Introducing a Pole Concept for Nodule Growth in the Thyroid Gland: Taller-than-Wide Shape, Frequency, Location and Risk of Malignancy of Thyroid Nodules in an Area with Iodine Deficiency

Abstract

Purpose: (i) To examine the criterion taller-than-wide (TTW) for the sonographic assessment of thyroid nodules in areas of iodine deficiency in terms of frequency, anatomical distribution within the thyroid gland and risk of malignancy. (ii) To develop a model for nodule growth in the thyroid gland. Methods: German multicenter study consisting of two parts. In the prospective part, thyroid nodules were sonographically measured in all three dimensions, location within the thyroid gland and contact to a protrusion-like formation (horn) in the dorsal position of thyroid gland was noted. In addition, further sonographic features such as the composition, echogenity, margins and calcifications were investigated. All nodules from the prospective part were assessed for malignancy as part of clinical routine at the decision of the treating physician adhering to institutionally based algorithms. In the retrospective part, only nodules with fine needle aspiration and/or histology were included. The risk of malignancy in TTW nodules was determined by correlating them with cyotological and histological results. Results: Prospective part: out of 441 consecutively evaluated thyroid nodules, 6 were found to be malignant (1.4%, 95% CI 0.6-2.7%). Among the 74 TTW nodules (17%), 1 was malignant (1%, 95% CI 0-4%). TTW nodules were more often located in the dorsal half of the thyroid than non-TTW nodules (factor 2.3, p = 0.01, 95% CI 2.1-2.5) and more often located in close proximity to a horn than non-TTW nodules (factor 3.0, p = 0.01, 95% CI 2.4-3.8). Retrospective part: out of 1315 histologically and/or cytologically confirmed thyroid nodules, 163 TTW nodules were retrieved and retrospectively analyzed. A TTW nodule was 1.7 times more often benign when it was dorsal (95% CI 1.1-2.5) and 2.5 times more often benign when it was associated with a horn (95% CI 1.2-5.3). The overall probability of malignancy for TTW nodules was 38% (95% CI 30-46%) in this highly preselected patient group. Conclusion: TTW nodules are common in iodine deficient areas. They are often located in the dorsal half of the thyroid gland and are frequently associated with a dorsal protrusion-like formation (horn) of the thyroid. Obviously, the shape of benign nodules follows distinct anatomical preconditions within the thyroid gland. The frequency of TTW nodules and their predominant benignity can be explained by a pole concept of goiter growth. The difference between the low malignancy risk of TTW nodules found on a prospective basis and the high risk found retrospectively may be the result of a positive preselection in the latter.

Keywords: TIRADS; TTW nodules; Zuckerkandl’s tubercle; pole concept of goiter growth; posterior horn; posteroinferior horn; risk of malignancy; taller-than-wide.

Figure 1

(a) Dimension craniocaudally: cranial–central–caudal. (b) Dimension ventrodorsally: ventral–dorsal. (c) Dimension horizontally: lateral–medial (isthmus).

Figure 1

(a) Dimension craniocaudally: cranial–central–caudal. (b) Dimension ventrodorsally: ventral–dorsal. (c) Dimension horizontally: lateral–medial (isthmus).

Figure 2

Stylized sagittal view of the thyroid gland. Note the posterior and the posteroinferior horn as protrusions at the back. These protrusions as well as the upper and lower extremities of the thyroid lobe and the isthmus are introduced as poles of the thyroid gland for the purpose of a pole concept of nodule growth to be developed in the Section 4.

Figure 3

Sagittal view of a non-enlarged thyroid lobe. At the lower portion, there is a downward and backward protrusion named the posteroinferior horn herein (blue arrow).

Figure 4

Frequency of TTW nodules. The y-axis shows the log (transversal diameter/sagittal diameter) and x-axis the log (transversal diameter/longitudinal diameter) for each nodule. The lower ends of the y-axis and the x-axis are named TTW (taller-than-wide) and TTL (taller-than-long). Red points represent malignant nodules.

Figure 5

Huge TTW nodule extending into a prominent posterior horn (arrowheads). Note the thyroid parenchyma extending along the cranial portion of the nodule (arrowheads) but not along the caudal portion (arrow) arguing for a pre-existing posterior horn. A pre-existing posterior horn may have channeled the way for nodule growth causing its taller than wide shape. The nodule was benign at cytology.

Figure 6

(a,b). The green circles represent typical benign nodule in the respective part of the thyroid gland. The size of each circle represents the frequency of nodules in the respective part (all parts together sum up to 100%, apart from rounding differences). The shape represents the typical relation of the sagittal diameter (tall) to the horizontal diameter (wide) of a typical nodule. Note that nodules at or in a horn are typically round (tall = wide) whereas nodules in other locations are elliptic (tall < wide) in particular in the thyroid isthmus (tall << wide). * Frequencies at the cranial, central, and caudal portions were 13%, 41%, and 25%, respectively. Configuration did not differ between these three locations for which reason they are given as one circle. ^$ including nodules at or in a horn.

Figure 6

(a,b). The green circles represent typical benign nodule in the respective part of the thyroid gland. The size of each circle represents the frequency of nodules in the respective part (all parts together sum up to 100%, apart from rounding differences). The shape represents the typical relation of the sagittal diameter (tall) to the horizontal diameter (wide) of a typical nodule. Note that nodules at or in a horn are typically round (tall = wide) whereas nodules in other locations are elliptic (tall < wide) in particular in the thyroid isthmus (tall << wide). * Frequencies at the cranial, central, and caudal portions were 13%, 41%, and 25%, respectively. Configuration did not differ between these three locations for which reason they are given as one circle. ^$ including nodules at or in a horn.

Figure 7

Pole concept of goiter growth. Nodule growth follows the shape of the respective pole. Overstressed depiction of nodule growth along the pole model.