Correspondence /

Encapsulated Follicular Variant of Papillary Thyroid Carcinoma

The Author's Reply

I am pleased to have the opportunity to respond to the letters by Hunt and colleagues and Baloch and colleagues. In my editorial,¹ I actually have not endorsed the terminology proposed by the Chernobyl pathologists group. I do not use these terms myself; for a fully encapsulated follicular lesion with no vascular or capsular invasion but with equivocal nuclear features of papillary thyroid carcinoma, I simply call it a "follicular adenoma." I raise the "well-differentiated tumor of uncertain malignant potential" terminology of the Chernobyl pathologists group only as an alternative for pathologists who opt to call equivocal follicular lesions of the thyroid the follicular variant of papillary carcinoma.² This approach is at least more honest, because it admits the lack of reliable data on the clinical outcome of such lesions. As pointed out by Renshaw and Gould³ in their editorial, use of the term hopefully may discourage clinicians from treating these lesions too aggressively. I certainly fully agree with Hunt and colleagues and with Baloch and colleagues that there is a need to obtain long-term follow-up data on such lesions so that evidence-based diagnostic criteria can be formulated.

The cases described in these 2 letters and the 5 cases recently reported by Baloch and LiVolsi⁴ as "encapsulated follicular variant of papillary thyroid carcinoma with bone metastases" may be raised as evidence to justify a diagnosis of papillary carcinoma for an encapsulated follicular-patterned lesion with focal or equivocal nuclear features of papillary carcinoma. However, we also should consider alternative interpretations of these cases. Could these cases be follicular carcinoma instead? Follicular thyroid carcinoma is well known to give rise to distant metastasis, sometimes after a very long interval from the initial diagnosis. Although the 2 cases described in the accompanying letters are said not to show invasion, we are not provided with information on the size of the thyroid tumors and how thoroughly they have been sampled to rule out vascular and capsular invasion. It is possible that the areas with nuclear features suggestive of papillary thyroid carcinoma may merely represent nonspecific nuclear clearing or, alternatively, an incidental and unrelated papillary carcinoma arising in a follicular carcinoma. Similarly, in the 5 cases of encapsulated follicular variant of papillary thyroid carcinoma with bone metastases described earlier by Baloch and LiVolsi,⁴ the cytoarchitectural features of the metastatic tumor in the bone (their Figure 2) are more reminiscent of those of follicular carcinoma. Furthermore, these tumors definitely show invasion-vascular in 3 and capsular in 1; thus, these are at least definite well-differentiated thyroid carcinomas.

We still do not know how best to interpret encapsulated and noninvasive follicular-patterned thyroid tumors showing focal or equivocal nuclear changes suggestive of papillary carcinoma. However, we do know that even if such encapsulated lesions show well-developed nuclear features of papillary thyroid carcinoma, the chances of developing distant metastasis are very low. Thus, I believe such lesions should not be lightly diagnosed as papillary carcinomas-at least not until more solid data are available. On the other hand, if there is definite vascular or capsular invasion, it is premature to discard the possibility of follicular carcinoma in favor of the follicular variant of papillary carcinoma, although it does not matter too much in terms of clinical management because the patient has a documented well-differentiated thyroid carcinoma anyway.

John K.C. Chan

Department of Pathology

Queen Elizabeth Hospital

Hong Kong, People's Republic of China

References