—  ENDOCRINE PATHOLOGY SOCIETY   —

Handling of Thyroid Follicular Patterned Lesions


Juan Rosai
Instituto Nazionale Tumori
Milan, Italy


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:
  1. 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;

  2. 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; [3, 5, 6]

  3. Among those with vascular invasion, there is a prognostic difference depending on the number of vessels involved.
The classification we propose, which we view as a refinement of the traditional scheme, is the following:

Follicular carcinoma
Encapsulated
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: [7]

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. [2] 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, [7] fully aware of the subjectivity and arbitrareness behind the decision as to whether a certain morphologic change is "obvious", "questionable" or "absent": [1, 4]

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.

References

  1. Chan JKC. Strict criteria should be applied in the diagnosis of the encapsulated follicular variant of papillary thyroid carcinoma. Am J Clin Pathol 117: 16-18, 2002.

  2. Fusco A, Chiappetta G, Hui P, Garcia-Rostan G, Golden L, Kinder BK, Dillon DA, Giuliano A, Cirafici AM, Santoro M, Rosai J, Tallini G. Assessment of RET/PTC oncogene activation and clonality in thyroid nodules with incomplete morphological evidence of papillary carcinoma. A search for the early precursors of papillary cancer. Am J Pathol 160: 2157-1944, 2002.

  3. Lang W, Choritz H, Hundeshagen H. Risk factors in follicular thyroid carcinomas. A retrospective follow-up study covering a 14-year period with emphasis on morphological findings. Am J Surg Pathol 10: 246-255, 1986.

  4. Lloyd RV, Erickson LA, Casey MB, Lam KY, Lohse CM, Asa SL, Chan JKC, DeLellis RA, Harach HR, Kakudo K, LiVolsi VA, Rosai J, Sebo JT, Sobrinho-Simoes M, Wenig BM, Lae ME. Observer variation in the diagnosis of follicular variant of papillary thyroid carcinoma. Am J Surg Pathol 28: 1336-1340, 2004.

  5. Thompson LDR, Wienke JA, Paal E, Frommell RA, Adair CF, Heffess CS. A clinicopathologic study of minimally invasive follicular carcinoma of the thyroid gland with a review of the English literature. Cancer 91: 505-524, 2001.

  6. van Heerden JA, Hai ID, Goellner JR, Salomao D, Ebersold JR, Bergstralh EJ, Grant CS. Follicular thyroid carcinoma with capsular invasion alone: A nonthreatening malignancy. Surgery 112: 1130-1138, 1992.

  7. Williams ED (on behalf of the Chernobyl Pathologists group). Two proposals regarding the terminology of thyroid tumors. Int J Surg Pathol 8: 181-184, 2000.