Follicular carcinomas account for 5% to 15% of primary thyroid cancers. They are more common in women (occurring in a ratio of 3:1) and manifest at an older age than papillary carcinomas, with a peak incidence between 40 and 60 years of age. Follicular carcinoma is more frequent in areas with dietary iodine deficiency (accounting for 25%–40% of thyroid cancers), while its incidence has either decreased or remained stable in iodine-sufficient areas of the world.
http://ebooksmedicine.net
Fig.20.16Follicularcarcinomaofthethyroidgland.Afewoftheglandularluminacontainrecognizablecolloid.
MORPHOLOGYOnmicroscopicexamination,mostfollicularcarcinomasarecomposedofuniformcellsformingsmallfollicles,reminiscentofnormalthyroid(
Fig.20.16 );inothercases,folliculardifferentiationmaybelessapparent.Aswithfollicularadenomas,Hürthlecellvariantsoffollicularcarcinomasmaybeseen.Follicularcarcinomasmaybewidelyinvasive,infiltratingthethyroidparenchymaandextrathyroidalsofttissues,orminimallyinvasive.Thelattertypearesharplydemarcatedlesionsthatmaybeimpossibletodistinguishfromfollicularadenomasongrossexamination.Thedistinctionbetweenfollicularadenomaandcarcinomarequiresextensivehistologicsamplingofthetumorcapsule–thyroidinterface,toexcludecapsularand/orvascularinvasion(
Fig.20.17 ).Asdiscussedearlier,invasivefollicularlesionsinwhichthenuclearfeaturesaretypicalofpapillarycarcinomasareregardedasfollicularvariantsofpapillarycancers.
Follicular carcinomas manifest most frequently as solitary cold thyroid nodules. In rare cases, they may be hyperfunctional. These neoplasms tend to metastasize through the bloodstream (hematogenous dissemination) to the lungs, bone, and liver. In contrast with papillary carcinomas, regional nodal metastases are uncommon. As many as one-half of patients with widely invasive carcinomas succumb to their disease within 10 years, while less than 10% of patients with minimally invasive follicular carcinomas die within the same time span. Follicular carcinomas are treated with surgical excision. Well-differentiated metastases may take up radioactive iodine, which can be used to identify and also ablate such lesions.
Anaplastic carcinomas are undifferentiated tumors of the thyroid follicular epithelium, accounting for less than 5% of thyroid tumors. They are aggressive, with a mortality rate approaching 100%. Patients with anaplastic carcinoma are older than those with other types of thyroid cancer, with a mean age of 65 years. Approximately one-fourth of patients with anaplastic thyroid carcinomas have a history of a well-differentiated thyroid carcinoma, and another one-fourth harbor a concurrent well-differentiated tumor in the resected specimen.
MORPHOLOGYAnaplasticcarcinomasmanifestasbulkymassesthattypicallygrowrapidlybeyondthethyroidcapsuleintoadjacentneckstructures.Onmicroscopicexamination,theseneoplasmsarecomposedofhighlyanaplasticcells,whichmaybelargeandpleomorphicorspindleshapedandinsomecasesmixtureofthetwocelltypes.
Fociofpapillaryorfolliculardifferentiationmaybepresentinsometumors,suggestingoriginfromabetter-differentiatedcarcinoma.