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.