Abstract
The management of thyroid nodules, one of the main clinical challenges in endocrine clinical practice, is usually straightforward. Although the most important concern is ruling out malignancy, there are grey areas where uncertainty is frequently present: the nodules labelled as indeterminate by cytology and the extent of therapy when thyroid cancer is diagnosed pathologically. There is evidence that the current available precision medicine tools (from all the “-omics” to molecular analysis, fine-tuning imaging or artificial intelligence) may help to fill present gaps in the future. We present here a commentary on some of the current challenges faced by endocrinologists in the field of thyroid nodules and cancer, and illustrate how precision medicine may improve their diagnostic and therapeutic capabilities in the future.
Introduction
The concept of precision medicine implies that every patient is unique [1,2]. Precision medicine emphasizes that physicians are not dealing with diseases but with particular individuals who are ill.
The easy access to health care resources and the wide use of periodic medical check-ups have led to the frequent diagnosis of thyroid nodules in many individuals who live in developed countries. There are no 2 identical thyroid nodules in the same way as there are no 2 identical thyroid cancers. A thyroid nodule (whatever it might be: benign or malignant) has a particular anatomical (size, echotexture, location, etc.) and molecular signature (harbour a number of specific gene mutations). Additionally that particular nodule exists in a distinctive individual, with a well-defined phenotypic and genotypic background. The molecular signature of 2 thyroid nodules might be very similar but the individuals in whom they are placed are precisely unique. Identifying these differences and tailoring their management are the main challenges that face precision medicine.
Thyroid Nodules
The development of highly sensitive thyroid imaging techniques in combination with the increased use of screening programmes has led to an overload in the number of patients with thyroid nodules visiting endocrine clinics. The challenge is to differentiate between malignant and benign lesions. The current assessment is based on cytology findings supported by ultrasound (US) features. Unfortunately, approximately one quarter of cytology results are either non-diagnostic or indeterminate [3]. Similarly, the information from US, or other techniques such as elastography, is far from an accurate identification of malignancy. Hence, we need better diagnostic methods with the aim to improve the precision in diagnosis and avoid unnecessary surgery (Fig. 1).
Algorithm for the management of thyroid nodules. An accurate diagnosis will more precisely define the extent of treatment required for every thyroid nodule. This approach should compile information gathered from imaging techniques, cytological features, molecular tests, and artificial networks. FNA, fine-needle aspiration.
Citation: European Thyroid Journal 6, 2; 10.1159/000457793
Refining Cytological Analysis
Categories III, IV, and V from the Bethesda classification comprise the spectrum of indeterminate cytological results [4]. Around 10-60% of these nodules are eventually confirmed to be malignant after surgery [5]. A number of ancillary approaches are under development with the aim to improve the diagnostic accuracy in these cases.
Immunocytochemistry
Several immunocytochemical markers have been proposed to differentiate benign from malignant nodules in fine-needle aspiration samples [6]. Some of them are listed in Table 1. Currently no immunomarker has demonstrated enough diagnostic accuracy to be used alone. However, the combined analysis of galectin-3 and Hector Battifora mesothelial-1 has shown acceptable sensitivity and specificity to identify malignant tumours [7]. In this regard, other promising markers are being studied, such as CD44 [8] or Ki-67 [9].
Immunocytochemistry markers studied to characterize thyroid nodules
Molecular Tests
The advances in understanding the molecular pathways in tumorigenesis have identified many of the genetic abnormalities involved in thyroid cell transformation [10,11]. The presence of a single mutation (such as BRAF) has shown high specificity but low sensitivity [12,13]. The new approaches in diagnostics based on genetic abnormalities are testing a number of genetic alterations simultaneously. Several platforms are currently marketed to improve the malignancy risk assessment in indeterminate nodules [14]. These techniques are widely used in the USA but not in Europe.
- The Afirma Gene expression classifier analyses the mRNA expression of a panel of 167 genes. Fine-needle aspiration samples of the screened nodules are labelled “benign” or “suspicious” [15]. Gene expression classification has been proposed as a “rule-out” test [16], that is, a test with a high sensitivity and high negative predictive value.
- ThyroSeq is a dynamic panel that analyses a panel of specific thyroid cancer-related mutations. The ThyroSeq approach has been proposed as a “rule-in” test [16,17,18], that is, a test with a high specificity.
- Rossetta genomics is a platform recently developed that analyses the expression of a combination of micro-RNA (miRNA) species. The platform includes a set of 24 miRNAs and is reported to improve the malignancy risk assessment [19].
Developing Advances in Imaging Techniques
Advances in US technique have improved the ability to recognize suspicious nodules [20]. In addition, other radiological and molecular imaging methods have also been tested to differentiate benign from malignant thyroid nodules. Potentially these techniques could improve the accuracy in cancer diagnosis at earlier stages.
Liquid Biopsy
Innovations in genetic technologies have enabled the detection of circulating tumour cells, free DNA or even miRNA [21]. The detection of such abnormalities would help in the diagnosis of thyroid cancer in patients with thyroid nodules, or in the follow-up of patients with thyroid cancer. Pupilli et al. [22] investigated the presence of the DNA BRAF mutation in plasma from 103 patients with nodular goitre and reported 65% specificity and 80% sensitivity to discriminate papillary thyroid carcinoma (PTC) from benign nodules. miRNAs have also been quantified in serum, and differences have been found in blood samples from patients with PTC compared with those from patients with benign thyroid nodules or controls [23,24].
The Help of Artificial Intelligence
Artificial neural networks are statistical machine learning models that emulate the processing performance of biological neurons [25]. Artificial neural network models process input data, learn from experiences, and discover relationships between variables to generate a final decision output [26]. Artificial neural networks have been constructed with data from cytological [27,28], clinical [29], or US [30] variables to differentiate benign from malignant thyroid nodules with accuracy up to 82%.
Differentiated Thyroid Cancer
The current management of differentiated thyroid cancer does not always tailor the treatment intensity to tumour aggressiveness. This imprecision in therapy challenges the current concept of precision medicine. The accurate characterization of the unit “patient-tumour” is the final goal to avoid unnecessary aggressive protocols in low-risk patients or to use the entire therapeutic arsenal in high-risk patients.
At present, more than 15 scoring systems have been proposed to characterize every individual case [31]. The most accepted prognostic factors are age, histological variant, initial extension of the disease, and size of the primary tumour. However, a significant percentage of patients are not correctly classified with these variables, indicating the necessity of better early markers of cancer risk assessment to obtain a fine-tuned prognostic characterization [32]. Furthermore, the currently used clinical scoring systems focus on disease-specific mortality, when, in fact, a system to predict recurrence is most relevant to the vast majority of thyroid cancer patients, since they will have low-risk disease.
Molecular Analysis
Molecular biology can offer the most effective way to achieve a personalized approach in differentiated thyroid carcinoma (DTC) patients.
Prognosis and Follow-Up
Currently, the significance of the BRAF mutation in PTC prognosis is a matter of debate [33,34,35]. New information illustrates that the combination of BRAF and TERT promoter mutations confers a particularly aggressive phenotype [36,37]. Other genetic alterations such as RET (rearranged during transfection)/PTC rearrangements [38], TP53 [17,39] or PI3K-AKT signalling pathway changes have been reported to influence the outcome of PTC as well [40]. In a meta-analysis Pak et al. [41] concluded that the determination of a single genetic mutation is a poor prognostic marker. The whole-genome characterization of PTC offers a more sophisticated tumour classification and a list of new potential prognostic markers [10].
Recent studies show that deregulation of miRNAs may be implicated in a number of thyroid cancer characteristics. Some miRNAs have been associated with aggressive features for recurrence in PTC [42,43,44].
Serum thyroglobulin is the widely used tumour marker in patients with DTC but its utility is frequently hampered by the presence of antithyroglobulin antibodies [45]. The analysis of thyroglobulin free DNA can bypass the antibodies and provide a non-invasive approach to assess tumour evolution [46]. This technique could more reliably detect postoperative residual disease and identify patients who should receive further therapy [47].
Treatment
Several issues related to thyroid treatment are the subject of intense debate, such as the extent of thyroidectomy, the indication for prophylactic neck lymph node dissection, or the usefulness of radio-iodine ablation (Table 2). At present we are not able to identify the “iodine-refractory thyroid cancer” (IRTC) tumours from the outset [48]. Initial tumour molecular characterization could tip the balance between conservative and aggressive management.
Topics of debate on differentiated thyroid cancer treatment
The targeted therapy era for thyroid cancer started nearly a decade ago. To date, 2 tyrosine kinase inhibitors (TKIs), sorafenib and lenvatinib, have been approved for treatment of IRTCs. Both compounds have demonstrated a significant improvement in progression-free survival compared to placebo [49]. It is not clear whether the mutational tumour status modifies the effectiveness of these drugs. In theory, a more specific target-based therapy might improve its efficacy. A number of clinical trials with different TKIs is currently in progress in patients with IRTCs [50].
Decreased expression of the sodium iodide symporter is the main cause of the IRTC phenotype. Although the aetiology of sodium iodide symporter loss of function is not completely understood, hyperactivation of some molecular pathways, such as the MAPK and the PI3K pathways, plays a primordial role [51]. Several approaches have been evaluated to restore the sodium iodide symporter action with the aim to reverse the refractoriness [52,53]. In this scenario, TKIs are used only for a few days, with the objective of increasing iodine uptake and permitting 131I therapy.
Despite the important role of TKIs in the management of advanced thyroid cancer, several concerns remain to be addressed (Fig. 2).
Topics that remain to be elucidated in the treatment of thyroid cancer with TKIs. TKI, tyrosine kinase inhibitor; Tg, thyroglobulin; TSH, thyrotropin.
Citation: European Thyroid Journal 6, 2; 10.1159/000457793
Some new exciting approaches are on the therapeutic horizon, including immunotherapy [54] or locally directed treatments such as radiofrequency and cryo-ablation for solid tumours or cementoplasty for bone metastasis [55].
Molecular Imaging
Several molecular imaging technologies will improve the accuracy of the radiological follow-up of DTC patients. Specific targeted probes will help to characterize the outcome of specific targeted drugs in a precise tandem model. The dual role (diagnostic and therapeutic or theranostics) of such molecular imaging techniques will expand the contribution of this to thyroid oncology.
Current positron emission tomography (PET) modalities are designed to visualize specific molecular and cellular processes such as glucose uptake (18FDG-PET), cell proliferation (18F-fluorothymidine) and, more recently, tumour hypoxia (18F-fluoromisonidazole) [56]. A step forward in PET technology is the development of probes with specific molecular targets such as monoclonal antibodies, pro-angiogenic molecules, or somatostatin receptors [57]. The role of these new techniques has not been established in thyroid cancer, beyond anecdotal cases [58,59,60]. These findings have prompted the development of peptide receptor radionuclide therapy with yttrium- or lutetium-labelled somatostatin analogues to treat advanced DTC tumours [60,61,62]. Additionally TKI- PET tracers have shown encouraging results [63]. Finally, 124I-PET has demonstrated clinical and functional value in the analysis of DTC metastases [64,65].
Magnetic resonance imaging has also been used to assess tumour angiogenesis by targeting integrin αvβ3 expression, and as a measure of the efficacy of anti-angiogenesis drugs [66]. With the same purpose, contrast-enhanced US with microbubbles targeting angiogenesis markers has been studied in a preliminary mouse model of thyroid cancer [67].
Medullary Thyroid Carcinoma
Currently, it is difficult to predict the course of medullary thyroid carcinoma (MTC); thus, it is challenging to tailor the appropriate treatment to every individual case of MTC. Usually MTC clinical behaviour is more aggressive than that of DTC, although some MTC patients with distant metastases have an indolent course. Total thyroidectomy, central neck dissection, and therapeutic dissection of involved lateral neck compartments are the current recommended surgical treatment for patients with MTC [68]. Recent data have shown that bilateral disease is identified in 5.6% of patients with sporadic disease, while multifocal disease was noted in 16.0% of patients, suggesting that total thyroidectomy should remain the standard of care for initial surgery, as less complete thyroid surgery may fail to address fully the primary site of disease [69].
The Question of Prophylactic Thyroidectomy
The specific RET mutation enables us to intervene in multiple endocrine neoplasia type 2 patients with a prophylactic thyroidectomy in susceptible individuals (Table 3) [68]. Although decisions based on the known genotype-phenotype are established on a solid rationale, there is great heterogeneity in the age of onset and aggressiveness of MTC among individuals with the same RET mutation. Undoubtedly, other epigenetic, metabolomic, and environmental factors will contribute to making more precise decisions on the timing of thyroidectomy.
Risk stratification and recommendations for prophylactic thyroidectomy in children with RET mutations
Contribution of Genetics
A precision approach to patients with MTC implies accurate identification of the molecular signature. To date prediction of phenotype through the genotype is far from perfect.
Somatic RET mutations in sporadic tumours may not necessarily drive tumorigenesis but rather appear to be important for the progression of disease. Often, tumours show mutational heterogeneity, that is, RET mutations may be found in subpopulations of tumour cells rather than the entire tumour [70,71]. Different amino acid substitutions at a particular RET codon may be associated with phenotypes with different tumour aggressiveness [72]. An important limitation of genomic analysis is epigenetic influences. Therefore our current practice is far from the accuracy that could be achieved by in-depth biological, psychosocial, environmental, and lifestyle information about an individual patient.
Imprecisions and Troubles in the Diagnosis and Follow-Up
Although the serum calcitonin (CT) level is considered a sensitive but non-specific tool for the diagnosis of MTC in patients with thyroid nodules, no clear CT threshold has been identified. Thus, there is a need for accurate markers to rule out MTC in these patients. Some studies have reported better performance of procalcitonin in comparison with CT [73].
Surveillance for hyperparathyroidism and pheochromocytoma is necessary in multiple endocrine neoplasia type 2A or familial MTC patients. Today it is not possible to predict who might suffer from these diseases [74]. Precision medicine should be able to discriminate patients requiring lifelong surveillance.
The follow-up of patients with persistent or recurrent MTC poses a major problem. Frequently, patients have elevated postsurgical tumour markers with negative imaging. A precise assessment of postsurgical serum CT elevations requires consideration of the particular conditions of the patient (presurgical CT values, renal function, thyroid auto-immune status, hypergastrinaemia, heterophilic antibodies, interfering medications, etc.), an accurate pre-analytical assessment (time from surgery and time of the day), as well as the performance of the particular assay.
In patients with elevated serum CT values, whole-body imaging evaluation is recommended [68]. Unfortunately, the sensitivity of the conventional imaging techniques is poor [75]. A precision medicine approach implies the use of radiotracers specifically captured by MTC cells. Nevertheless, results of the few comparative studies have been variable, and we still do not know whether PET imaging using new radiotracers, such as 18F-dihydrophenylalanine and 68Ga-labelled somatostatin analogues, will offer us better performance in clinical practice [76].
Need to Improve Our Current Treatment Options
Today 2 TKIs, vandetanib [77] and cabozantinib [78], are offered to patients with advanced metastatic MTC. Both drugs are not selective and target multiple kinases. Other TKIs are being tested in clinical trials in patients with advanced MTC. These drugs target several tyrosine kinases and their effects are not precise and specific. Additionally a characteristic of cancer cells is instability. Targeting a single pathway results in the development of drug resistance, and patients may then need a number of synergistic or sequential therapies.
Several unanswered questions still remain [68,79]. It is not known whether targeted agents improve overall survival. Specific criteria for selection of the appropriate drug in a particular patient are lacking, and the dose and treatment timing are not clearly established. In the future, progress in pharmacogenomics will allow us to anticipate what drug will be suitable for a given patient. Nowadays, a wide range of pharmacogenomic tests are beginning to be recognized as having significant potential to alter standard medical practice [80].
Innovative Options for Treatment Need More Precise Knowledge
Information about tumour mutations can allow a more precise selection of the target therapy. RAS and RET mutations appear to be mutually exclusive [81]. The Raf-1/MEK/ERK pathway has been implicated in the metastatic phenotype [82], whereas inactivation of glycogen synthase kinase-3 has been reported to be associated with growth suppression in MTC cells [83]. The deregulation of the PI3K/Akt/mTOR pathway seems to contribute to the tumorigenic activity of RET proto-oncogene mutations. Targeting this pathway may represent an attractive potential therapeutic approach.
Targets for therapeutic agents could be independent of the tumour mutational status. The abnormal expression or deregulation of angiogenesis and cell proliferation factors, including VEGFR, EGFR, cMET, and FGFR4, may contribute to the progression and divergent responses to different drugs. An interesting approach might be to change the methylation status of the tumour, since it is known that MTC is characterized by general hypomethylation [84]. Histone deacetylase inhibitors have also been shown to suppress proliferation of MTC cell lines [85].
In patients with advanced tumours that do not respond to currently accepted therapies, a personalized approach will be an option in the future. Novel strategies include the use of tumour vaccines [86], anti-CEA-pretargeted radioimmunotherapy [87], and radiolabelled octreotide [88].
Conclusions
The correct diagnostic identification of malignant nodules is a major challenge in thyroidology. Although some interesting advances have been made in the last 2 decades, novel and promising imaging techniques, combined with more accurate molecular examination of tissue samples, will reduce the diagnostic uncertainty of this prevalent entity. This approach will also allow tailoring the treatment to the patients' needs.
Personalized medicine also depends on using specialized approaches and multidisciplinary health care teams to promote health, patient education and satisfaction, and customized disease prevention, detection, and treatment strategies.
Disclosure Statement
The authors declare no conflict of interests.
Footnotes
verified
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