Definition of Non Differentiated, Non Anaplastic Carcinomas of the Thyroid: The Japanese Experience
Kyorin University School of Medicine
Papillary and follicular carcinomas have been considered as separate and independent categories.
There seem to be at least two problems with this conventional histologic classification. The first is
that there is no definite difference in prognosis between the two types of carcinoma. Concerning the
cancer prognosis, it is not necessary to classify papillary and follicular carcinomas as two different
The second problem is that there are some cases with a poor result even after adequate surgical
treatment. Sometimes the poor outcome is apparently caused by anaplastic transformation, but in other
cases this type of dedifferentiation does not occur. Among the latter cases, local recurrence sometimes
develops, but not all recurrences are fatal. If the recurrence is caused by inappropriate resection of
the tumour, the policy regarding extirpation of thyroid tumours should be reconsidered.
Against this background, we proposed as long ago as 1983 that "poorly differentiated carcinoma"
should be added to the classification of thyroid carcinomas arising from the follicular epithelium. On
the basis of histopathological and clinicopathological studies, poorly differentiated carcinoma was
designated as an aggressive type of papillary and follicular carcinoma of thyroid. The histological
criteria and biological characteristics are described below.
According to our criteria, both papillary and follicular carcinomas consists histologically of
glandular and non-glandular structures, using the terms "papillary" and "follicular" in their
descriptive, not their diagnostic sense. Papillary structures present as papillary protrusions into the
lumen lined by an intact layer of follicular cells. Follicular structures also show a lumen bounded an
intact cell layer, but without any papillary projections. Non-glandular components included solid,
trabecular and scirrhous patterns. The solid pattern shows a sheetlike arrangement of cancer cells.
However, squamous metaplasia consisting of cells without cytological features of malignancy does not
occur in poorly differentiated carcinoma. The trabecular pattern involves a cord-like arrangement of
cancer cells. A few cancer cells are isolated and scattered in the stroma in the scirrhous pattern.
Papillary and follicular carcinomas composed entirely of glandular components except for squamous
metaplasia, as described above, are designated "well differentiated carcinoma". If additional
non-glandular components are found in papillary and follicular carcinomas on histological examination,
the tumour types of histological combinations in poorly differentiated carcinoma: either an admixture of
glandular and non-glandular components, or a tumour composed exclusively of non-glandular elements.
In poorly differentiated carcinoma, both glandular and non-glandular components are commonly
found in the same case. The two components are generally intermingled, not sharply demarcated. In most
cases, the glandular component is usually seen in the peripheral area of a tumour, with an irregular
border between this component and the surrounding tissues.
We also investigated the possibility of histological grading of thyroid papillary carcinoma by
cytology. According to our experience, the cytological characteristics of poorly differentiated
papillary carcinoma can be described as follows: (1) Clusters are of small to medium size. Generally
the cells are scattered separately. (2) Cells of the cluster are in an irregular arrangement with tight
and / or loose connections between cells. (3) Rather large nuclei with anisocytosis, compared to their
well-differentiated counterpart. (4) Clusters with a well-differentiated component sometimes intermingle
with other cells on the same slide.