thyroid lesions containing gross fat include heterotopic fat nests , thyrolipoma , teratoma , liposarcoma , and thyroid cancers of the papillary or follicular type ( 234567 ) . most of these cases are only described upon obtaining pathological data after surgery ; radiologic methods are of limited use ( 89 ) . with the approval of the institutional review board of our hospital , we reported the case of an intrathyroid nodule composed mainly of gross fat detected by computed tomography ( ct ) and confirmed by lobectomy as a follicular variant of papillary thyroid cancer ( fvptc ) with mature fat . a 58-year - old woman presented with an incidentally detected mass in the thyroid on routine ultrasonography ( us ) at a primary clinic . the patient had a 10-year history of diabetes mellitus and hypertension , and was currently taking medication . physical examination revealed a firm mass of approximately 3 cm in size in the right anterior neck . laboratory tests revealed that serum anti - microsomal antibody , anti - thyroglobulin antibody , thyroglobulin , calcitonin , t3 , free t4 , and thyroid - stimulating hormone levels were within normal ranges . ultrasonography revealed an ovoid lesion with a smooth margin of approximately 2.9  2  1.3 cm in size . the nodule was generally hyperechoic , with the peripheral portion more hyperechoic than the central area . the perithyroidal fat , muscle , and vertebral bodies were therefore not clearly visualized ( fig . doppler us did not show any vascular signals in the nodule . based on these findings a board - certified radiologist performed us - guided fine needle aspiration using a freehand technique with a 23-gauge needle and a 5-ml disposable plastic syringe . cytologic smears were made on glass slides and immediately fixed in 95% alcohol for both papanicolaou staining and may - grunwald - giemsa staining . cytology revealed atypia of undetermined significance or a follicular lesion of undetermined significance with negative for braf mutations and positive for an nras 61 mutation . the thyroid ct protocol at our hospital includes pre - contrast , early , and delayed phases . early phase images ( 40 seconds ) were scanned from the hyoid bone to the thoracic inlet to cover the entire thyroid and level iii - vi lymph nodes . delayed scans ( 90 seconds ) covered the area from the skull base to the upper mediastinum . all scans were acquired with a slice thickness of 2.5 mm and reconstruction increment of 2.5 mm . pre - contrast ct showed markedly low attenuation ( mean ct number , -80 hounsfield units [ hu ] ) , in a well - defined mass with fine reticular and thick septa - like structures in the parenchyma of the right mid - to - upper portion of the thyroid ( fig . 1b ) . to measure the enhancement pattern of the tumor , 25 mm of the region of interest was placed in the area of the fatty mass , including the reticular and thick septa - like soft tissue lesions . the tumor was enhanced in the early phase ( mean ct number , 16 hu ) and was washed out in the delayed phase ( mean ct number , -13 hu ) ( fig . the tumor was ovoid in the longitudinal direction , had a lobular appearance in the axial plane , and was completely encapsulated in the thyroid parenchyma , with no evidence of extrathyroidal extension . based on these findings , we suspected a fat - containing , neoplastic lesion of the thyroid gland , such as thyrolipoma , teratoma , lipoma , low - grade liposarcoma , or a follicular tumor with stromal fat . considering the results from cytopathology and the gene mutation analysis it showed a yellowish , homogeneous , and well - demarcated nodular lesion measuring 2.5  2.2  1.5 cm at the upper - to - mid pole . microscopic examination revealed a well - circumscribed , thin , fibrous , and capsulated nodule with mature fat . there were atypical follicular cells with nuclear enlargement , nuclear overlapping , chromatin clearing , and nuclear grooves without papillary structures in the nodule ( fig . on immunohistochemistry , the tumor cells stained positive for hbme-1 ( dako , glostrup , denmark , 1:100 ) and galectin-3 ( novocastra , newcastle upon tyne , uk , 1:200 ) . finally , we tested for the v600e and k601e mutations in exon 15 of the braf gene and codons 12 , 13 , and 61 of the kras and nras genes . in our case , the tumor was a yellowish , homogeneous , and well - demarcated nodular lesion on gross examination , with negative value of attenuation on ct , suggesting fat . however , it was difficult to recognize the tumor as a fat containing mass on us examination . the hyperechoic nature of the mass and the curtain - like hyperechoic shadowing posterior to the mass may suggest rich fatty component . we suspected that the hyperechoic shadowing might have been due to a reverberating artifact caused by innumerable fat globules and septa of the tumor , similar to the artifact frequently observed along the thick abdominal walls that consist of several layers of fat globules surrounded by connective tissues , skin , and fascia . computed tomography and gross specimen findings of our case were very similar to those of thyrolipoma , a benign fat - containing tumor ( 810 ) . thyrolipoma is considered a variant of follicular adenoma , which is characterized by a well - circumscribed and encapsulated nodule resulting from the proliferation of thyroid follicles admixed with mature fat ( 1011 ) . this case could have been misdiagnosed as a follicular neoplasm or thyrolipoma , as the tumor cells did not show a papillary growth pattern upon microscopic examination and the gross features were very similar to thyrolipoma . two reports provided the ct and magnetic resonance imaging findings of thyrolipoma ( 812 ) . the mass predominantly had fat attenuation and distinct margins in the thyroid gland . however , the authors only performed pre - contrast ct on the lesion . therefore , it is uncertain whether thyrolipoma and thyroid cancer with massive stromal fat can be differentiated by ct with contrast enhancement . in our case , the tumor was enhanced in the early phase and slightly washed out in the delayed phase . fat - containing thyroid cancer of the papillary or follicular type , have only been reported from pathological studies ( 2456 ) . fat tissue was observed only by microscopy , so it was assumed that small amounts of fat tissue could not be detected by ct . we also assumed that the detectability of the fat component is variable , as it depends on the total number of fat cells in the mass . mature teratoma can be recognized if the fat - containing mass contains calcification , bone or cartilage , and cystic components . however , this is not possible , as radiological images do not always exhibit all these attributes ( 7 ) . ct images of liposarcoma ( 3 ) in the thyroid gland are very similar to those in our case , and differentiating between teratoma , thyrolipoma , liposarcoma , and papillary thyroid cancer with massive fat may be impossible using radiology alone . some investigators have insisted that fat cells are derived from displaced remnants of embryonic structures in the thyroid . in amyloid goiter , schrder and bcker ( 1 ) postulated that adipose tissue may be derived from metaplasia of stromal fibroblasts due to either impaired circulation and diffusion or tissue hypoxia caused by amyloid deposition . derienzo and truong ( 5 ) suggested that the fat cells in thyrolipoma and follicular carcinoma might be neoplastic tissue because 1 ) fat did not exist in the normal thyroid , 2 ) fat was a major component of the tumor , 3 ) there was no evidence of degenerative changes such as dystrophic calcification , cystic or hemorrhagic degeneration , or amyloid deposition , 4 ) fat was widely distributed in the tumor , and 5 ) there were similar fat - containing tumors in other organs such as thymolipoma , parathyroid adenoma , and fat - containing fibroadenoma of the breast . our case showed similar microscopic features to theirs in agreement with the suggestion that the fat cells might be neoplastic cells themselves . in summary these findings included a hyperechoic mass with a smooth margin with a reverberating artifact and no detectable doppler signal on us . ct revealed a well - encapsulated fat - attenuation mass with enhancing septa - like structures . fvptc should be included in the differential diagnosis of fat - containing focal lesions in the thyroid , in addition to thyrolipoma , teratoma , liposarcoma , and small focal fat nests near the capsule . moreover , fine needle aspiration cytology with gene analysis may be helpful in determining the nature of fatty tumors before surgery .
thyroid cancer may have small adipose structures detected by microscopy . however , there are no reports of thyroid cancer with gross fat evaluated by radiological methods . 
 we reported a case of a 58-year - old woman with a fat containing thyroid mass . 
 the mass was hyperechoic and ovoid in shape with a smooth margin on ultrasonography . 
 on computed tomography , the mass had markedly low attenuation suggestive of fat , and fine reticular and thick septa - like structures . 
 the patient underwent a right lobectomy . 
 the mass was finally diagnosed as a follicular variant of papillary thyroid cancer with massive stromal fat .