Interobserver Agreement of Planar and SPECT Tc99m-MIBI Scintigraphy for the Assessment of Hypofunctioning Thyroid Nodules

Abstract

Introduction Thyroid scintigraphy with ^99mTc-methoxyisobutylisonitrile (MIBI) is a helpful tool for the risk stratification of thyroid nodules (TN). Whereas a nodule with low or hypointense MIBI uptake has a low risk for malignancy, a hyperintense uptake may indicate a malignant nodule, which requires surgical resection. The appropriate diagnostic or therapeutic regimen of an isointense nodule with an uptake similar to the paranodular tissue is discussed controversially. Aim of this study was to assess the interobserver agreement (IA) for the assignment of TN to the three categories: hypo-, iso-or hyperintense.

Methods Retrospective analysis of planar and SPECT images of MIBI scintigraphy was performed in 36 randomly selected patients with hypofunctioning TN and histological diagnosis. Four observers with different levels of experience in MIBI-scintigraphy analyzed MIBI uptake and assigned the nodules to the appropriate category. To assess the IA, Fleiss‘ Kappa was calculated.

Results The study cohort included 11 patients with papillary thyroid carcinoma (diameter 20.3 mm) and 25 patients with benign nodules (diameter 24.8 mm). The IA for all nodules using planar images was 0.76 compared to 0.80 for SPECT images. The IA was better in the subgroup of malignant nodules for planar images as well as SPECT images (Kappa 0.91 and 0.90, respectively) compared to benign nodules (0.65 and 0.76, respectively). Using SPECT images, only one thyroid carcinoma presented with hypointense uptake, the remainder with hyper- or isointense uptake. In contrast, benign nodules were found in all categories.

Conclusion MIBI scintigraphy shows a good IA for the interpretation of thyroid carcinoma. The IA is further improved if MIBI scintigraphy is performed in SPECT technique.

Zusammenfassung

Einleitung Die Szintigraphie mit ^99mTc-MIBI hilft bei kalten Knoten eine Risikostratifizierung vorzunehmen. Dabei spricht eine Minderspeicherung von MIBI (hypointenser Befund) mit hoher Wahrscheinlichkeit für einen benignen Knoten (MIBI negativer Befund), hingegen empfiehlt sich eine OP zum Malignitätsausschluss bei einer Mehranreicherung (hyperintenser Befund, MIBI positiver Befund). Aktuell wird die MIBI-Anreicherung im Knoten identisch zum paranodulären Gewebe (isointenser Befund) noch kontrovers beurteilt. Das Interobserver-Agreement ist für die Beurteilung der Befunde entscheidend.

Methoden Retrospektive Analyse der planaren Bilddaten und der SPECT-Aufnahmen der MIBI-Untersuchungen von 36 randomisierten Patienten mit mindestens einem hypofunktionellen Knoten und vorliegender Histologie. Vier Untersucher (Erfahrungslevel: drei mit 2 und einer mit 4 Jahren) beurteilten die Knoten hinsichtlich des MIBI-Speichermusters: hypo-, iso- und hyperintens. Zur Beurteilung des Interobserver-Agreements wurde das Fleiss‘ Kappa berechnet.

Ergebnisse Es wurden 11 papilläre Karzinome (20,3 mm) und 25 benigne Knoten (24,8 mm) beurteilt. Der Übereinstimmungswert (Kappa-Wert) für das Gesamtkollektiv lag für die planaren Bilder bei 0,76, für die SPECT-Bilder bei 0,80. Werden nur die Karzinome betrachtet, lag das Kappa höher (planar 0,91; SPECT 0,90) als bei den benignen Knoten (planar 0,65; SPECT 0,76). Während nur ein Karzinom im SPECT als hypointens beurteilt wurde, zeigten die übrigen ein iso- und hyperintenses Muster. Die benignen Knoten verteilten sich über allen drei Kategorien.

Zusammenfassung Die MIBI-Szintigraphie zeigt insbesondere bei der Beurteilung von Schilddrüsenkarzinomen eine gute Übereinstimmung, die durch SPECT noch verbessert werden kann.

Key words

MIBI-scintigraphy - interobserver agreement - hypofunctioning thyroid nodules - SPECT-Imaging

Schlüsselwörter

MIBI-Szintigraphie - interobserver-Variabilität - hypofunktionelle Schilddrüsenknoten - SPECT-Bildgebung

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