Using the 3-tiered system for categorizing workup of incidental thyroid nodules detected on CT, MRI, or PET/CT: How many cancers would be missed?

Background: A 3-tiered system has been proposed by radiologists for the reporting and workup of incidental thyroid nodules (ITN) detected on computed tomography (CT), magnetic resonance imaging (MRI), or positron emission tomography/computed tomography (PET/CT). It has been shown to reduce the workup rate, but there remains concern about missed malignancies. This study aims at estimating the proportion of incidental cancers that would be missed relative to all thyroid cancers if the 3-tiered system were applied to ITN seen on CT, MRI, or PET/CT. We also aim to characterize these missed incidental cancers by histology and tumor stage. Methods: A retrospective review was performed of 680 consecutive patients with thyroid cancer who underwent surgery between January 2003 and December 2012. Medical records were reviewed to identify incidental thyroid cancers detected on imaging. Patients with incidental cancers detected on CT, MRI, or PET/CT were categorized according to the system as 3-tiered system-positive and 3-tiered system-negative. The system recommends that only 3-tiered system-positive ITN undergo further workup with ultrasound. Three-tiered system-negative cancers were the cancers that would have been missed if the system were used in practice. These tumors are described by tumor type, size, and stage to determine the potential impact if the 3-tiered system were adopted. Results: One hundred and one patients had imaging-detected incidental cancers, of whom 64 met the inclusion criteria and were originally detected on CT, MRI, or PET/CT. Eight were 3-tiered system-negative, which represents 13% of the 64 incidental cancers that could be categorized and 1.2% of all thyroid cancers treated at our institution in the 10-year period. Three-tiered system-negative tumors were all papillary in histology and had a median size of 12mm (interquartile range 10-12mm). Six tumors (75%) were American Joint Committee on Cancer (AJCC) stage I, one was AJCC stage II, and one was AJCC stage III at diagnosis. Conclusions: Based on thyroid cancers diagnosed during a decade, incidental malignancies missed by the 3-tiered system represent 1.2% of all thyroid malignancies. Three-tiered system-negative incidental cancers were all small papillary cancers. Given that few cancers would be missed and most are less aggressive, we propose that the 3-tiered system could be adopted in clinical practice to guide the workup of ITN identified on CT, MRI, and PET/CT.