PAPILLON COURSE on THYROID ULTRASOUNDSection 2 The nodular goiterPart 6 The shape of the nodule
|
OPENING |
|||||||||||||||||||||||||||||||
Taller-than-wide shape (TWS) or nonparallel orientation (NPO) is regarded as the most specific sign among suspicious ultrasound (US) characteristics. On the other hand, the sensitivity of NPO is substantially lower than that of microcalcification or hypoechogenicity. This sign is relatively easy to define, the interobserver agreement is good. Although irregular borders also rely on the shape of the nodule, when we talk about the shape of a nodule, most authors consider only NPO and not irregular |
borders. Nevertheless, these two suspicious characteristics together with extrathyroidal extension rely on the infiltrative growth of papillary thyroid cancer (PTC). The diagnostic performance is worse in the event of medullary thyroid cancer (MTC), and this sign is useless in the event of follicular thyroid cancer (FTC). |
||||||||||||||||||||||||||||||
DEFINITION AND POSSIBLE EXPLANATIONS OF NONPARALLEL ORIENTATION |
|||||||||||||||||||||||||||||||
TWS means that the antero-posterior diameter exceeds the width of the nodule measured in transverse scan. Both types of NPO are evaluated in the axial plane by comparing the height or depth ('tallness') and width or length of a nodule measured parallel and perpendicular to the ultrasound beam, respectively. NPO is usually evident on visual inspection and rarely requires formal measurements. Although authors are on different views whether an equal distance is included or not included, this has very limited importance in the everyday practice. |
As will be presented later, the guidelines differ whether they include the TLS among NPO. The widely accepted explanation of this finding indicates that the malignant nodules grow across the normal tissue plane in a centrifugal manner, while benign nodules do the same in a parallel fashion (1-3). The finding that TWS occurs more frequently in benign lesions on CT scan than on US, led to another reasonable explanation. Benign nodules are easier to compress than malignant and we compress them in the US while do not in the case for CT (4).
|
||||||||||||||||||||||||||||||
SOME CONCERNS ABAOUT THE NONPAPARALLEL ORIENTATION |
|||||||||||||||||||||||||||||||
| Although regarding NPO, a 'yes' or 'no' answer is accepted by the guideline, in the reality there is a continuous spectrum of this ratio. An arbitrarily chosen value, i.e., a yes or no answer does not reflect the reality. Indeed, higher the ratio of width to depth more likely the lesion being papillary cancer. (This problem is very similar to that in the event of echogenicity of the nodule.) However, there is no point in deviating from this somewhat lazy approach. A qualitative approach (i.e. a ratio of depth and width) would very likely better reflect reality than the long-used quantitative one 'yes' or 'no'), but the time spent on measurement would hardly pay off. , if an experienced investigator On the other hand , an experienced examiner gains an overall impression of the nodule, a shape difference close to the nonparallel also plays a role in this, so if we assign significance to a depth-to-width ratio of 1.05, we have also attach significance to a ratio of 0.95. Moreover, considering the |
growing number of publications on the use of artificial intelligence in thyroid US, it is worth mentioning that the NPO and the echogenicity of the nodule are those two characteristics which are relatively easy to measure.
More important is the second one. As with all other suspicious characteristics, NPO is useful in the diagnosis of PTC and to a lesser extent in MTC while useless in FTC. We always should bear in mind that when guideline speak about the usefulness of suspicious characteristics for diagnosing thyroid cancers, they think mostly or only of PTC. Otherwise, they would be wrong. Unfortunately, reading the guidelines and scoring systems they do not always reveal this very important distinction. The third issue is self-evident: there is no US characteristics which had both an ideal sensitivity and specificity, and this is also true for NPO. |
||||||||||||||||||||||||||||||
NONPARALLEL ORIENTATION IN THE MOST FREQUENT TYPES OF THYROID MALIGNANCIES |
|||||||||||||||||||||||||||||||
A taller-than-wide shape is an insensitive but highly specific indicator of malignancy (5-7). The specificity of NPO is about 90% in most studies (8). Nonparallel orientation has the highest specificity among suspicious US characteristics. Remonti et al. found the highest specificities for absence of elasticity, microcalcifications, irregular margins, and a 'taller-than-wide' shape (86.2, 87.8, 83.1, and 96.6%, respectively) (9). In a study which assessed the risk of malignancy in 2,000 thyroid nodules (63.4% benign and 36.6% malignant) (10). The following odds ratios (OR) were observed for malignancy: deeply hypoechoic (OR, 6.8; a relevant finding not specifically considered in the 2014 meta-analysis), spiculated or microlobulated margins (OR, 5.9), microcalcifications (OR, 3.3), solid nodule (OR, 2.6), slightly hypoechoic (OR, 2.6), nonparallel (taller than wide) growth (OR, 2.3), and irregular shape (OR, 1.3) (10). In the study of Campanella et al., the highest risk of malignancy was associated with a TWS (DOR of 10.2; 95% CI: 6.7-15.3), an absent halo sign (7.1; 95% CI: 3.7-13.7), microcalcifications (6.8; 95% CI: 4.7-9.7), and irregular margins (6.1; 95% CI: 3.1-12.0) (11). It is worth noticing the huge difference in OR in these large-scale studies, in the secondly |
mentioned work (11), the 2.3 OR was the worst while in the firstly mentioned study (10), the 10.2 OR was the best among suspicious characteristics. Although the above-cited authors extend their conclusions to 'thyroid cancers', indeed these conclusions are relevant only in the event of PTC. NPO is significantly more frequent in MTC compared with benign lesions, but the frequency is much lower than in PTC (12). NPO has no relevance in the diagnosis of FTC. Table 1 summarizes the occurrence of NPO in the 3 most common subtype of thyroid carcinomas (13-37). Similar to other characteristics, the NPO occurs significantly more frequently in papillary cancer compared with follicular carcinoma. The occurrence of NPO in MTC is between that of FTC and PTC but it is closer close to the latter. The broad range of the observed frequencies of NPO means that even in the event of a seemingly easy-to-judge characteristics, the interobserver agreement is far from the ideal. |
||||||||||||||||||||||||||||||
|
|||||||||||||||||||||||||||||||
THE ROLE OF NONPARALLEL ORIENTATION IN VARIOUS TIRADS |
|||||||||||||||||||||||||||||||
TWS belongs to the most suspicious features in all TIRADS (38-42). The presence of TWS automatically groups the lesion among the most suspicious subgroup in the AACE and in the European TIRADS, while to classify a lesion showing TWS into the most suspicious category requires that the nodule should be hypoechoic nodules in the ATA and Korean TIRADS. Regarding the ACR TIRADS, the presence of TWS worth three points. Following the logic of this system, it means that such lesions should be categorized into category 4 or 5 depending on the presence of |
other suspicious signs. (See Table 2.) There is an important difference between the TIRADS. It affects the handling of nodules presenting TLS. Three TIRADS involve TLS among nonparallel orientation (40-42) while two do not (38-39). It seems to be more justified to include TLS among NPO (43). |
||||||||||||||||||||||||||||||
|
|||||||||||||||||||||||||||||||
PATTERNS CAUSING DIFFERENTIAL DIAGNOSTIC PROBLEMS IN THE EVENT OF NONPARALLEL ORIENTATION |
|||||||||||||||||||||||||||||||
Compared with other suspicious characteristics, NPO is much less prone to interpretation difficulties. In the everyday practice we meet two types of deceptive patterns (see Table 3). The first is caused by normal anatomical situation. The thyroid not infrequently presents itself NPO. Baek et. al found NPO in 15% of diffuse thyroid diseases (44). If the thyroid has taller-than-long-shape than the normal growth of a nodule simply follows the shape of the thyroid. The second affects nodules located in the upper pole of the lobe. To a lesser extent, nodules located in the lower pole are also affected. The thyroid as a rotational ellipsoid normally narrows in its poles which hinders the usual extension of the nodule into the transverse direction. We meet the third anatomical situation in nodules that form between the trachea and the carotid artery are much more difficult to grow sideways and therefore much more likely present taller-than-wide shape. The second group of issues is caused by pathological conditions. The |
sideway growth can be hindered by a neighboring nodule. This is particularly true for those lesions which are sandwiched between two other lesions. A coarse calcification has two different deceptive impacts. The sideway extension of a nodule can be hindered if there is a macrocalcification close to the lesion. The other issue arises in nodules presenting macrocalcifications. The dorsal acoustic shadow makes the depth deceptively larger. The next problem is caused by the thyroiditis. A suspicious sign can be interpreted only in the context of pathological nodules and not in discrete lesions of more active foci of thyroiditis. In contrast with other deceptive patterns, in the event of thyroiditis not the presence of nonparallel orientation is the matter, but the presence of pathological nodule. Nevertheless, the NPO of a lesion can be of help in distinguishing foci of thyroiditis from potentially malignant nodules (see case). The guidelines fail to mention that the nonparallel orientation has very limited if any role in dominantly cystic nodules. A relatively rapidly growing fluid frequently makes the orientation of the nodule irregular. In such cases, not the tumorous proliferation but the accumulation of cystic content is responsible to the NPO. So, dominantly cystic nodules are the exceptions, NPO has limited if any significance in such lesions. |
||||||||||||||||||||||||||||||
|
|||||||||||||||||||||||||||||||
If a nodule is in close proximity to another lesion or more importantly, it is sandwiched between two other nodules than this location has influence to the shape. The neighboring nodule or nodules can prevent the extension of the lesion into the usual direction, i.e. into the medial/lateral or into the lower/upper direction. This may cause nonpathological form on NPO. |
As with all other US characteristics, we should not forget that we speak of pathological nodules. Therefore, suspicious characteristics including NPO have no relevance in those discrete lesions of Hashimoto's thyroiditis which are not pathological nodules. |
||||||||||||||||||||||||||||||
References
