Handling of Thyroid Follicular Patterned Lesions
Instituto Nazionale Tumori
Traditionally, follicular carcinoma of the thyroid has been divided into minimally and widely
invasive, the former being identified by the presence of focal capsular and/or blood vessel invasion in
what is otherwise an intact capsule. It seems to us that a clinically more significant division of
follicular carcinoma is one that will take into account the following considerations:
The classification we propose, which we view as a refinement of the traditional scheme, is the
- There are two major categories of follicular carcinoma, of widely different prognostic import: the encapsulated tumors, in which one has to look for invasion; and the widely invasive tumors, in which one has a look for a capsule. Significantly, most of the latter are poorly differentiated tumors;
- Among the encapsulated neoplasms, there is a marked difference in prognosis among those that show invasion of the capsuleonly, and those that show vascular invasion, with or without capsular invasion;
- Among those with vascular invasion, there is a prognostic difference depending on the number of vessels involved.
| Follicular carcinoma|
| With capsular invasion only |
| With limited(<4) vascular invasion |
| With extensive (=4) vascular invasion |
| Widely invasive|
A related problem concerns the handling of the well-differentiated follicular neoplasms in which
the capsular interruption is "questionable", in the sense of involving only the inner half or being
represented by tumor islands embedded within. A group of pathologists interested in thyroid tumors and
colloquially known as the Chernobyl Pathologists Group recommended the adoption of the following
terminology for this situation: 
|For tumors showing definite capsular invasion:|
| ||Follicular carcinoma [to be further subdivided as per the above scheme];|
|For tumors showing questionable capsular invasion:|
| ||Follicular tumor of uncertain malignant potential (FT-UMP) if papillary carcinoma-type nuclear changes are absent or|
| ||Well-differentiated tumor of uncertain malignant potential (WDT-UMP) if those nuclear changes are questionable|
This takes us to the last problem, which is that of nodules having a follicular pattern of growth but
exhibiting the nuclear features of the papillary family of neoplasms, i.e., clearing, pseudoinclusions,
and grooves. If these features are well-developed and widespread, the diagnosis (follicular variant of
papillary carcinoma) is easy. The problem is what to do when they are not. It is possible that lesions
in which the nuclear changes are "questionable" represent an early development of papillary carcinoma in
a preexisting benign lesion, as suggested by the fact that in microdissection experiments the RET/PTC
rearrangements are restricted to these foci.  However, it is abundantly clear that such
lesions will be cured by a conservative operation in nearly every instance. On the basis of this
observation and with the purpose of avoiding all the responses that the term carcinoma induces in the surgeon and the patient, the Chernobyl Pathologists Group
proposed that the following terminology be adopted,  fully aware of the subjectivity and
arbitrareness behind the decision as to whether a certain morphologic change is "obvious", "questionable"
|For lesions having obvious nuclear changes:|
| ||Papillary carcinoma, follicular variant, regardless of the status of the capsule.|
|For lesions having questionable nuclear changes:|
| ||Well-differentiated carcinoma, NOS, if there is definite capsular invasion.|
| ||Well-differentiated tumor of uncertain malignant potential (WDT-UMP), if capsular invasion is questionable or absent.|
The two types designated as carcinoma are to be further subdivided using
the same scheme as the one above proposed for follicular carcinoma.
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