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Reply: ¹⁸F-FDG PET of Thyroid Nodules with Inconclusive Cytologic Results

Radboud University Nijmegen Medical Centre, Nijmegen, The Netherlands

REPLY: We thank Martínez-Duncker and Hurtado-López for their comments about our paper on ¹⁸F-FDG PET in thyroid nodules with inconclusive cytologic results (1). We agree with them that several other radiopharmaceuticals besides ¹⁸F-FDG PET can be considered for evaluation of solitary thyroid nodules. They refer to their recent paper on the use of ^99mTc-MIBI for this purpose (2). Like ¹⁸F-FDG PET, ^99mTc-methoxyisobutylisonitrile (MIBI) was positive in all patients with thyroid cancer (1,2). For ¹⁸F-FDG PET, this is in line with data in the literature (3,4). However, several studies in the literature do report false-negative ^99mTc-MIBI findings in thyroid cancer (5,6).

Furthermore, their study (2) was designed differently from ours (1). First, 78 of 130 patients (60%) underwent diagnostic fine-needle aspiration biopsy (FNAB). These patients do not require any additional imaging—neither ¹⁸F-FDG PET nor ^99mTc-MIBI. This type of group was not included in our study. The remaining 52 patients (40%) had inconclusive FNAB results (2). This is a relatively large percentage of patients, because inconclusive aspirates typically are reported in up to 20% of cases (7,8). The number of patients in the inconclusive-FNAB group who ultimately had thyroid cancer was also relatively high (18/52, or 35%) (2), whereas approximately 20% is typical (9). In our series, the prevalence of thyroid cancer was 14% (1).

When they separately analyzed the results of ^99mTc-MIBI imaging in the 52 patients with inconclusive FNAB results (2), 20 of 52 patients (38%) showed negative findings on ^99mTc-MIBI scintigraphy and were diagnosed with benign disease. In our study (1), ¹⁸F-FDG PET findings were negative in 25 patients (57%), all of whom had benign thyroid nodules.

In conclusion, ¹⁸F-FDG seems to be less prone to accumulation in benign thyroid nodules than is ^99mTc-MIBI, thereby allowing better selection of patients with inconclusive FNAB results who require surgical intervention. Furthermore, unlike ¹⁸F-FDG PET, the literature contains evidence that false-negative ^99mTc-MIBI findings do occur in thyroid cancer. For these reasons, we do not support the recommendation of Martínez-Duncker and Hurtado-López that ^99mTc-MIBI scans should be the first study performed in the diagnostic algorithm of patients with thyroid nodules and inconclusive cytologic results to avoid unnecessary thyroid surgery. However, a definite conclusion in favor of either ¹⁸F-FDG PET or ^99mTc-MIBI can be drawn only in a prospective comparative study with appropriately selected patients, especially in view of the high percentage of inconclusive FNAB results and the high prevalence of thyroid cancer in their series (2).

FOOTNOTES

COPYRIGHT © 2006 by the Society of Nuclear Medicine, Inc.

References

1. de Geus-Oei L-F, Pieters GFFM, Bonenkamp JJ, et al. ¹⁸F-FDG PET reduces unnecessary hemithyroidectomies for thyroid nodules with inconclusive cytologic results. J Nucl Med. 2006;47:770–775.[Abstract/Free Full Text]
2. Hurtado-Lopez LM, Arellano-Montano S, Torres-Acosta EM, et al. Combined use of fine-needle aspiration biopsy, MIBI scans and frozen section biopsy offers the best diagnostic accuracy in the assessment of the hypofunctioning solitary thyroid nodule. Eur J Nucl Med Mol Imaging. 2004;31:1273–1279.[Medline]
3. Kresnik E, Gallowitsch HJ, Mikosch P, et al. Fluorine-18-fluorodeoxyglucose positron emission tomography in the preoperative assessment of thyroid nodules in an endemic goiter area. Surgery. 2003;133:294–299.[Medline]
4. Bloom AD, Adler LP, Shuck JM. Determination of malignancy of thyroid nodules with positron emission tomography. Surgery. 1993;114:728–734.[Medline]
5. Kresnik E, Gallowitsch HJ, Mikosch P, et al. Technetium-99m-MIBI scintigraphy of thyroid nodules in an endemic goiter area. J Nucl Med. 1997;38:62–65.[Abstract/Free Full Text]
6. Demirel K, Kapucu O, Yucel C, et al. A comparison of radionuclide thyroid angiography, (99m)Tc-MIBI scintigraphy and power Doppler ultrasonography in the differential diagnosis of solitary cold thyroid nodules. Eur J Nucl Med Mol Imaging. 2003;30:642–650.[Medline]
7. Carmeci C, Jeffrey RB, McDougall IR, Nowels KW, Weigel RJ. Ultrasound-guided fine-needle aspiration biopsy of thyroid masses. Thyroid. 1998;8:283–289.[Medline]
8. Belfiore A, La Rosa GL. Fine-needle aspiration biopsy of the thyroid. Endocrinol Metab Clin North Am. 2001;30:361–400.[Medline]
9. Utiger RD. The multiplicity of thyroid nodules and carcinomas. N Engl J Med. 2005;352:2376–2378.[Free Full Text]

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