You will be presented with the full text of a biomedical research paper. Please write an abstract for this paper. Your response should include the abstract and no additional text.

Paper text:
pigmented paraganglioma is a special type of paraganglioma , and it is rare in the thyroid .
we report a case of a 41-year - old woman who had complained of a mass in the thyroid gland .
histology revealed tumor cells arranged in a nest - like or organoid pattern , separated by delicate fibrovascular septa .
two distinct components were observed . in the first , which constituted the majority of the tumor cells , no pigments were observed . in the second
, a few cells with pigment showed intercellular substance , but the structure was unclear . using immunohistochemistry , cells in the first component were confirmed to be diffuse strong positive for synaptophysin , but negative for chromogranin a , pan - cytokeratin , calcitonin , and thyroglobulin .
about 1% of tumor cells were stained by ki-67 . in the margins of the tumor , a few cells
were observed to be positive for hmb-45 and melan a after bleaching by oxalic acid .
using electron microscopy , a few cells containing many round melanin bodies with greater electron density granules of nonuniform size were observed .
primary melanotic paraganglioma of the thyroid gland is a rare , low malignant potential tumor . to the best of our knowledge ,
a comprehensive search of pubmed showed that only 38 cases had been reported in the english literature since 1974 .
pigmentary paraganglioma is a special type of paraganglioma that usually occurs in the uterus , orbit , heart , urinary bladder , and temporal horn , with reported cases also in the mediastinum , lumbar spine , retroperitoneum , and others . after reviewing ,
, we report this special presentation of thyroid paraganglioma with melanin , which was confirmed by immunohistochemistry and electron microscopy .
a 41-year - old woman was hospitalized for a left thyroid mass , which had been found using type b ultrasonography .
there was no enlargement of the lymph nodes in the neck on physical examination , and the color of the skin surface was normal .
the mass was located in the left lobe of the thyroid , with an irregular shape , which indicated a malignant tumor .
the maximum diameter of the mass was 2.4  cm , detected by ultrasonic examination .
scattered clusters of atypical cohesive epithelioid cells were found in the mass of the thyroid by fine - needle aspiration .
the hematoxylin - eosin staining of the specimen , together with frozen section and extensive immunohistochemistry , confirmed a diagnosis of paraganglioma of the thyroid gland with melanocytic differential .
the patient underwent a mass excision , and no lesion was found using computed tomography or magnetic resonance imaging .
four lymph nodes , which showed no metastatic carcinoma ( 0 of 4 ) , were isolated from the left neck .
the mass specimen was measured to be 3.5  cm    2.3  cm    2.0  cm . on the cut surface ,
all the visible area of the tumor was solid without cyst changes , with grayish white or black color , and the margins of the tumor were slightly irregular .
two distinct components were found in the black area of the tumor through microscopic observation .
the first component , the majority of the mass , was composed of solid sheets and clusters of cohesive epithelial cells , with no pigment observed in the cytoplasm ( figure 1a , b ) .
the second component comprised cells covered by pigment , with the intercellular substance and structure unclear ( figure 1c , d ) .
the tumor cells in the first component were arranged in a nest - like distribution without any gland cavity . the tumor consisted of monotonous sheets of cells with fluent and light - dyed cytoplasm , but without significant atypia .
tumor cells were round or polygonal , and the cytoplasm was basophilic . no mitotic figure was observed .
there was no tumor thrombus observed in the vascular tissue after multiple dissections . using immunohistochemistry , the tumor cells in the first component tested diffuse strong positive for synaptophysin ( syn ) ( figure 2 ) , but negative for chromogranin a ( cga ) , pan - cytokeratin , calcitonin and thyroglobulin .
these pigments were negative for periodic acid - schiff ( pas ) and prussian blue , but positive for fontana - masson .
of the cells surrounding the tumor , few were positive for hmb-45 and melan a after bleaching by oxalic acid ( figure 3a , b ) .
the tumor cells were negative for calcitonin and thyroglobulin ( figure 3d , e ) .
( a ) , ( b ) few cells were positive for hmb-45 and melan a after the tissue was bleached by oxalic acid .
( c ) the stromal cells were positive for s-100 after the tissue was bleached by oxalic acid .
electron microscopy showed that the tumor was composed predominantly of polyhedral cells that had the features of paraganglion chief cells , although they exhibited greater nuclear pleomorphism than normal chief cells .
half of the chief cells had abundant cytoplasm with dispersed organelles and secretory granules ( figure 4 ) ; the other half had less cytoplasm and more closely approximated organelles and granules .
the cytoplasm contained numerous mitochondria and many dense granules of the neurosecretory of endocrine type ( figure 4 ) .
these granules were of uniform electron density , membrane - limited , and 100 to 150  m in diameter .
although the cytoplasmic margins of adjacent cells were difficult to evaluate , they appeared to interdigitate with one another . as is typical of paragangliomas , the tumor cells were closely related to the capillaries present within the stroma
a few had less cytoplasm and fewer organelles , and these also had many round melanin bodies with greater electron density granules of nonuniform size ( figure 4 ) .
( a ) electron micrograph showing granules of uniform electron density , membrane - limited , and 100 to 150  m in diameter ; most had halos or clear spaces between the granule and the surrounding limiting membrane .
( b ) many round melanin bodies with greater electron density granules of uniform size were observed .
the pathologic diagnosis was of primary pigmentary paraganglioma of the thyroid gland without focal necrosis and vessel tumor thrombus .
no recurrence or distance metastasis was observed after 19  months of follow - up .
paraganglioma usually occurs in the area where paraganglia are distributed , such as the posterior peritoneum and the head and neck region .
paraganglioma of the head and neck region mainly occurs in the carotid body , jugulotympanic , vagus nerve body , and auris media .
previous articles have reported that no paraganglioma occurred in the thyroid and the precise pathogenesis of thyroid paraganglioma is unknown .
we postulate that ectopic carotid body or jugulotympanic paraganglia could be a plausible origin of the lesion .
another possibility is that paraganglioma may originate from entrapped neuroendocrine progenitor cells of the dispersed neuroendocrine system , resulting from aberrant migration of the neural crest during embryogenesis .
less than 38 cases have been reported since tavassoli first reported two pigmental paraganglioma of the uterus .
pigmented paraganglioma has been reported across a wide age range ( 17 - 57  years ) , and no gender differences were noted .
it has arisen in the bladder , posterior peritoneum , mediastinum , spine , and other sites .
. an unusual feature of this case was the presence of significant amounts of pigment . based on histochemical staining or electron microscopy ,
herein , we present a case of thyroid paraganglioma with extensive melanin melanosis in the stroma . using electron microscopy ,
it was an interesting and unique case of paraganglioma with melanocytic differentiation without evidence of melanoma of the skin .
these pigments are negative for pas and prussian blue , but positive for fontana - masson .
electron microscopy showed that the tumor cells had large , pleomorphic granules with varying electron density , size , and shape , which are identified as neuromelanin . to the best of our knowledge ,
although primary melanoma of the thyroid gland has not been previously reported , paraganglioma with melanocytic differentiation was confirmed by morphologic and immunohistochemical staining , and melanoma was definitely excluded .
however , in previous reports , it was made by morphology , special staining , and electron microscopy , which could not distinguish paraganglioma and medullary carcinoma . because the head and neck paraganglioma is a nonfunctional tumor , there is no obvious clinical presentation other than a cervical lump .
thyroid paraganglioma , medullary thyroid carcinoma , and hyalinizing trabecular tumor ( htt ) are difficult to distinguish only by histological morphology , so they are usually misdiagnosed before immunohistochemistry . as a result ,
all thyroid paragangliomas were identified as medullary thyroid carcinoma . meanwhile , some paragangliomas are extremely similar to medullary carcinoma , the histological type of which is called  paraganglioma - like medullary carcinoma . 
it features high levels of serum calcitonin and more or less amyloid in tumor stroma .
immunohistochemistry shows that apart from neuroendocrine markers such as neuron - specific enolase , cga , and syn , the tumor also expresses calcitonin , carcinoembryonic antigen , and cytokeratins .
even s-100-positive cells occurred in medullary carcinoma around the tumor nest , which has similar presentation with paraganglioma .
however , it is difficult to confuse paraganglioma - like medullary carcinoma and paraganglioma . comparatively speaking ,
the tumor cells are arranged as funicular or acini , resembling paraganglioma , and rare thyroid collagens are observed
. nevertheless , immunohistochemistry shows tumor cells are positive for thyroglobulin and thyroid transcription factor-1 , and may express cytokeratins , but are negative for calcitonin .
genetic features suggest htt is closely related to thyroid papillary carcinoma and may be the early period or histological type of papillary carcinoma , so immunohistochemistry can differentiate htt from paraganglioma . of greatest concern to clinical doctors is whether paraganglioma is malignant or not because it relates closely to resection range and continuous therapy .
however , with neuroendocrine tumors , benign and malignant are hard to differentiate for the pathologist .
the prognosis of paraganglioma needs to be comprehensively assessed by surgical approach ( biopsy , local resection , or complete resection ) , tumor size , operative margin , cell atypia , count of mitotic figure , necrosis , lymph node metastasis , vascular invasion , etc .
paraganglioma in the thyroid has been depicted as a benign progression in previous reports , although one reported that thyroid paraganglioma was malignant because it invaded the thyroid cartilage and penetrated through trachea wall into the tracheal mucosa .
its infiltrative growth mode seemed malignant , but others suspected it originated from cervical soft tissue rather than the thyroid .
primary pelanotic paraganglioma of the thyroid gland is a rare low malignant potential tumor in the thyroid .
the tumor presented focal necrosis and high mitotic index , and intravascular tumor thrombus was observed .
although these presentations could not indicate definite malignancy for neuroendocrine tumors , frequent follow - up is essential for this type of tumor .
background : pigmented paraganglioma is a special type of paraganglioma , and it is rare in the thyroid.case presentation : we report a case of a 41-year - old woman who had complained of a mass in the thyroid gland . 
 histology revealed tumor cells arranged in a nest - like or organoid pattern , separated by delicate fibrovascular septa . 
 two distinct components were observed . in the first , which constituted the majority of the tumor cells , no pigments were observed . in the second 
 , a few cells with pigment showed intercellular substance , but the structure was unclear . using immunohistochemistry , cells in the first component were confirmed to be diffuse strong positive for synaptophysin , but negative for chromogranin a , pan - cytokeratin , calcitonin , and thyroglobulin . 
 about 1% of tumor cells were stained by ki-67 . in the margins of the tumor , a few cells 
 were observed to be positive for hmb-45 and melan a after bleaching by oxalic acid . 
 the stromal cells were positive for s-100 . using electron microscopy , a few cells containing many round melanin bodies with greater electron density granules of nonuniform size 
 were observed . 
 the diagnosis of primary melanotic paraganglioma of the thyroid gland was made.conclusion:primary melanotic paraganglioma of the thyroid gland is a rare , low malignant potential tumor . 
 to the best of our knowledge , this is the first case described .