Do hyperechoic thyroid nodules on B-ultrasound represent calcification?

Abstract

Objective

To explore the correlation between hyperechoic thyroid nodules observed on B-ultrasound and histological calcification seen in paraffin-wax sections.

Methods

Records of patients who underwent surgical removal of thyroid nodules diagnosed on preoperative B-ultrasound were analysed retrospectively. Calcification present on B-ultrasound was compared with calcification seen in postoperative pathology specimens.

Results

Of the 1 655 patients included in the study, 518 had malignant and 1 137 had benign thyroid nodules. Calcification on B-ultrasound was seen in 366 patients with malignant, and 414 with benign nodules. Calcification was confirmed on histology in 209 and 127 of these patients, respectively, giving a sensitivity and specificity for B-ultrasound in diagnosing calcification (compared with histology) of 95.87% and 47.67%, respectively, in thyroid cancer and 90.71%, and 71.21% respectively in benign thyroid nodules. Microcalcification was seen in 483 patients on B-ultrasound and in 186 on histology, of whom 294 (60.87%) and 152 (81.72%), respectively, had thyroid cancer.

Conclusions

B-ultrasound is a useful and accurate test for detecting calcification in thyroid nodules, with a high sensitivity. There is a close association between calcification (especially microcalcification) and thyroid cancer on both B-ultrasound and pathological examination.

Keywords

Thyroid nodule thyroid cancer calcification ultrasound

Introduction

The worldwide incidence of thyroid cancer has increased noticeably, and thyroid cancer has become the most common primary malignant tumour of the neck.^1–5 Ultrasound assessment of thyroid nodule features such as calcifications, hypoechogenicity, irregular margins and ‘taller than wide’ dimensions has emerged as the principal imaging test to evaluate underlying malignancy. Thyroid nodules with hyperechoic foci, with or without acoustic shadowing, which are termed ‘calcifications’, have received increasing attention.^6,7 Thyroid calcification can occur in benign or malignant thyroid disease, but a higher percentage of nodules complicated by calcification has been described in thyroid cancer compared with benign disease.⁸ Park et al.⁹ reported that the diagnostic specificity of ultrasound-suggested microcalcification in thyroid cancer was 85–94% and the positive predictive value was 69–71%. Moon et al.¹⁰ reported that the sensitivity and specificity of microcalcification on ultrasound for malignancy were 44.2% and 90.8%, respectively.

It is important to know how many hyperechoic thyroid nodules seen on B-ultrasound also show calcification on histological examination. To our knowledge, little has been published in the literature on this subject. The present study explored the correlation between hyperechoic thyroid nodules observed on B-ultrasound and histological calcification seen in histological (paraffin-wax) sections.

Patients and methods

Patients

Records of consecutive patients who had undergone surgical removal of thyroid nodules, diagnosed on preoperative B-ultrasound at the Department of General Surgery, Huashan Hospital, Fudan University, Shanghai, China, between April 2009 and April 2011, were analysed retrospectively. The principal indications for surgical removal of thyroid nodules are suspicion of cancer, such as the presence of calcifications, hypoechogenicity, irregular margins or ‘taller than wide’ dimensions and/or preoperative aspiration biopsy cytology, symptoms of pressure, hyperthyroidism, substernal extension and cosmetic deformity.

As only archival surgical specimens and ultrasonography results were examined, patients involved in the study were not required to provide informed consent. The study protocol conformed with the Declaration of Helsinki and was approved by the Science and Technology Committee of Baoshan District, Shanghai, China, and the Ethics Committee of the Renhe Hospital (Baoshan District, Huashan Branch), Fudan University, Shanghai, China.

Ultrasound examination

All patients had undergone routine colour Doppler ultrasonography of the thyroid, with ultra-wideband linear sensors at 5–10 MHz or 5–12 MHz, using ATL5000 (ATL/Philips, Andover, MA, USA), Logiq 9 (GE Healthcare, Piscataway, NJ, USA) or Acuson Sequoia™ 512 (Siemens, Erlangen, Germany) ultrasound machines. B-ultrasound examination focused on whether the thyroid nodules were complicated by various types of calcification. Any calcification observed was classified as (i) microcalcifications (multiple punctate bright echoes, with or without acoustic shadowing); (ii) intranodular coarse calcification (hyperechoic structure >2 mm in diameter with acoustic shadowing); (iii) peripheral calcification (coarse calcifications located around the nodule or large area of calcification without acoustic shadowing); (iv) single calcified spot (a single coarse calcification <5 mm in diameter without visualization of the surrounding tumour);^11,12 or (v) mixed type (more than one type of calcification coexisting within the same nodule or lobe).

Histological examination

Postoperative pathology studies had been performed in all patients to confirm the diagnosis. Briefly, tissue samples were fixed in 30% formalin, dehydrated in ethanol, and embedded in paraffin wax. For routine pathological examination, all specimens were sliced continuously into 4-µm thick sections, stained with haematoxylin and eosin, and examined by two independent pathologists with experience in thyroid pathology. Calcification was identified in gross specimens and using light microscopy.

Statistical analyses

Data were compared using the Student's t-test and correlations were analysed using the χ² test. A P-value of <0.05 was considered to be statistically significant. All data analyses were performed using SPSS® software version 10.0 (SPSS Inc., Chicago, IL, USA).

Results

A total of 1 655 patients were included in the study. Of these, 427 were males and 1 228 were females, giving a male-to-female ratio of 1 : 2.88. The patients ranged in age between 16 and 83 years.

Of the 1 655 patients, 518 had malignant thyroid nodules (130 were males and 388 were females, with a male-to-female ratio of 1 : 2.98). The remaining 1 137 patients had benign thyroid nodules (297 were males and 840 were females, with a male-to-female ratio of 1 : 2.83). There was no significant difference in the male-to-female ratio between patients with malignant nodules and those with benign nodules. Those aged 41–60 years accounted for 60.97% of all the patients and for 63.51% of all those with malignant thyroid nodules.

Pathological examination of the malignant thyroid nodule specimens revealed 501 cases of papillary carcinoma, nine cases of follicular adenocarcinoma, six cases of medullary carcinoma, one case of squamous-cell carcinoma and one case of lymphoma. In the benign thyroid nodule specimens, pathological examination revealed 663 cases of nodular goitre, 417 cases of follicular adenoma, 52 cases of Hashimoto's disease, two cases of subacute thyroiditis, two cases of hyalinizing trabecular tumour and one case of eosinophilic adenoma.

Calcification rates on preoperative B-ultrasound

Of the 518 patients with malignant nodules, 366 (70.66%) were complicated with calcification on preoperative B-ultrasound, including 294 (56.76%) with microcalcification. Of the 1 137 patients with benign nodules, 414 (36.41%) were complicated with calcification on preoperative B-ultrasound, including 189 (16.62%) with microcalcification. The calcification and microcalcification complication rates were significantly different in benign and malignant thyroid nodules (P < 0.01 for both parameters).

Amongs the 414 patients with benign nodules and calcification on preoperative B-ultrasound, the calcification complication rates were 24.70% (103 of 417) in patients with thyroid adenoma, 43.44% (288 of 663) in patients with nodular goitre, and 44.23% (23 of 52) in patients with Hashimoto's disease. Of the patients with benign nodules and calcification, microcalcification was seen in 57.28% (59 of 103) of those with thyroid adenoma, 43.40% (125 of 288) of those with nodular goitre, and 21.74% (five of 23) of those with Hashimoto's disease. There were significant differences in both the calcification complication rates and the microcalcification complication rates between those with thyroid adenoma, nodular goitre and Hashimoto's disease (P < 0.01).

Comparison of ultrasound and histological findings

Of the 518 patients with thyroid cancer, 366 (70.66%) had hyperechoic thyroid nodules on preoperative B-ultrasound. Of these, 209 patients were confirmed as having calcification on histological examination of paraffin-wax sections (Table 1). Of the 152 patients with thyroid cancer without calcification on preoperative B-ultrasound, nine were confirmed as having calcification on histological examination of paraffin-wax sections. This gave a sensitivity and specificity for B-ultrasound in diagnosing calcification, compared with histology, in malignant thyroid nodules of 95.87% and 47.67%, respectively, with positive and negative predictive values of 57.10% and 94.08%, respectively. Of the 518 patients with thyroid cancer, a total of 218 (42.08%) were confirmed as having calcification based on paraffin-wax sections.

Table 1.

Calcification on B-ultrasound and paraffin-wax sections in thyroid nodules, according to calcification type.

	Calcification type^a						Statistical significance
	All	Micro	Coarse	Peripheral	Solitary	Mixed	Statistical significance
Benign nodules
Calcification on B-ultrasound, n	414	189	142	32	14	37
Calcification on paraffin-wax section, n	127	34	49	10	6	28
Positive predictive value of B-ultrasound, %	30.68	17.99	34.51	31.25	42.88	75.68	P < 0.01^b
Malignant nodules
Calcification on B-ultrasound, n	366	294	23	6	2	41
Calcification on paraffin-wax section, n	209	152	18	4	2	33
Positive predictive value of B-ultrasound, %	57.10	51.70	78.26	66.67	100.0	80.49	P < 0.01^c
All nodules
Calcification on B-ultrasound, n	780	483	165	38	16	78
Malignancy rate, %	60.87	13.94	15.79	12.50	52.56	P < 0.01^d
Calcification on paraffin-wax section, n	336	186	67	14	8	61
Malignancy rate, %	81.72	26.87	28.57	25.00	54.10	P < 0.01^e

Micro, microcalcifications; Coarse, intranodular coarse calcification; Peripheral, peripheral calcification; Solitary, single calcified spot; Mixed, more than one type of calcification co-existing.

χ²= 51.498; ^cχ²= 18.586; ^dχ²= 133.195; ^eχ²= 78.870.

Of the 1 137 patients with benign thyroid nodules, 414 (36.41%) had calcification on preoperative B-ultrasound. Of these, 127 patients were confirmed as having calcification on histological examination of paraffin-wax sections (Table 1). Of the 723 patients with benign thyroid nodules without calcification on preoperative B-ultrasound, 13 were confirmed as having calcification on histological examination of paraffin-wax sections. This gave a sensitivity and specificity of B-ultrasound in diagnosing calcification, compared with histology, in benign thyroid nodules of 90.71% and 71.21%, respectively, with positive and negative predictive values of 30.68% and 98.20%, respectively. Of the 1 137 patients with benign thyroid nodules, a total of 140 (12.31%) were confirmed as having calcifications based on paraffin-wax sections.

For both benign and malignant thyroid nodules, the positive predictive value of B-ultrasound in diagnosing calcification, compared with paraffin-wax sections, was significantly different (P < 0.01) for different types of calcification (Table 1). Of the 483 patients with microcalcification on B-ultrasound, 294 (60.87%) had thyroid cancer, whereas of the 186 patients with microcalcification on paraffin-wax sections, 152 (81.72%) had thyroid cancer.

Discussion

With the widespread use of high-frequency ultrasound and in-depth research on thyroid calcification, correlations between calcification and thyroid cancer have received increasing attention.^11,13–16 A close association has been demonstrated between calcification and thyroid cancer.¹⁷ Of the various forms of calcification, microcalcification seems to be most closely correlated with thyroid cancer (especially papillary cancer), and is therefore regarded as a specific marker of malignancy.¹⁸ However, it is not known how many thyroid nodules that are hyperechoic on B-ultrasound also show histological calcification on paraffin-wax sections. In the present study, calcification was seen on B-ultrasound in 70.66%, and on paraffin-wax sections in 42.08%, of patients with thyroid cancer. In contrast, calcification was seen on B-ultrasound in 36.41%, and on paraffin-wax sections in 12.31%, of patients with benign thyroid nodules. There was a statistically significant difference in the percentage of patients with malignant thyroid disease compared with benign thyroid nodules, on both B-ultrasound and paraffin-wax sections (P < 0.01 for both).

It is not fully understood why thyroid cancer is often complicated by calcification, and in particular microcalcification. Concentric lamellated calcified structures known as psammoma bodies are often associated with papillary neoplasms in various organs,^19–22 and papillary carcinoma is the most common type of thyroid cancer. Das et al.²³ found that surviving thyroid tumour cells were able to release precursors of psammoma bodies. Psammoma bodies are mainly related to microcalcification,²¹ which supports the idea that calcification may be linked with the development of malignant tumours and confirms the close association between microcalcification and thyroid cancer. The presence of small calcium-binding proteins,²⁴ proteins from the bone morphogenetic protein (BMP) family, especially BMP-11²⁵ and osteopontin (which is produced by macrophages and has been shown to be associated with the development of psammoma bodies in papillary carcinoma of the thyroid²⁶) could explain the differences in cation concentration, particularly calcium, in psammoma bodies.²⁵ Further studies on osteopontin also support the theory of calcification due to tumour development and progression.^25,27 Although microcalcification is considered to be a specific marker for papillary thyroid cancer because of its association with psammoma bodies, microcalcification is not equivalent to psammoma body formation. In a study on inorganic elements in psammomas from thyroid cancer, Olivera Merlin et al.²⁸ found that psammoma bodies from thyroid papillary cancers contained at least eight inorganic elements; they suggested that differences in the concentrations of inorganic elements may be due to variations in environmental pollution and indicate a potential relationship between papillary thyroid cancer and environmental pollution.

Nodular goitre and thyroid adenoma can both produce calcium oxalate crystals that have a radiological appearance resembling microcalcification; therefore, these diseases are difficult to differentiate on ultrasound. In addition, the appearance of microcalcification on B-ultrasound can also be caused by colloid, pathologically confirmed fibrous nodules and nodules complicated by intracystic haemorrhage. This may be one reason why the agreement rates between microcalcifications observed on B-ultrasound and those confirmed on paraffin-wax sections in malignant and benign lesions in our present study were lower than those observed for other types of calcification.

Thyroid calcification can occur in both benign and malignant thyroid disease. Ultrasound represents a genuine diagnostic advance as it is useful for detecting calcification and thyroid cancers in both solitary and multiple nodules. In the present study, all types of sonographically detected calcification were associated with an increased risk of malignancy, but this was especially true for microcalcification. However, from the perspective of cancer screening, all types of calcification (not just microcalcifications) should be thoroughly investigated.

The series included in the present study may reflect a higher proportion of malignant nodules than the expected rate in the general population, as the data are from inpatients requiring surgical intervention. Furthermore, results of the present study may be affected by the quality of the ultrasound equipment used, the transducer frequency, and whether ultrasound interpretation was based on static or real-time imaging. No single ultrasound feature is adequately sensitive to identify all malignant thyroid nodules; rather, ultrasound can identify certain features that may confer a heightened risk for malignancy, indicating the need for fine-needle aspiration.²⁹

In conclusion, B-ultrasound is a useful and accurate method for detecting calcification in thyroid nodules, with a high sensitivity. There is a close association between calcification (especially microcalcification) and thyroid cancer on both B-ultrasound and pathological examination.

Footnotes

Declaration of conflicting interest

The authors declare that there are no conflicts of interest.

Funding

This research received no specific grant from any funding agency in the public, commercial, or not-for-profit sectors.

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