Metastatic lesions to the thyroid gland are generally considered rare, possibly due to a high oxygen and iodine environment which may impair the ability of metastatic cells to settle and develop. Abundant high velocity blood flow through the thyroid gland also possibly plays a role in impeding the ability for metastatic cells to gain a foothold[3
]. Perhaps unsurprisingly, when primary thyroid pathology occurs which results in structural change, this has been associated with an increased incidence of metastases to the thyroid gland. Multinodular goiters and adenomatous change have both been associated with an increased incidence of metastases to the thyroid gland[7
]. It is possible our patient had a pre-existing primary thyroid carcinoma at the time of initial surgery for the primary rectal malignancy which altered the local thyroid environment rendering conditions more favorable for metastatic rectal adenocarcinoma cells to settle.
With the advent of improved diagnostic imaging technology such as 18F-FDG PET, an increasing number of incidental cases of metastatic disease to the thyroid gland are likely to be detected. Well differentiated primary thyroid carcinomas such as papillary and follicular carcinomas are generally not 18F-FDG avid on PET scanning and usually only become 18F-FDG avid if they de-differentiate.
It has been reported in the literature incidentally PET detected 18
F-FDG avid primary thyroid malignancies are generally a more aggressive variant of primary thyroid cancer which harbour a higher rate of unfavourable prognostic factors and are often less well differentiated[8
]. In our case, the coexistent focus of metastatic colorectal adenocarcinoma within a primary poorly differentiated thyroid cancer somewhat complicates the issue, as both tumours are likely to be 18
To our knowledge there have been no cases of metastatic malignancy within a primary thyroid malignancy reported previously in the literature. As such, there is scant evidence in the literature regarding the most appropriate management strategy for such a patient. Conventional management of a primary thyroid malignancy usually involves total surgical thyroidectomy followed by radioiodine therapy to ablate the thyroid remnant, decrease risk of recurrence and enable adequate follow up using I-131 whole body scintigraphy and stimulated thyroglobulin levels [9
In our case, prognosis is more likely to be dependent on the patient's metastatic rectal adenocarcinoma rather than the primary thyroid malignancy. Stage IV metastatic rectal adenocarcinoma portends a poor prognosis, with five year survival rates of between 4%–8%[14
]. More specific to our case, in a review of 12 patients with metastatic rectal carcinoma to the thyroid between 1990 to 1993 by Fujita et al [3
], in the one patient without metastases to any other organ but the thyroid, survival was just 4 years. Primary early stage non anaplastic thyroid carcinoma even if poorly differentiated has a better prognosis compared to metastatic rectal cancer. Poorly differentiated primary follicular thyroid carcinoma has been reported to have a 5 year survival rate of 63% in a recent published series of 40 patients[16
As a result, the primary treatment focus in our patient was tailored towards treating metastatic rectal carcinoma and less so the primary thyroid malignancy. It is also of note, many poorly differentiated thyroid carcinomas are not particularly radioiodine avid and it is quite possible the impact of radioiodine therapy on reducing recurrence rates may be greatly diminished in this setting.