Incidental Findings in Nuclear Medicine Scans

Thyroid "hot spots" incidentally detected by whole body Fluorodeoxyglucose-Positron Emission Tormography (FDG-PET) scan

Fluorodeoxyglucose (FDG) whole body positron emission tormography (PET) scan is being used more often in the diagnostic follow-up or work-up of patients. In such conditions, positive PET scans with unanticipated hot spots within the thyroid region could be given the definition of thyroid FDG-PET incidentaloma, a name analogous to unexpected sonographic thyroid modules. A description of eight consecutive patients referred to the endoctrine department due to thyroid "hot spots" is given, incidentally detected by whole body FDG-PET scan. By applying ultrasound, histology reports, and fine needle aspiratory cytology (FNAC) in an experiment, an identification of pathology underlying thyroid FDG-PET incidentaloma will be attempted. FNAC showed a hint for surgery in all patients. Surgery has been carried out in 7 patients. There was a correct identification of malignancy in five patients; two having medullary thyroid carcinomas, one with lymph node invasion, and three having papillary thyroid carcinomas with attack through the thyroid capsule in two of the PTC cases. In two patients having a positive FDG-PET scan, FNAC showed indication of follicular neoplasms, and final reports of histology indicated follicular adenoma. In the last patient, FNAC disclosed a follicular lesion, but no surgery has been carried out so far. Summarily, a small series of successive thyroid FDG-PET incidentaloma cases is obtainable and suggests a high rate of clinically relevant malignancies (Wang, et al., 2000).

Introduction

The discovery of thyroid incidentalomas takes place through various ways. Normally, they are detected in the process of an imaging study, such as ultrasound (U.S.) or computerized tormography (CT) for suspected non-thyroid diseases. U.S. studies account for prevalence of a nodule ranging from 19 to 46% in the general population. The risk of carcinoma in insubstantial thyroid nodules range from 1.5 to 10% (Burguera & Hossein, 2000). Fluorodeoxyglucose (FDG) positron emission tormography (PET) is different from conventional imaging methods, such as CT and U.S., that depend on alterations of the morphology for tumor detection. It is actually a functional imaging method that depends on in vivo visualization of lesional glucose metabolism. Malignat and inflammatory lesions indicate elevated rates of glycolysis and glucose uptake (Fischman, 1993). In thyroidology, the clinical value of FDG-PET is set up in the follow-up, and localization of reappearances in a subgroup of thyroid cancers, that is, papillary and follicular thyroid cancer reappearances with increased thyroglobulin levels and negative radioiodine scan of the whole body (Wang et al., 2000), and metastatic medullary thyroid cancer.

On the other hand, there is no reputable function of FDG-PET in the diagnosis of exceedingly prevalent cold thyroid nodules. To distinguish recurrent benign from uncommon malignant lesions, the preferred approach now is fine needle aspiration cytology (FNAC). FDG-PET is progressively used in the diagnostic work-up and in the follow-up of patients in departments of medicine and oncology. In the process of a whole body FDG-PET for a non-thyroid malignancy or a para-neoplastic event, a scan with an unpredicted hot spot inside the thyroid area is seldom found. I would like to describe this phenomenon as a thyroid FDG-PET incidentaloma. This communication depicts the first sequence of eight successive thyroid FDG-PET incidentaloma cases.

Subjects and Methods

Subject A

A 51-yaer-old male was referred to a gastro-enterologist due to an elevated carcinoembryonic antigen (CEA) value of 12.4µg/liter, the normal value should be less than 3µg/liter. The results for Clinical examination, CT scan of the abdomen, gastroscopy, and ileocoloscopy showed negative. A FDG-PET scan exposed a hot spot at the right side in the thyroid area. A consequent calcitonin value was eminent at 158ng/liter while the normal value should be less than10ng/liter. Thyroid U.S. indicated a hypoechoic nodule in the right lobe. FNAC guided by Ultrasound suggested a medullary thyroid carcinoma (MTC). An assessment to rule out pheochromocytoma (urinary catecholamine excretion and adrenal U.S.) resulted negative. Overall thyroidectomy and bilateral careful lymph node dissectionexposed a gray 1-cm diameter lesion in the right upper pole. This tumor comprised of sheets of large polygonal cells having eosinophilic cytoplasm divided by fibrous stroma. Positive immune-staining for CEA and calcitonin proved the diagnosis of MTC, lymph nodes did not have metastasis. At post-surgery assessment which was done 3 months later, serum CEA and calcitonin were normal.

Subject B

A 71-yr-old female got back to her gastro-enterologist for postsurgical assessment 3 months after a supposed curative resection of a pT2N0M0 colonic carcinoma. The hemoglobin value had normalized. The CEA value, however, continued raised at 56.8 µg/liter in comparison to the preoperative value of 42.7µg/liter. A CT scan carried out on the abdomen discovered neither local reappearance nor liver metastasis. A FDG-PET scan indicated a cervical hot spot related to the right lower thyroid lobe. A high calcitonin 853ng/liter compared to the normal which is less than 10ng/liter, the U.S. image of a hypoechoic nodule (20 mm) in the right thyroid lobe and the outcome of FNAC (immunocytochemistry for CEA and calcitonin was positive) held up the suspicion of MTC. A preoperative assessment for pheochromocytoma was negative: adrenals were usual on the earlier performed CT scan, and the secretion of urinary catecholamines was usual An entire thyroidectomy and bilateral careful lymph node dissection exposed a gray tumor having a diameter of 22mm in the right lobe. It comprised of large polygonal cells divided by fibrovascular septa. Congo Red stain indicated amyloid deposits in these septa. Tumor cells marked positive for chromogranin and calcitonin verifying the diagnosis of MTC. A single ne lymph node in the region of the right periodic nerve had an MTC metastasis. Further radiotherapy (50 Gy) of the neck was consequently carried out. Afterward, CEA had gone back to normal (1µg/liter) and calcitonin stayed a little high at13ng/liter.

Subject C

A 53-yr-old female was brought before the rheumatology department for examination of anemia. A mixed connective tissue disease was earlier alleged A normochromic anemia having a hemoglobin value of 80 g/liter could not be explained. Bone marrow aspiration and biopsy indicated weak erythropoiesis and a preponderance of myeloid precursors. These non-pathognomonic modifications suggested the possibility of a thymoma or a paraneoplastic occurence. A CT scan of the thorax exhibited a clearly demarcated mass in the left anterior mediastinum relating to a thymoma or lymph nodes. An FDG-PET scan indicated two hot spots, one relating to the left anterior mediastinal mass and a second one relating to the right thyroid lobe. Thyroid U.S. indicated an uneven hypoechoic nodule in the right thyroid lobe and FNAC of this nodule pointed to a papillary thyroid carcinoma. A joint surgical process for resection of the thoracic and cervical tumors was carried out. After sternotomy, a large mass, having a diameter of a 35 mm was discovered at the back of the left clavicle and first rib, attacking the pleura and the left lung. A total thyroidectomy and cervical lymph node dissection followed the resection of this mass. The mediastinal mass comprised of a large lymph node having an undifferentiated tumor with numerous psammoma bodies on one side and on the other side foci of squamous differentiation. The diagnosis of a thyroid papillary carcinoma with attack of the thyroid capsule and a cervical lymph node was established; the mediastinal tumor persisted as a diagnostic difficulty, a dedifferentiated metastasis of the thyroid carcinoma was in all likelihood. For that reason, post- therapeutic scans indicated uptake in the thyroid bed and in the left mediastinum. At the final CT scan of the chest, progression of tumor in the mediastinum was present.

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Subject D

A 52-yr-old women put before the hematology department for a follow-up visit. Morbus Hodgkin had been diagnosed for nodular sclerosing type, stage II B. with iliacal and inguinal lymph nodes. After six rounds of chemotherapy, which included Adriamycin, Bleomycin, Velbe, and Dacarbazin scheme, she stayed put in absolute reduction. Nevertheless 11 months following the preliminary diagnosis she complained of joint pain, particularly at the left hip and a FDG-PET scan with particular concentration to this area was ordered. It was negative with the exception of diffuse thyroidal uptake. Thyroiditis or attack by the lymphoma was suggested as the likely causes of the thyroidal uptake. On clinical inspection bilateral thyroid nodularity was detected and established by U.S.. FNAC of the right and left nodules indicated very cellular aspirates containing cell groups and handful follicles. A small number of mitoses were observed. A few atypical cells had an uneven lengthened nucleus, colloid was not present A small number of small and larger lymphoid cells were observed. The cytological diagnosis was follicular neoplasia and a complete thyroidectomy was carried out to rule out follicular carcinoma. The histologic diagnosis was follicular adenoma with Hashimoto thyroiditis.

Subject E

A 57-yr-old woman presented at the rheumatology department due to anemia, joint pains, and myalgia. She had refused invasive processes, and steroids had been prescribed for a resumed polymyalgia rheumatica, but upon narrowing the steroids the clinical picture depreciated with a toxic anemia (hemoglobin value of 70 g/liter) and loss of weight. Bone marrow target and biopsy were unconvincing. An FDG-PET scan was ordered to discover the primary location of inflammation or to spot a vague primary tumor. The FDG-PET scan indicated improved bone marrow uptake and a left cervical hot spot. FNAC of an hypoechoic nodule in the left thyroid lobe pointed to papillary carcinoma. Total thyroidectomy was carried out. The diagnosis of a PTC was ascertained. Even though small, measuring 13 mm in diameter, there was attack via the thyroid capsule in the strap muscles. Anemia endured, and a second bone marrow target and biopsy suggested the second difficulty: numerous myeloma.

Subject F

A 63-yr-old women presented to the oncology department complaining of low back pain in December 1999. Her past medical history comprised of a right mastectomy in 1988 and left breast carcinoma removal and adjunctive irradiation in 1996. Spinal MRI images pointed to metastatic lesions to the sacrum and the eighth dorsal vertebra. In the lack of proof of other metastatic lesions, an FDG-PET scan was carried out to ascertain the findings of MRI. It indicated a sacral lesion and an unpredicted right anterior cervical lesion. The patient was referred for FNACof an hypoechoic nodule in the right thyroid lobe having some microcalcifications. FNAC was poorly cellular and indicated sheets of distended follicular cells with large eccentric nuclei. Intranuclear pseudoinclusions were observed in some of the cells. A small amount of colloid was present. Cell margins were emphasized. The diagnosis of papillary carcinoma was put forward. Total thyroidectomy was carried out and final histology has been debated. Atypia, intranuclear pseudoinclusions, and nuclear grooves were there in some cells but were less prominent in comparison to the nuclear atypia in the cytology sample. Negative immunohistochemistry for thyroid peroxidase and positive immunostaining for HBME1 confirmed further the final diagnosis of a PTC.

Subject G

A56-yr-old man was referred to the pneumology department due to small peripheral lung nodules on chest x-ray noticed in the course of preoperative screenings for a hernia. A FDG-PET scan was ordered to find an unknown primary tumor or other metastatic lesions. A hot spot in the right lung top or anterior mediastinum was spotted. A thoracoscopic lung biopsy of the peripheral nodules was carried out and indicated varied dust fibrosis. The interest was then drawn to the thyroid lesion and the patient was referred for FNAC of an unpalpable retroclavicular nodule in the inferior right thyroid lobe. Cytology disclosed sheets and groups of Hurthle cells with a large quantity of cytoplasm, distended nuclei, and anisokaryosis. Colloid was profusely available. The diagnosis of follicular neoplasm, most likely Hurthle cell adenoma, was made. During this Hurthle cell tumor, surgery was considered and ascertained the diagnosis of a Hurthle cell adenoma.

Subject H

An 80-yr-old man was referred for examination of a large necrotic, pointing to a liver metastasis. Ten years before, in 1991 a left upper lung lobectomy was carried out for a squamous cell carcinoma. A FDG-PET scan indicated a hot spot in the liver and a second one in the right anterior cervical region. FNAC of a hypoechoic nodule in the right thyroid lobe was carried out: cell groups and some rosettes were there. The nuclei of the follicular cells were a little blown up A small quantity of colloid was available, and multinucleated giant cells and calcifications. It was categorized as a follicular neoplasm, most likely follicular adenoma. A liver biopsy indicated a large cell undifferentiated carcinoma. Immunostaining was positive for keratine and negative for thyroglobulin. The histology specimen of the lung carcinoma resected in 1991 was reworked and diagnosed as a well differentiated squamous cell carcinoma. The liver mass was thought of as a likely metastasis of the lung tumor. The thyroid lesion is left without treatment.

FDG-PET method

The PET imaging was carried out with a CTI-Siemens HR+ scanner (Knoxville, TN) having an axial field of view of 15 cm, and a spatial resolution of 6 mm. All patients fasted during 6 hours prior to the tracer administration. Sixty minutes after the iv injection of 6.5 megabecquerels/kg 18F-FDG with a possiblea maximum of 555 megabecquerels, a whole-body emission scan was carried out. The raw imaging data were recreated in a 128x128 matrix using an in-house iterative recreation algorithm without reduction correction.