive for broad spectrum keratin AE1/AE3, keratin 7, TTF-1, PAX8, HMBE-1, BRAFV600E and thyroglobulin, while napsin A was negative. Thus these findings were consistent with a metastatic PTC (Image 2a).
Computed tomography staging demonstrated a mildly bulky left thyroid lobe, mildly prominent left inferior neck lymph node measuring 12mm, multiple <5mm nodules within both lung fields, and tiny cystic foci within the liver and kidneys. Thyroid ultrasound showed a 21x30x22mm heterogenous hypoechoic nodule with irregular margins and microcalcifications in the inferior pole of the left thyroid lobe (TI-RADS 5) (Image 3 ). He was euthyroid, with TSH 1.11mIU/L and FT4 15.9pmol/L. Thyroglobulin level was 514 ug/L with negative thyroglobulin antibodies <1.0 IU/ml.
FDG-PET indicated a focus of moderately avid tracer uptake in the left inferior pole of the thyroid gland (SUVmax 4.8) and left-sided cervical lymph nodes, the largest measuring up to 11mm (SUVmax 2.4). There was relatively reduced uptake in the surgical bed with mildly increased uptake at the resection margins, likely reflecting post-operative changes. There were pulmonary nodules in both lung fields, with the greatest tracer avidity measuring SUVmax 0.9, and no suspicious hilar or mediastinal lymphadenopathy. FNA of the TI-RADS 5 thyroid lesion demonstrated mainly papilliform fragments of malignant cells (Bethesda VI) consistent with PTC.
Multidisciplinary Team meeting recommended the following treatment sequence: 1) total thyroidectomy and left neck dissection, followed by 2) stereotactic radiosurgery to right cerebellar cavity (27 Gray over 3 fractions) and 3) Radioactive iodine ablation with recombinant TSH stimulation.
Intra-operatively, the tumour nodule detected on thyroid ultrasound and PET scan was determined to be from the left neck level VI rather than the thyroid gland proper, and it was almost completely replaced by a 30mm nodule of PTC of classical type. There was infiltration into fibrofatty tissue and skeletal muscle with perineural and multifocal lymphovascular invasion. There was no identifiable normal thyroid or lymph node tissue in this tumour nodule (Image 2b/c ). The cells stained positively forBRAFV600E , TTF-1 and thyroglobulin. ALK and pan-TRK were negative (Image 2d ). The total thyroidectomy on the other hand did not demonstrate evidence of PTC. There were changes of multinodular goitre and interestingly an incidental 4mm focus of calcitonin-positive medullary carcinoma arising from C-cell hyperplasia in the mid left lobe (Image 4a/b ). As the tumour measured <10mm, this was regarded as medullary microcarcinoma (microMTC) as per WHO classification of Endocrine tumours4.
Since the microMTC was an incidental finding, no pre-operative calcitonin was performed, but a post-operative calcitonin was negative at <5 ng/L (<20). Just prior to the RAI dose, stimulated thyroglobulin was 916 ug/L. The patient underwent 4.22 GBq radioactive iodine ablation with prednisolone to prevent transient oedema at the old surgical site.