Thyroid incidentalomas with increased focal 18F-FDG uptake in 18F-FDG PET/CT of a patient with multiple primary cancers.

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Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. The images or other third party material in this article are included in the article's Creative Commons license, unless indicated otherwise in a credit line to the material. If material is not included in the article's Creative Commons license and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this license, visit http://creativecommons. org/licenses/by/4.0/.  7), corresponding to a stage II disease, as shown on the maximum intensity projection image (A; blue arrow). In addition, multifocal intense 18 F-FDG uptake in the right breast (SUV max 14.4) and hypermetabolic axillary lymph nodes on the right side (SUV max 11.3)highly suspicious for a second primary malignancywere detected (A; green arrows). Consecutive fine-needle aspiration cytology of all lesions confirmed the simultaneous diagnosis of multifocal breast cancer (G2, HER2positive) with axillary metastases on the right side. Moreover, the initial 18 F-FDG PET/CT scan revealed two focal lesions in the right thyroid lobe (SUV max 11.0; A-D; red arrows). Given the confirmed diagnosis of two tumor entities and the lack of a therapeutic consequence, initial histopathological examination of the two thyroidal lesions was not performed. After three cycles of neoadjuvant chemotherapy (3 × 5-fluorouracil, epirubicin, and cyclophosphamide (FEC); 3 × docetaxel, trastuzumab, and pertuzumab) and additional radiation therapy to the neck, the patient received a follow-up 18 F-FDG PET/CT scan that revealed a complete response both of the Hodgkin's lymphoma (according to Lugano 2014 criteria) and the metastasized breast cancer (in terms of RECIST and PERCIST) with concomitant reactive activation of the bone marrow and of the spleen (H). Interestingly, the two previously hypermetabolic thyroid lesions also showed a complete response (E-G)ultimately indicating a malignant origin, e.g., Hodgkin's lymphoma of the thyroid, breast cancer metastases to the thyroid gland or a third primary thyroid tumor. While the incidental finding of a focal thyroid 18 F-FDG uptake in 18 F-FDG PET/CT is rare and only occurs at a frequency of 1.1-4.2% [1], thyroid incidentalomas carry a significant risk of malignancy that is reported to be 23.0-63.6% [1]. This risk of malignancy is especially high when thyroid lesions show focal 18 F-FDG uptake [2,3], i.e., when the PET scan (rather than the CT image) shows a suspicious finding and when their SUV max is above 4.2 [2].
Histopathological evaluation of thyroid incidentalomas shows papillary thyroid carcinoma to be the most prevalent thyroid malignancy, whereas metastases to the thyroid gland are mostly derived from renal cell carcinoma (in a clinical setting) or lung cancer (in autopsy series). Hodgkin's lymphoma of the thyroid shows a female preponderance, but is extremely rare, and breast cancer metastases to the thyroid are seldomly reported. However, an association between thyroid cancer and breast cancer has been described in the literature. For the evaluation of a thyroid incidentaloma, both PET (focal 18 F-FDG uptake, high SUV max ) and CT (low attenuation) can be helpful [2] while ultrasound is still the mainstay to stratify the risk of malignancy. Still, prompt histopathological examination should be performed for definitive diagnosis. Here, a biopsy of the thyroid incidentalomas would have been obligatory in case of persistence or progression under treatment