An added benefit of sequential stimulated diagnostic radioiodine whole body scans (WBS) and 18F-Fluorodeoxyglucose (FDG) Positron Emission Tomography/Computed Tomography (PET/CT) scans in the management of higher-risk differentiated thyroid cancer (DTC)

Objectives Diagnostic radioiodine WBS and FDG PET/CT scans both play an important role in the management of higher-risk DTC. Generally one or the other is performed as part of the management of DTC patients, but for several years our institution has offered a protocol in which both of these scans are performed sequentially with stimulation either by thyroid hormone withdrawal (THW) or with the use of recombinant human Thyroid Stimulating Hormone (rhTSH). This protocol was performed on a small subset of our higher-risk patients in which the referring endocrinologist felt that the added information and the abbreviated nature of the scan combination were favored over the usual standard of care. Following adequate stimulation with either THW or with two intramuscular injections of rhTSH, the patient undergoes a PET/CT of the whole body followed by a dedicated, delayed PET/CT of the neck. Upon successful completion of this scan, the patient is administered radioiodine (RAI), 123I or 131I, orally and then returns 24 hours following a 123I dose or 48 hours after a 131I dose to receive a WBS. Baseline and stimulated thyroglobulin (Tg) levels are obtained to assist with diagnosis. These studies are usually temporally separated, often done on separate weeks and with repeated stimulation, particularly with the use of rhTSH.

Methods Thirty randomly selected studies were analyzed over a 3-year period from November 2011 through October 2015. All studies were performed on a single PET/CT camera and one of two gamma cameras. Collected data included demographic information, PET/CT and WBS results, Tg levels, additional radiographic data, and clinical information in an effort to ascertain the benefits of the combined scans over either scan alone.

Results The patients who underwent this sequential protocol were predominantly female (22/30) and ranged in age from 20 to 78 years (mean 49.4). All had DTC, with 27 diagnosed with papillary thyroid cancer (PTC), including one with Struma Ovarii, two with follicular thyroid cancer (FTC), and one with components of both PTC and FTC. All but one underwent rhTSH stimulation. The majority were TNM stage 1 (20/30) at initial diagnosis, although five were stage 2, one was stage 3, and four were stage 4. The primary indication for the combination of studies was the clinical or pathological aggressive features of the disease, and the time from diagnosis to scan ranged from 0.3 to 18 years (mean 6.9). All but two had been treated with 131I in the past. Residual or recurrent disease was diagnosed either clinically or by tissue confirmation in eleven patients, one of whom was also diagnosed with a new secondary malignancy (papillary urothelial carcinoma). PET/CT proved very sensitive, accurately detecting all of these cases. PET/CT proved relatively nonspecific, identifying multiple suspicious findings not subsequently confirmed as disease. WBS proved less sensitive, detecting only four of those with disease, although only three false positives were noted with WBS. WBS demonstrated a benefit accurately showing a good response to therapy and predicting RAI distribution following therapy. Stimulated Tg levels accurately detected six patients with disease; three of the patients not detected in this manner had Tg antibodies, although one patient with biopsy proven PTC demonstrated no detectable Tg stimulation.

Conclusions Sequential stimulated WBS and PET/CT provides a valuable tool at our institution for rapidly and efficiently evaluating higher-risk DTC patients. PET/CT proved highly sensitive, if unspecific by itself. WBS was relatively insensitive, unsurprising given the patient population, although did demonstrate other clinical benefits. Tg levels were relatively specific, but complicated by antibodies. We continue to offer this exam as a valued tool for our endocrinologists.