Incidental Primary Intrathoracic Goiter: Dual-Isotope Scintigraphy and Early-MIBI SPECT/CT.

Primary intrathoracic goiter is an uncommon congenital entity resulting from over decent ectopic thyroid tissue. As compared with secondary intrathoracic goiter, primary entities are discrete from orthotopic thyroid tissue and may lead to potentially serious complications such as malignancy and shortness of breath. Intrathoracic goiters have been described as showing mild or absent uptake of 99m Tc-pertechnetate on planar scintigraphy. We present an incidental primary intrathoracic goiter found in a patient undergoing evaluation with multimodal scintigraphy and early 99m Tc-sestamibi single-photon emission computed tomography/computed tomography (SPECT/CT) for localization of parathyroid adenomas. The mass was inconspicuous on TcO 4- scintigraphy but methoxyisobutylisonitrile-avid on early planar and SPECT/CT. World Association of Radiopharmaceutical and Molecular Therapy (WARMTH). This is an open access article published by Thieme under the terms of the Creative Commons Attribution-NonDerivative-NonCommercial License, permitting copying and reproduction so long as the original work is given appropriate credit. Contents may not be used for commercial purposes, or adapted, remixed, transformed or built upon. ( https://creativecommons.org/licenses/by-nc-nd/4.0/ ).

Introduction

Primary intrathoracic goiter is a rare congenital entity arising from accessory and/or ectopic thyroid tissue that is discrete from an orthotopic thyroid gland and may lead to complications such respiratory obstruction and malignancy. 1 These entities are typically managed surgically due to an increased risk for respiratory complications and malignancy. 2 3 4 On the contrary, secondary thyroid goiter is a more common entity that arises from orthotopic thyroid tissue. 5 Differential diagnoses that should be considered when encountering incidental mediastinal masses in adult patients include thymoma and lymphoma. 6

Intrathoracic thyroid goiter may be inconspicuous on planar 99m Tc-pertechnetate (TcO 4- ) scintigraphy, relative to normal thyroid tissue, 7 although it may demonstrate early blood pool activity. 8

Case Report

An elderly woman was referred to our institution for evaluation of primary hyperparathyroidism with a parathyroid hormone (PTH) level of 105 pg/mL (upper limit of normal [ULN]: 65), and hypercalcemia with calcium 10.1 mg/dL (ULN: 10.5). Initial diagnostic imaging evaluation with high-resolution ultrasound was unremarkable for localization of parathyroid adenomas.

Nuclear medicine was consulted for the localization of parathyroid adenoma, and the patient was prepared for evaluation with dual-isotope scintigraphy with 99m Tc-sestamibi (methoxyisobutylisonitrile, MIBI), TcO 4- , and early-MIBI single-photon emission computed tomography/computed tomography (SPECT/CT). Our institutional protocol, including early MIBI SPECT/CT and subsequent TcO 4- evaluation, is based on a higher reported accuracy for the evaluation of parathyroid adenomas, 9 and increased value of MIBI-SPECT/CT for anatomic localization and surgical planning. 9 10 11 The patient received approximately approximately 740 MBq of 99m Tc-sestamibi followed by 10 and 120 minutes planar scanning in anterior projection, in addition to early-MIBI SPECT/CT at 15 minutes following administration of radiotracer. Three hours following administration of MIBI, the patient received approximately 370 MBq of 99m Tc-pertechnetate followed by 15 minutes delayed planar scanning in anterior projection. The protocol for the study was based on Society of Nuclear Medicine practice guidelines. 12

Early-MIBI planar scintigraphy ( Fig. 1 ) showed a large moderately-avid mediastinal lesion inseparable from the thyroid. On delayed imaging, the mediastinal mass revealed equivocal areas of minimal retention. Differential diagnoses included lymphoma, thymoma, and primary intrathoracic goiter. Additionally seen was a more prominent left lower thyroid on MIBI, as compared with TcO 4- , with near-complete washout on delayed MIBI, suggestive of a parathyroid adenoma localized in this area.

Fig. 1

Dual-isotope planar scintigraphy with early ( A ) and delayed ( B ) 99m Tc-sestamibi (methoxyisobutylisonitrile, MIBI), and 99m Tc-pertechnetate (TcO 4- ) ( B ) of an elderly woman undergoing evaluation for hyperparathyroidism. Early-MIBI planar scintigraphy ( A ) showed a large moderately-avid mediastinal lesion ( arrowheads ) adjacent to the thyroid gland ( arrow ). Delayed images ( B ) showed equivocal areas of minimal retention in the mediastinal mass ( yellow arrows ); differential diagnoses included lymphoma, thymoma, and primary intrathoracic goiter. Early-MIBI planar scintigraphy ( A ) also showed a relatively thicker left lower thyroid pole that was discordant with TcO 4- images ( C ) and demonstrated minimal retention on delayed-MIBI ( B ), suggestive of a left lower thyroid pole localized parathyroid adenoma.

Early-MIBI SPECT/CT ( Fig. 2 ) localized the previously seen lesion on planar MIBI to a mediastinal mass, which now appeared discrete from the thyroid gland. Also revealed was a parathyroid nodule ( Fig. 3 — arrows at A1 and A2) posterior to the lower portion of the left thyroid pole corresponding to the area of increased activity in the left lower thyroid seen on planar scan.

Fig. 2

Early-methoxyisobutylisonitrile single-photon emission computed tomography/computed tomography (MIBI SPECT/CT) performed as part of the protocol for localizing parathyroid adenoma(s). Early-MIBI SPECT maximum intensity projection ( A ), and transcoronal fused SPECT/CT (B1) and contrast-enhanced CT images (B2) showed a large mediastinal mass discrete from the thyroid gland with heterogeneous uptake of MIBI ( arrowheads ). Additionally, there was relatively more intense activity in the left lower thyroid pole ( arrows ) that corresponded to the discordant uptake seen on TcO 4- planar images.

Fig. 3

Transaxial fused and CTimages at the level of the lower thyroid gland show an MIBG-avid nodule posterior to the left thyroid lobe (A1-A2; arrows ). Transaxial fused and CT images of the chest showed a heterogeneously MIBG-avid mediastinal mass (B1, arrowheads ) with heterogeneous contrast enhancement (B2, arrowheads ). Post surgical changes are shown with removal of the left thyroid nodule (A3) and mediastinal mass (B3).

The patient underwent surgical resection of the left lower thyroid and mediastinal mass ( Fig. 3 ). PTH levels decreased to 53 pg/mL (ULN: 65). Histopathologic evaluation of the mediastinal mass revealed thyroid parenchyma with smaller areas of thymic and parathyroid tissue, and evaluation of the left lower parathyroid nodule revealed hypercellular parathyroid tissue.

Discussion

This patient was clinically diagnosed with primary hyperparathyroidism and referred to our nuclear medicine department for the localization of parathyroid adenoma(s). During our imaging evaluation, the patient was found to have an incidental MIBI-avid mediastinal mass with minimal equivocal retention that was nonavid on TcO4-.

Prior case reports have described primary intrathoracic thyroid with minimal or absent uptake of TcO4-, although we found no cases comparing their appearance with early and delayed MIBI and SPECT/CT, as shown in this case. The nearly-absent uptake of TcO4- is likely secondary to a poor expression of sodium iodide symporters in the congenital neoplasm, as compared with normal thyroid tissue.

Primary intrathoracic thyroid goiter results from enlargement of ectopic thyroid tissue within the thoracic cavity. Unlike secondary thyroid goiter, primary entities derive their blood supply from intrathoracic vessels and are associated with potentially serious complications.