Category: Thyroid

Monitor: 22

22 - BROWN FAT: A COPY CAT

Thursday, Apr 25
11:30 AM – 12:00 PM

Objective :

Fluorine-18 fludeoxyglucose positron emission tomography (FDG-PET) utilizes increased glucose uptake to detect metabolically active tissue. We present a case in which its use to evaluate for suspected malignancy may have been affected by the presence of uptake by brown adipose fat (BAT).


Methods : n/a


Results : We present a 50 year-old underweight female (BMI 14.7 kg/m2) with stage I T3,N1b,M0 papillary thyroid cancer (PTC) in 2012, treated with total thyroidectomy and modified right neck dissection, followed by radioactive iodine ablation (RIA) with 157 mCi of 131-I. Following loco-regional recurrence in 2014, she underwent resection of two PTC-positive lymph nodes in the left central compartment and RIA with 150 mCi of 131-I. There was no abnormal uptake on post-treatment scan. Over the last 6 years, despite TSH suppression to < 0.1 mU/l, thyroglobulin (Tg) levels increased with a doubling time ~24 months. Recent thyrogen stimulated whole body scan was negative with a Tg  209 ng/mL. Neck ultrasound showed no suspicious lymphadenopathy and a 6mm right thyroid bed lesion which was biopsy-positive for PTC.  FDG-PET showed hypermetabolic brown fat in the bilateral cervical, supraclavicular and retrosternal regions limiting sensitivity at those sites.


Discussion :

BAT can cause high uptake of 18 F-FDG on PET, which may interfere with interpretation of the scan either by mimicking a tumor or hindering the ability to detect uptake by a tumor in the given area. To localize tumors, markers for cellular activity are used in nuclear medicine with increased glucose uptake indicating possible tumor presence. On FDG-PET, hypermetabolic BAT is often recognized by increased uptake in the neck, supraclavicular, mediastinum, paravertebral, and suprarenal areas. In our patient with a high stimulated TG but only a 6mm identified area of tumor, it is believed that BAT may be obscuring metastatic disease. Our patient fits the profile for increased BAT as an underweight woman with cancer and iatrogenic hyperthyroidism. Understanding factors that influence BAT activation can help us implement interventions to reduce activation such as preparing patients in a warm room, scanning patients early in the morning or administering medications such as propranolol.


Conclusion :

BAT activation on FDG-PET can be a false positive in the supraclavicular area which is the site of potential metastatic PTC lymph nodes or a false negative in young females with low BMI in sites where brown adipose tissue is commonly seen. In cases with high clinical suspicion, the presence of BAT showing high 18 F-FDG uptake should not exclude malignancy and efforts should be undertaken to decrease BAT activity on further imaging.

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Priya Jaisinghani

Resident
Rutgers Robert Wood Johnson Medical School
Edison, New Jersey

Internal Medicine Resident at Rutgers Robert Wood Johnson

Sara E. Lubitz

Associate Professor
Rutgers Robert Wood Johnson Medical School
Westfield, New Jersey

Sara Lubitz, MD is an Associate Professor at Rutgers Robert Wood Johnson Medical School where she serves as Program Director for the Endocrinology Diabetes and Metabolism Fellowship. She earned her medical degree from Rutgers New Jersey Medical School (formerly UMDNJ) and completed Internal Medicine residency, chief residency, and Endocrinology fellowship at New York University. Areas of clinical interest include pituitary, thyroid and adrenal disease. She serves as Medical Director of the Pituitary Center at Robert Wood Johnson University Hospital and runs clinical trials in Acromegaly, Cushing’s disease and hypoparathyroidism. Dr. Lubitz is a member of the AACE/APDEM joint liaison committee working on supporting endocrine fellows-in-training as future clinicians and colleagues and is on the board of directors of the NJ Chapter of AACE.

David A. Cohen

Assistant Professor of Medicine, Vice Chair of Education
Rutgers Robert Wood Johnson Medical School

Dr. David A Cohen is an Assistant Professor of Medicine at the Robert Wood Johnson Medical School in New Brunswick, NJ. His clinical interestes within endocrinology are thyroid disease and male hypogonadism. He also serves as a hospitalist on the medical teaching service for the Robert Wood Johnson Internal Medicine Residency Program. Academically, he serves as the Vice Chair of Education for the Department of Medicine. Academically, his research interests within education are in active learning, concept video creation, and health care disparities.

Jeffrey Kempf