Undifferentiated Forms of Thyroid Cancer

According to this article, diffuse sclerosing is classified as 'intermediate' differentiation - so is not classified as poorly or dedifferentiated. In this 'middle' category, it is considered more aggressive than its' well differentiated (ie papillary or follicular) cousins:
http://www3.interscience.wiley.com/cgi-bin/abstract/107639325/ABSTRACT

I do think it is important to remember that even the most easily treatable variants of thyroid cancer may have the ability to mutate, dedifferentiate/become less differentiated over time, if left untreated and/or poorly treated.

It is a cellular progression, where the malignant cells mutate from being very similar to thyroid tissue (and therefore easily and rapidly will uptake radioactive iodine, and are highly treatable) to more abherant cellular behaviours that are less inclined to absorb radioactive iodine, therefore requiring alternate methods of treatment.

Science does not yet know why some of us will experience this change in the cells, nor why it occurs more rapidly in some of us, and not others.

On a positive note, though, even poorly differentiated thyroid cancers are being 'restimulated' to uptake radioactive iodine again.

Alot of work is being done in this field by Dr. Ain, who participates with, and operates this information group:
http://health.groups.yahoo.com/group/thyroidcancerhelp/

A number of articles do outline that most diffuse sclerosing variants are still treatable by conventional methods, ie still uptake radioactive iodine, though.

Just to clarify, 'Undifferentiated' terminology is usually applied strictly to anaplastic thyroid cancer, whereas other, less aggressive variants are often considered as 'dedifferentiated', or 'intermediate to poorly differentiated'; in contrast, the majority of thyroid cancers (standard papillary and follicular) are considered as 'well differentiated'; there is some good information on these distinctions in this article (Medscape registration is required, but is free):
http://www.medscape.com/viewarticle/536312

Hope a bit of this helps. It is important to know and understand that there are definitely treatment options, even in the event of dedifferentiated/intermediate or poorly differentiated tumors, and lastly, that diffuse sclerosing is not considered 'undifferentiated', and is normally considered treatable - but must be followed with an aggressive treatment course, to ensure the cells do not have an opportunity to mutate any further. All the best to you.