Levels of chemical element contents in thyroid as potential biomarkers for cancer diagnosis(a preliminary study)

Vladimir Zaichick, Sofia Zaichick

1Radionuclide Diagnostics Department, Medical Radiological Research Centre, Obninsk 249036, Russia.

2Laboratory of Dr. Gabriela Caraveo Piso, Feinberg School of Medicine, Northwestern University, Chicago, IL 60611-4296, USA.

Abstract Aim: Thyroid cancer is an internationally important health problem. The aim of this exploratory study was to evaluate whether significantchanges in the thyroid tissue levels of Al, B, Ba, Br, Ca, Cl, Cu, Fe, I, K, Li, Mg, Mn, Na, P,S, Si, Sr, V, and Zn exist in the malignantly transformed thyroid.Methods: Thyroid tissue levels of twenty chemical elements were prospectively evaluated in 41 patients with thyroid malignant tumors and 105 healthy inhabitants. Measurements were performed using a combination of non-destructive and destructive methods: instrumental neutron activation analysis and inductively coupled plasma atomic emission spectrometry, respectively. Tissue samples were divided into two portions. One was used for morphological study while the other was intended for trace element analysis.Results: It was found that contents of Al, B, Br, Ca, Cl, Cu, K, Mg, Mn, Na, P, S, and Si were significantly higher(approximately 3.2, 4.6, 9.3, 1.8, 2.3, 3.6, 1.6, 1.6, 1.6, 1.2, 2.5, 1.1, and 2.8 times, respectively) while content of I lower (nearly 26 times) in cancerous tissues than in normal tissues.Conclusion: There are considerable changes in chemical element contents in the malignantly transformed tissue of thyroid.

Keywords: Thyroid malignant tumors, intact thyroid, chemical elements, biomarkers for cancer diagnosis,instrumentalneutron activation analysis, inductively coupled plasma atomic emission spectrometry

INTRODUCTION

Thyroid cancer (TC) is the most common endocrine malignancy. TC incidence has dramatically increased in the recent decades[1]. During the same period no other cancer has increased as much as TC. With the worldwide increase in the incidence of TC, it has become the fifth most common cancer in women[2-4]. In some countries, the incidence of TC has increased extremely fast, and it has been the most common cancer over the last years[5].

Although the etiology of TC is unknown, several risk factors including deficiency or excess of such micronutrient as I have been well identified[6-17]. It was also reported that the incidence of TC and mortality from this disease increases progressively with advancing age[18,19]. For example, a 37-fold increase in hazard ratio from age ＜ 40 years to age ＞ 70 years was shown in the study of 3,664 TC patients that received surgery and adjuvant treatment at Memorial Sloan Kettering Cancer Center from the years 1985 to 2010[19].

Besides I involved in thyroid function, other trace elements have also essential physiological functions such as maintenance and regulation of cell function, gene regulation, activation or inhibition of enzymatic reactions, and regulation of membrane function. Essential or toxic (mutagenic, carcinogenic) properties of trace elements depend on tissue-specific need or tolerance, respectively[20]. Excessive accumulation or an imbalance of the trace elements may disturb the cell functions and may result in cellular degeneration, death or malignant transformation[20-22].

In our previous study a significant positive correlation between age and some chemical element contents in the thyroid was observed[23-28]. It was concluded that an age-dependent excess of intra-thyroidal I and Zn concentration is probably one of the factors acting in both initiation and promotion stages of thyroid carcinogenesis[9,24,25], as it was earlier shown by us for I in thyroid and for Zn in prostate gland[29-34].Moreover, it seems fair to suppose that besides I and Zn,many other chemical elementsalso play arolein the pathophysiology of the thyroid.

This work had two aims. Thefirst was to assess the Al, B, Ba, Br, Ca, Cl, Cu, Fe, I, K, Li, Mg, Mn, Na, P, S,Si, Sr, V, and Zn mass fraction contents in TC tissue using a combination of non-destructive and destructive methods: instrumental neutron activation analysis with high resolution spectrometry of short-lived radionuclides (INAA-SLR) and inductively coupled plasma atomic emission spectrometry, respectively. The second aim was to compare the levels of chemical elements in the malignant thyroid with those in intact(normal) gland of apparently healthy persons.

METHODS

All patients suffering from TC (n = 41, mean age Mean ± SD was 46 ± 15 years, range 16-75) were hospitalized in the Head and Neck Department of the Medical Radiological Research Centre. Thick-needle puncture biopsy of suspicious nodules of the thyroid was performed for every patient, to permit morphological study of thyroid tissue at these sites and to estimate their chemical element contents. In cases of surgical operated patients with TC the specimens of resected materials were also used for morphological and chemical investigation. In all cases the diagnosis has been confirmed by clinical and morphological results obtained during studies of biopsy and resected materials. Histological conclusions for malignant tumors were: 25 papillary adenocarcinomas, 8 follicular adenocarcinomas, 7 solid carcinomas, and 1 reticulosarcoma.

Normal thyroids for the control group samples were removed at necropsy from 105 deceased (mean age 44 ±21 years, range 2-87), who had died suddenly. Samples were obtained within 48 h after a sudden death. The majority of deaths were due to trauma. A histological examination in the control group was used to control the age norm conformity, as well as to confirm the absence of micro-nodules and latent cancer.

All tissue samples were divided into two portions using a titanium scalpel[35]. One was used for morphological study while the other was intended for chemical element analysis. After the samples intended for chemical element analysis were weighed, they were freeze-dried and homogenized[36].

The pounded samples weighing about 5-10 mg (for biopsy) and 100 mg (for resected materials) were used for chemical element measurement by INAA-SLR. The samples for INAA-SLR were sealed separately in thin polyethylenefilms washed beforehand with acetone and rectified alcohol. The sealed samples were placed in labeled polyethylene ampoules. The content of Br, Ca, Cl, I, K, Mg, Mn, and Na were determined by INAASLR using a horizontal channel equipped with the pneumatic rabbit system of the water-water-reactorspecial research nuclear reactor (Branch of Karpov Institute, Obninsk). Thyroid samples irradiated by neutrons were measured using a gamma spectrometer. The gamma spectrometer included the 98 cm3Ge(Li)detector with on-line computer-based multichannel analyzer system (NUC 8100, Hungary) and provided a resolution of 1.9 keV on the60Co 1332 keV line.

After INAA-SLR investigation the thyroid samples were taken out from the polyethylene ampoules and used for inductively coupled plasma-atomic emission spectrometry (ICP-AES). The samples were decomposed in autoclaves. For this 1.5 mL of concentrated HNO3(nitric acid at 65%, maximum of 0.0000005% Hg; GR, ISO,Merck, Darmstadt, Germany) and 0.3 mL of H2O2(pure for analysis) were added to each thyroid samples,which were placed in one-chamber autoclaves (Ancon-AT2, Ltd., Moscow, Russia) and then heated for 3 h at 160-200 °C. After autoclaving, they were cooled to room temperature and solutions from the decomposed samples were diluted with deionized water (up to 20 mL) and transferred to plastic measuring bottles.Simultaneously, the same procedure was performed in autoclaves without tissue samples (containing only HNO3+ H2O2+ deionized water), and the resultant solutions were used as control samples. Sample aliquots were used to determine the Al, B, Ba, Ca, Cu, Fe, K, Li, Mg, Mn, Na, P, S, Si, Sr, V, and Zn mass fractions by ICP-AES using the spectrometer ICAP-61 (Thermo Jarrell Ash, USA). The determination of the ChE content in aqueous solutions was made by the quantitative method using calibration solutions (High Purity Standards, USA) of 0.5 and 10 mg/L of each element. The calculations of the ChE content in the probe were carried out using software of a spectrometer (ThermoSPEC, version 4.1).

Information detailing the NAA-SLR and ICP-AES methods used and other details of the analysis were presented in our earlier publications concerning chemical element contents in human thyroid, scalp hair,and prostate[7,23,27,37-42].

To determine contents of the elements by comparison with a known standard, biological synthetic standards(BSS) prepared from phenol-formaldehyde resins were used[43]. In addition to BSS, aliquots of commercial,chemically pure compounds were also used as standards. Ten sub-samples of certified reference material(CRM) International Atomic Energy Agency (IAEA) H-4 (animal muscle) and five sub-samples of CRM of the Institute of Nuclear Chemistry and Technology (INCT, Warszawa, Poland), INCT-SBF-4 Soya Bean Flour, INCT-TL-1 Tea Leaves, and INCT-MPH-2Mixed Polish Herbs were treated and analyzed in the same conditions as those for thyroid samples to estimate the precision and accuracy of results.

A dedicated computer program for INAA mode optimization was used[44]. All thyroid samples were prepared in duplicate, and mean values of chemical element contents were used. Mean values of chemical elements contents were used infinal calculation for the Br, Fe, Rb, and Zn mass fractions measured by two methods. Using Microsoft Office Excel, a summary of the statistics, including, arithmetic mean, standard deviation, standard error of mean, minimum and maximum values, median, percentiles with 0.025 and 0.975 levels was calculated for chemical element contents. The difference in the results between two age groups was evaluated by the parametric Student’s t-test and non-parametric Wilcoxon-Mann-Whitney U-test.

Table 1. Instrumental neutron activation analysis with high resolution spectrometry of short-lived radionuclides data of chemical element contents in the IAEA H-4 (animal muscle) reference material compared to certified values (mg/kg on dry mass basis)

Table 2. Inductively coupled plasma-atomic emission spectrometry data of chemical element contents in certified reference materials(M ± SD, mg/kg on dry mass basis)

RESULTS

Table 1 depicts our data for Br, Ca, Cl, K, Mg, Mn, and Na mass fractions in ten sub-samples of CRM IAEA H-4 (animal muscle) and the certified values of this material.

Table 2 presents our data for Al, B, Ba, Ca, Cu, Fe, K, Li, Mg, Mn, Na, P, S, Si, Sr, V, and Zn mass fractions infive sub-samples of INCT-SBF-4 Soya Bean Flour, INCT-TL-1 Tea Leaves and INCT-MPH-2Mixed Polish Herbs CRMs and the certified (or informative) values of this material.

The comparison of our results for the Ca, K, Mg, Mn, and Na mass fractions (mg/kg, dry mass basis) in the normal human thyroid obtained by both INAA-SLR and ICP-AES methods is shown in Table 3.

Table 4 presents certain statistical parameters (arithmetic mean, standard deviation, standard error of mean,minimal and maximal values, median, percentiles with 0.025 and 0.975 levels) of the Al, B, Ba, Br, Ca, Cl,Cu, Fe, I, K, Li, Mg, Mn, Na, P, S, Si, Sr, V, and Zn mass fraction in normal and cancerous thyroid tissue.

Table 3. Comparison of the mean values (M ± SEM) of the chemical element mass fractions (mg/kg, on drymass basis)in the normal human thyroid (males and females combined) obtained by both instrumental neutron activation analysis with high resolution spectrometry of short-lived radionuclides and inductively coupled plasma-atomic emission spectrometry methods

The comparison of our results with published data for Al, B, Ba, Br, Ca, Cl, Cu, Fe, I, K, Li, Mg, Mn, Na, P, S,Si, Sr, V, and Zn mass fraction in normal and cancerous thyroid[45-74]is shown in Table 5.

The ratios of means and the difference between mean values of Al, B, Ba, Br, Ca, Cl, Cu, Fe, I, K, Li, Mg, Mn,Na, P, S, Si, Sr, V, and Zn mass fractions in normal and cancerous thyroid are presented in Table 6.

DISCUSSION

Precision and accuracy of results

A good agreement of our results for the Al, B, Ba, Br, Ca, Cl, Cu, Fe, I, K, Mg, Mn, Na, P, S, Sr, V, and Zn mass fractions with the certified values of CRM IAEA H-4, INCT-SBF-4, INCT-TL-1, and INCT-MPH-2[Tables 1 and 2] as well as the similarity of the means of the Ca, K, Mg, Mn, and Na mass fractions in the normal human thyroid determined by both INAA-SLR and ICP-AES methods [Table 3] demonstrates an acceptable precision and accuracy of the results obtained in the study and presented in Tables 4-6.

The mean values and all selected statistical parameters were calculated for twenty chemical elements (Al, B,Ba, Br, Ca, Cl, Cu, Fe, I, K, Li, Mg, Mn, Na, P, S, Si, Sr, V, and Zn) mass fractions [Table 4]. The mass fraction of Al, B, Ba, Br, Ca, Cl, Cu, Fe, I, K, Li, Mg, Mn, Na, P, S, Si, Sr, V, and Zn were measured in all, or a major portion of normal and cancerous tissue samples.

Comparison with published data

The means obtained for Al, B, Ba, Br, Ca, Cl, Cu, Fe, I, K, Mg, Mn, Na, P, S, Si, Sr, V, and Zn mass fraction,as shown in Table 5, agree well with the medians of mean values reported by other research for the human thyroid, including samples received from persons who died from different non-thyroid diseases[45-65].The mean obtained for Li is two orders of magnitude lower than the median of previously reported data.Moreover, it is outside the range of previously reported means. The mean obtained for V is one order of magnitude higher than the median of previously reported data, but it is inside the previously reported range of means. A number of values for chemical element mass fractions were not expressed on a dry mass basis by the authors of the cited references. Hence we calculated these values using published data for water 75%[75]and ash 4.16% on dry mass basis[76]contents in thyroid of adults.

In cancerous tissues [Table 3] our results were within the range of means published for Br, Ca, Cu, Fe, I, Mg,Mn, and Zn contents. The obtained means for V was approximately three orders of magnitude lower median of previously reported mean [Table 5]. The obtained mean for Cl was almost one order of magnitude higher than the only reported result and the mean for K was some higher than the median of previously reported means and also higher than the upper level of the range of these means [Table 5]. No published data referring Al, B, Ba, Li, Na, P, S, Si, and Sr contents of cancerous thyroid tissue were found.

The ranges of means of Al, B, Ba, Br, Ca, Cl, Cu, Fe, I, K, Li, Mg, Mn, Na, P, S, Si, Sr, V, and Zn levels reported in the literature for normal and for untreated cancerous thyroid vary widely [Table 5]. This can beexplained by a dependence of element content on many factors, including the region of the thyroid, from which the sample was taken, age, gender, ethnicity, mass of the gland, and the cancer stage. Not all these factors were strictly controlled in cited studies. Another leading cause, in our opinion, of inter-observer variability can be attributed to the accuracy of the analytical techniques, sample preparation methods, and inability of taking uniform samples from the affected tissues. It was insufficient quality control of results in these studies. In many reported papers tissue samples were ashed or dried at high temperature for many hours. In other cases, thyroid samples were treated with solvents (distilled water, ethanol, formalin etc.). There is evidence that by using these methods some quantities of certain trace elements are lost as a result of this treatment, which concerns not only such volatile halogen as Br, but also other trace elements investigated in the study[36,77,78].

Table 4. Some statistical parameters of Al, B, Ba, Br, Ca, Cl, Cu, Fe, I, K, Li, Mg, Mn, Na, P, S, Si, Sr, V, and Zn mass fraction(mg/kg, dry mass basis) in normal and cancerous thyroid

Table 5. Median, minimum and maximum value of means Al, B, Ba, Br, Ca, Cl, Cu, Fe, I, K, Li, Mg, Mn, Na, P, S, Si, Sr, V, and Zn contents in the normal and cancerous thyroid according to data from the literature in comparison with our results (mg/kg,dry mass basis)

Effect of malignant transformationon chemical element contents

From Table 6, it is observed that in cancerous tissue the mass fractions of Al, B, Br, Ca, Cl, Cu, P, and Si are approximately 3, 5, 9, 2, 2, 4, 2, and 3 times, respectively, higher than the mass fractions of K, Mg, Mn, Na,and S, which are almost 57%, 61%, 57%, 24%, and 14%, respectively, higher than in normal tissues of the thyroid. In contrast, the mass fraction of I is almost 26 times lower. Thus, if we accept the chemical element contents in thyroid glands in the control group as a norm, we have to conclude that with a malignant transformation the levels of Al, B, Br, Ca, Cl, Cu, K, Mg, Mn, Na, P, and S in thyroid tissue significantly increased whereas the levels of I drastically decreased.

Table 6. Differences between mean values (M ± SEM) of Al, B, Ba, Br, Ca, Cl, Cu, Fe, I, K, Li, Mg, Mn, Na, P, S, Si, Sr, V, and Zn mass fraction (mg/kg, dry mass basis) in normal and cancerous thyroid

Role of chemical elements in malignant transformation of the thyroid

Characteristically, elevated or reduced levels of chemical elements observed in cancerous tissues are discussed in terms of their potential role in the initiation and promotion of TC. In other words, using the low or high levels of the chemical element in cancerous tissues researchers try to determine the carcinogenic role of the deficiency or excess of each chemical element in investigated organ. In our opinion, abnormal levels of many chemical elements in tumor could be the cause and also the effect of malignant transformation.From the results of such kind of studies, it is not always possible to decide whether the measured decrease or increase in chemical element level in pathologically altered tissue is the reason for alterations or vice versa.

Al

The trace element Al is not described as essential, because no biochemical function has been directly connected to it. At this stage of our knowledge, there is no doubt that Al overload impacts negatively on human health, including the thyroid function[79].

B

Trace element B is known to influence the activity of many enzymes[80]. Numerous studies have demonstrated beneficial effects of B on human health, including anti-inflammatorystimulus - which reduces levels of inflammatory biomarkers, such as high-sensitivity C-reactive protein and tumor necrosis factor α;as well as raises levels of antioxidant enzymes, such as superoxide dismutase (SOD), catalase, and glutathione peroxidase[81]. Why B content in cancerous thyroid is higher than normal level and how an excess of B acts on thyroid are still to be cleared.

Br

This is one of the most abundant and ubiquitous of the recognized trace elements in the biosphere. Inorganic bromide is the ionic form of bromine which exerts therapeutic as well as toxic effects. An enhanced intake of bromide could interfere with the metabolism of iodine at the whole-body level. In the thyroid gland the biological behavior of bromide is more similar to the biological behavior of iodide[82]. In our previous studies,we found a significant age-related increase of Br content in human thyroid[23,26-28]. Therefore, a goitrogenic and, probably, carcinogenic effect of excessive Br levels in the thyroid of old females was assumed. On the one hand, elevated levels of Br in TC tissues, observed in the present study, support this conclusion. But, on the other hand, bromide compounds, especially KBr, NaBr, and NH4Br are frequently used as sedatives in Russia[83]. It may be the reason for elevated levels of Br in specimens of patients with TC. Nevertheless, the accumulation of Br in neoplastic thyroid tissues could possibly be explored for diagnosis of TC.

Ca

In addition to the elevated Br level, an excess in Ca mass fractions in thyroid tissue may contribute to harmful effects on the gland. Many reviews and numerous papers raise the concern about role of Ca in the prostate, breast, lung and other organ malignant transformation[84-94].

Cl

Cl is a ubiquitous, extracellular electrolyte essential to more than one metabolic pathway. Cl exists in the form of chloride in the human body. In the body, it is mostly present as sodium chloride. Therefore, as usual,there is a correlation between Na and Cl contents in tissues and fluids of human body. It is well known that Cl mass fractions in samples depend mainly on the extracellular water volume, including the blood volumes, in tissues[95]. Cancerous tissues are predominantly highly vascularized lesions. Thus, it is possible to speculate that thyroid malignant tumors are characterized by an increase of the mean value of the Cl mass fraction because the level of tumor vascularization is higher than that in normal thyroid tissue. Overall, the elevated levels of Cl in neoplastic thyroids could possibly be explored for diagnosis of TC.

Cu

Cu is a ubiquitous element in the human body which plays many roles at different levels. Various Cuenzymes (such as amine oxidase, ceruloplasmin, cytochrome-c oxidase, dopamine-monooxygenase,extracellular SOD, lysyl oxidase, peptidylglycineamidating monoxygenase, Cu/Zn SOD, and tyrosinase)mediate the effects of Cu deficiency or excess. Cu excess can have severe negative impacts. Cu generates oxygen radicals and many investigators have hypothesized that excess copper might cause cellular injury via an oxidative pathway, giving rise to enhanced lipid peroxidation, thiol oxidation, and, ultimately, DNA damage[96-98]. Thus, Cu accumulation in thyroid parenchyma with age may be involved in oxidative stress,dwindling gland function, and increasing risk of goiter or cancer[26,28]. The significantly elevated level of Cu in thyroid malignant tumors, observed in the present study, supports this speculation. However, an overall comprehension of Cu homeostasis and physiology, which is not yet acquired, is mandatory to establish the exact role of Cuin the thyroid malignant tumors etiology and metabolism. Anyway, the accumulation of Cu in neoplastic thyroids could possibly be explored for diagnosis of TC.

I

Compared to other soft tissues, the human thyroid gland has higher levels of I, because this element plays an important role in its normal functions, through the production of thyroid hormones (thyroxin and triiodothyronine) which are essential for cellular oxidation, growth, reproduction, and the activity of the central and autonomic nervous system. Malignant transformation is accompanied by a loss of tissuespecific functional features, which leads to a significant reduction in I content associated with functional characteristics of the human thyroid tissue. Drastically low level of I content in neoplastic thyroids could possibly be explored for diagnosis of TC.

K

An uncontrollable cell proliferation characterizes the malignant tumors. Therefore, morphological structures of TC tissue differ from the structure of normal thyroid parenchyma. Because K is mainly an intracellular electrolyte, an elevated level of K content in the TC tissue might reflect the increase of ratio “mass of transformed thyroid cell - mass of follicular colloid”. Nevertheless, the accumulation of K in neoplastic thyroids could possibly be explored for diagnosis of TC.

Mg

Mg is abundant in the human body. This element is essential for the functions of more than 300 enzymes(e.g., alkaline phosphatases, ATP-ases, phosphokinases, the oxidative phosphorylation pathway). It plays a crucial role in many cell functions such as energy metabolism, protein and DNA syntheses, and cytoskeleton activation. Moreover, Mg plays a central role in determining the clinical picture associated with thyroid disease[99]. Experimental data have shown that high doses of magnesium increase the activity of the thyroid gland[100]. Magnesium deficiency can influence bioavailability and tissue distribution of selenium which then appears diminished[101]. From these data, one can conclude that Mg is involved in the thyroid function.If so, significant reduction in Mg content can be associate with TC, because malignant transformation is accompanied by a loss of thyroid-specific functional features. However, it is well known that malignant tumors usually have higher Mg levels than normal tissues[102-107], possibly caused by the “retention” of Mg by the tumor[108], as a result of the high Mg requirement of growing cells. In addition, cultured proliferating cells have long been known to contain more magnesium than quiescent cells, and experimental conditions that decreased magnesium availability affected cell proliferation rate[109]. Thus, the elevated levels of Mg in neoplastic thyroids could possibly be explored for the diagnosis of TC.

Mn

The trace element Mn is a cofactor for numerous enzymes, playing many functional roles in living organisms. The Mn-containing enzyme, Mn-SOD, is the principal antioxidant enzyme which neutralizes the toxic effects of reactive oxygen species (ROS). It has been speculated that Mn interferes with thyroid hormone binding, transport, and activity at the tissue level[110]. There is the opinion that Mn deficiencies in humans are rare and humans maintain stable tissue levels of this trace element[111]. It was reported that intracellular Mn content was positively correlated with Mn-SOD, suggesting that the intracellular Mn level is associated with Mn-SOD activity[112]. However, an overall comprehension of Mn homeostasis and physiology, which is not yet acquired, is mandatory to establish Mn exact role in the thyroid malignant tumors etiology and metabolism. Anyway, the accumulation of Mn in neoplastic thyroids could possibly be explored for diagnosis of TC.

Na

The knowledge concerning ion regulation in many normal and abnormal cell processes has had a rapid development. It was found, among other regulations, that sodium-calcium exchange is associated with the cytoskeleton and the cell membrane. A hypothesis was eventually established that a wide variety of pathological phenomena ranging from acute cell death to chronic processes, such as neoplasia, have a common series of cellular reactions[113]. In accordance with this hypothesis, concentrations of sodium were found to be enhanced in human and animal neoplastic tissues[114,115]. Moreover, the hypothesis that physiological and biochemical changes are associated with proliferating malignant tumors may cause an increase in total tissue sodium concentration was tested with non-invasive, quantitative23Na magnetic resonance imaging in patients with benign and malignant breast tumors. It was shown that elevated Na concentrations in breast lesions appear to be a cellular-level indicator associated with malignancy[116]. In addition, Na is mainly an extracellular electrolyte and its elevated level in malignant tumors might be linked with a high tumor vascularization (see Chlorine). Anyway, it seems that the accumulation of Na is a generic property of malignant tumors.

P

P is necessary for several, various biological roles in the signal transduction of cells and energy exchange of human body. About 80%-90% of phosphorus is founded in teeth and bones in the form of hydroxyapatite.Calcium phosphates are one of the main constituents of mineral deposits in aortic wall and tissues[117]. Thus,the high P level in TC can be intimately linked with tumor calcification[86-96].

SProteins contain between 3% and 6% of sulfur amino acids. Sulfur amino acids contribute substantially to the maintenance and integrity of the cellular systems by influencing the cellular redox state and the capacity to detoxify toxic compounds, free radicals and ROS[118]. ROS are generated during normal cellular activity and may exist in excess in some pathophysiological conditions, such as inflammation. Therefore exploring fundamental aspects of sulfur metabolism such as the antioxidant effects of sulfur-containing amino acids[119]may help elucidate the mechanism by which the S content increases in TC. Thus, it might be assumed that the elevated S level in cancerous thyroid reflects an increase in concentration of ROS in malignant tissue.

Ourfindings show that mass fraction of Al, B, Br, Ca, Cl, Cu, I, K, Mg, Mn, Na, P, and S are significantly different in TC as compared to normal thyroid tissues [Table 6]. Thus, it is plausible to assume that levels of these chemical elements in thyroid tissue can be used as tumor markers. However, this subject needs in additional studies.

Limitations

This study has several limitations. Firstly, analytical techniques employed in this study measure only twenty element (Al, B, Ba, Br, Ca, Cl, Cu, Fe, I, K, Li, Mg, Mn, Na, P, S, Si, Sr, V, and Zn) mass fractions. Future studies should be directed toward using other analytical methods which will extend the list of chemical elements investigated in normal and cancerous thyroid tissue. Secondly, the sample size of TC group was relatively small. It does not allow us to carry out the investigations of chemical element contents in TC group using differentials like gender, histological types of tumors, stage of disease, and dietary habits of healthy persons and patients with TC. Lastly, the generalization of our results may be limited to Russian population.Despite these limitations, this study provides evidence on cancer-specific tissue Al, B, Br, Ca, Cl, Cu, I, K,Mg, Mn, Na, P, and S level alteration and shows the necessity to continue chemical element research of malignant thyroid tumors.

DECLARATIONS

Acknowledgments

The authors are extremely grateful to Profs. Vtyurin BM and Medvedev VS, Medical Radiological Research Center, Obninsk, as well as to Dr. Choporov Yu, Head of the Forensic Medicine Department of City Hospital, Obninsk, for supplying thyroid samples. We are also grateful to Dr. Karandaschev V, Dr. Nosenko S, and Moskvina I, Institute of Microelectronics Technology and High Purity Materials, Chernogolovka,Russia, for their help in ICP-MS analysis.

Authors’ contributions

Collected thyroid samples, designed the INAA and ICP-AES of samples, carried out the statistical analysis of results: Zaichick V

Managed the literature searches, wrote the first draft of the manuscript, translated the manuscript into English: Zaichick S

Read and approved thefinal manuscript: Zaichick V, Zaichick S

Availability of data and materials

Data were obtained in Radionuclide Diagnostic Department, Medical Radiological Research Center,Obninsk 249036, Russia. The data are available in electronic format as Excel and Wordfiles upon request.

Financial support and sponsorship

None.

Conflicts of interest

All authors declared that there are no conflicts of interest.

Ethical approval and consent to participate

All studies were approved by the Ethical Committees of the Medical Radiological Research Centre, Obninsk.All procedures performed in studies involving human participants were in accordance with the ethical standards of the institutional and/or national research committee and with the 1964 Helsinki declaration and its later amendments or comparable ethical standards.

Consent for publication

Not applicable.

Copyright

? The Author(s) 2018.