Papillary Thyroid Microcarcinoma: Insights from a Cohort of 257 Thyroidectomized Patients

Abstract Papillary thyroid microcarcinoma management evolved, and less aggressive strategies are now considered. Questions, however, remain on these tumors’ behavior, particularly on developing countries' real ground healthcare scenarios. Our aim is to gather insights on the natural history of papillary thyroid microcarcinoma on patients treated with thyroidectomy in Brazil. Consecutive patients diagnosed with papillary thyroid microcarcinoma had their clinical characteristics, interventions, and outcomes described. Patients were classified as incidental or nonincidental based on the diagnosis after or before surgery, respectively. A sum of 257 patients were included, 84.0% of which were women, and the mean age was of 48.3±13.5 years. The mean tumor size was of 0.68±0.26 cm, 30.4% were multifocal, 24.5% had cervical metastasis, and 0.4% distant metastasis. The nonincidental and incidental tumors differed in tumor size (0.72±0.24 and 0.60±0.28 cm, respectively, p=0.003) and in presence of cervical metastasis (31.3% and 11.9%, respectively, p<0.001). Male sex, nonincidental diagnosis, and younger age were independent predictors of cervical metastasis. After 5.5 years (P25–75 2.5–9.7) of follow-up, only 3.8% of patients had persistent structural disease (3.4% cervical). Predictors of persistent disease at multivariate analysis included cervical metastasis and multicentricity. In conclusion, incidental and nonincidental papillary thyroid microcarcinoma patients of the population studied displayed excellent outcomes. Cervical metastasis and multicentricity were frequent findings and prognostic factors for persistent disease.


Introduction
In the last decades, thyroid cancer incidences increased worldwide [1][2][3][4][5], up to 15-fold in countries that adopted screening programs [3], and with substantial variability between and within populations. Mortality rates, however, remained stable or even decreased in most countries [1][2][3]6]. Papillary thyroid carcinoma (PTC), the histologic type that accounts for more than 85 % of thyroid carcinomas, has been shown to be responsible for most of thyroid cancer incidence's growth [1,2]. Almost half of new PTC cases can be due to small tumors with 10 mm or smaller diameter, called micro-carcinomas (PTMC) [1]. Possibly due to enhanced healthcare access and increased sensitivity of diagnostic imaging methods [7,8], this data raised concerns on the risk of overdiagnosing PTC [5,9], especially in PTMC cases, in which the tumors present favorable prognosis.
Given the surgical risks and potential consequences associated with thyroidectomies -such as vocal cord paralysis (VCP), hypoparathyroidism (Hypo-PT), and need for levothyroxine substitutioncurrent recommendations point to the increasing relevance of more conservative treatment options. In this context, thyroid Papillary thyroid microcarcinoma management evolved, and less aggressive strategies are now considered. Questions, however, remain on these tumors' behavior, particularly on developing countries' real ground healthcare scenarios. Our aim is to gather insights on the natural history of papillary thyroid microcarcinoma on patients treated with thyroidectomy in Brazil. Consecutive patients diagnosed with papillary thyroid microcarcinoma had their clinical characteristics, interventions, and outcomes described. Patients were classified as incidental or nonincidental based on the diagnosis after or before surgery, respectively. A sum of 257 patients were included, 84.0 % of which were women, and the mean age was of 48.3 ± 13.5 years. The mean tumor size was of 0.68 ± 0.26 cm, 30.4 % were multifocal, 24.5 % had cervical metastasis, and 0.4 % distant metastasis. The nonincidental and incidental tumors differed in tumor size (0.72 ± 0.24 and 0.60 ± 0.28 cm, respectively, p = 0.003) and in presence of cervical metastasis (31.3 % and 11.9 %, respectively, p < 0.001). Male sex, nonincidental diagnosis, and younger age were independent predictors of cervical metastasis. After 5.5 years (P25-75 2.5-9.7) of follow-up, only 3.8 % of patients had persistent structural disease (3.4 % cervical). Predictors of persistent disease at multivariate analysis included cervical metastasis and multicentricity. In conclusion, incidental and nonincidental papillary thyroid microcarcinoma patients of the population studied displayed excellent outcomes. Cervical metastasis and multicentricity were frequent findings and prognostic factors for persistent disease. lobectomy and active surveillance (AS) emerge as safe and effective options for selected cases, with excellent clinical outcomes in long-term follow-up [10,11]. Moreover, these new approaches brought relevant discussion of PTMC decision-making process and its impact in several issues, including quality of life and healthcare costs [12,13]. Nevertheless, current PTMC treatment approaches are predominantly aggressive worldwide, regarding surgical decisions and radioiodine (RAI) use, despite new evidence and guidelines [14]. The applicability, patient selection, and decision-making between treatment options according to prognostic factors and potential benefits and risks, especially in real clinical care settings, are still a matter of study.
In this study, we aim to gather insights on PTMC management and treatment perspectives in a cohort study of PTMC patients who underwent total thyroidectomy. We also explored if the clinical course and outcomes were different in PTMC classified as incidental and as nonincidental based on the diagnosis after and before surgery, respectively.

Patients and study design
Patients from a cohort of differentiated thyroid cancer (DTC) were followed at a tertiary care, university teaching hospital in southern Brazil. From 2000 to 2019, all consecutive patients with a histological diagnosis of PTC with a tumor size ≤ 10 mm were included. Patients were classified with nonincidental diagnosis when the index nodule investigated had malignancy confirmed by fine needle aspiration biopsy or high suspicion for malignancy prior to surgical intervention decision, with confirmatory histopathological results. Incidental diagnosis was defined as diagnosis of PTC made in histopathological examination after thyroid surgery for other reasons.

Treatment protocol and follow-up
All patients underwent thyroidectomy, some of them were referred to our Thyroid Unit after undergoing surgery in other institutions. Decisions regarding cervical lymph node dissection were made at the discretion of the surgical team at the Institution where the procedure was performed. At our hospital, lymphadenectomy is indicated for patients with lymph node (LN) metastasis identified before the surgery or in the presence of suspicious LN during the surgery. The radioiodine (RAI) administration protocol used activities prescribed at the attending physician's discretion. RAI was administered in a stimulated thyrotropin (TSH) condition of endogenous hypothyroidism (TSH > 30 mUI/l), after withdrawing levothyroxine for at least 3-4 weeks [15]. A post-treatment whole body scan (post-treatment WBS) was performed seven to ten days after RAI administration [16].
In the first evaluation, the following data were recorded for each patient: demographics, tumor characteristics including histological features, extension and lymph node involvement, and treatment (e. g., surgery, RAI remnant ablation, and other interventions). Each patient was classified using the 8th edition of the TNM/ AJCC staging system (I, II, III, or IV) [17]. The "no evidence of lymph node metastasis" (N0) status was determined by preoperative cytological evaluation of LN or pathological examination of patients with LN resection (TNM 8th Edition N0a) or preoperative and/or postoperative neck US imaging (TNM 8th Edition N0b).
Distant metastasis (M1) was considered present when there was a lesion outside the cervical bed on imaging computed tomography (CT), or scintigraphy with histological confirmation, or post-treatment WBS uptake and/or elevated thyroglobulin (Tg). The risk of persistent/recurrent disease was assessed based on the proposed risk stratification system by the 2009 American Thyroid Association (ATA) guidelines, with patients classified into three risk groups: low, intermediate, and high [18].
The follow-up protocol called for an initial assessment at 3 to 6 months after the initial treatment, which included a physical examination of the neck and measurements of the serum Tg levels under TSH suppression and anti-thyroglobulin antibody (TgAb). In a second evaluation, 6 to 12 months after the initial treatment, physical examination of the neck and serum TSH, Tg and TgAb were reassessed. Some patients also had the serum level of Tg evaluated under stimulated TSH condition of endogenous hypothyroidism (TSH > 30 mIU/l) (sTg) at the end of the first year of follow-up. Neck US was performed in this first year of follow-up. At this point, the patient was classified according to disease response to initial therapy (see below in the outcomes section). Patients classified as having an excellent response were scheduled for annual visits, during which a physical examination of the neck and measurements of TSH, Tg, and TgAb were performed. Patients with indeterminate response or persistent disease were scheduled for the same examination at least twice a year. Additional imaging studies (e. g., X-ray, bone scintigraphy, CT) were performed, as needed, whenever the clinical or laboratory findings raised the suspicion of persistent or recurrent disease. Duration of follow-up was defined as the time between the first surgery and the last medical visit to the clinic.

Outcomes
Dynamic response stratification (DRS) status after treatment was defined based on clinical examination, Tg and/or sTg levels, neck US, post-RAI WBS (when available), and additional imaging exams when indicated. Patients were classified into four categories: excellent response, indeterminate response, biochemical incomplete response, or structural incomplete response [19,20].
In patients submitted to RAI, excellent response was defined as negative imaging and Tg < 0.2 ng/ml or sTg < 1.0 ng/ml. Biochemical incomplete response was defined as negative imaging and Tg > 1.0 ng/ml or sTg > 10.0 ng/ml or rising TgAb levels. Structural incomplete response was defined as structural or functional evidence of disease with any Tg or TgAb level. Indeterminate response was defined as non-specific findings on imaging studies or Tg between 0.2 to 1.0 ng/ml, sTg between 1.0 and 10.0 ng/ml, or positive TgAb with stable or declining levels [19].
In patients who underwent total thyroidectomy (TT) but did not receive RAI, excellent response was defined as negative imaging and Tg < 0.2 ng/ml or sTg < 2.0 ng/ml. Biochemical incomplete response was defined as negative imaging and Tg > 5.0 ng/ml, sTg > 10.0 ng/ml, or increasing Tg levels over time or rising TgAb levels. Structural incomplete response was defined as structural or functional evidence of disease with any Tg or TgAb level. Indeterminate response was defined as non-specific findings on imaging studies or Tg between 0.2 to 5.0 ng/ml, sTg between 2.0 and 10.0 ng/ml, or positive TgAb with stable or declining levels [19].
For patients who underwent lobectomy, excellent response was defined as negative imaging and stable Tg level < 30 ng/ml with undetectable TgAb. Biochemical incomplete response was defined as negative imaging and Tg level > 30 ng/ml, increasing Tg level values over time with similar TSH levels, or increasing TgAb levels. Structural incomplete response was defined as structural or functional evidence of disease with any Tg or TgAb level. Indeterminate response was defined as non-specific findings on imaging studies, positive TgAb levels stable, or declining in the absence of structural or functional disease [20].
For treatment response analysis, persistent disease was defined as either incomplete biochemical, cervical structural incomplete, or distant structural incomplete responses. Indeterminate response was combined with excellent response outcome, due to their similar prognosis after treatment, with 80-85 % of these patients remaining either stable or improving to excellent response [21,22].
Regarding surgical complications, permanent Hypo-PT was defined as hypocalcemia and low parathormone levels (PTH < 20 pg/ ml) for more than one year after surgery; this period was chosen, rather than the six months period of current guidelines [23], aiming for more conservative and accurate event rate report of longterm complications. Patients with no preoperative voice symptoms who presented with complaints of deficiency or quality alterations on voice for more than one year after surgery were considered to have persistent subjective voice disturbance (PSVD). Any surgical complication was defined as permanent Hypo-PT and/or PSVD.

Laboratory analysis
Serum Tg measurements were conducted using immunoradiometric assays: radioimmunoassay (COBRA II)  Serum PTH was evaluated by chemiluminescent microparticle immunoassay method through the ARCHITECT ci 4100 equipment (Abbott Diagnostics, Abbott Park, IL, USA), with reference values (RV) of 15.0-68.3 pg/ml; intra-assay and inter-assay coefficients of variation were 6.1 and 6.4 %, respectively. Total serum calcium was evaluated by the NM-BAPTA method and corrected by albumin levels [corrected calcium = 0.8 × (4.0 -serum albumin) + serum calcium] with RV 8.6 to 10.0 mg/dl. Serum phosphorus was evaluated by the Molybdate UV and colorimetric (xylidyl blue) methods, with RV 2.5-4.5 mg/dl. The electrolyte tests were made using Cobas 8000 c702 equipment (Roche Diagnostics, Indianapolis, IN, USA).
After each new assay had been implemented, the necessary procedures for standardization and validation were performed.

Statistical analysis
The clinical and laboratory data are reported as the mean ± standard deviation (SD) values or as the median and percentiles 25 and 75 (P25-75) for continuous variables, and as absolute numbers and percentages for categorical variables. Statistical analyses were performed using Pearson chi-square and Fisher's exact test for categorical variables, and Student's t-test and Mann-Whitney U-test for continuous variables, as appropriate. For multivariate analysis models, Poisson regression was used, in which relative risk (RR) represented likelihood of associated events with a confidence interval (CI) of 95 %.
All tests were two-tailed, and all analyses were performed using the Statistical Package for Social Science Professional software version 20.0 (IBM Corp., Armonk, NY). A two-tailed p < 0.05 was considered statistically significant. The e!Sankey software (iPoint-systems Gmbh., Reutlingen, Germany) was used to build figures.

Patients
A total of 257 (23.5 %) patients amongst the 1091 patients of the institutional DTC cohort had PTC smaller or equal to 10 mm. Of these 257 PTMC patients, 84.0 % were females, and the mean age of diagnosis was 48.3 ± 13.5 years (▶table 1).

Clinical outcomes and lymph node metastasis predictors
The  Information table 1S. Predictors of persistent disease at univariate analysis for the 20 patients with this classification at last follow-up included male sex (p = 0.041), LN metastasis (p < 0.001), more advanced ATA risk stratification system classi-fication (p < 0.001), and persistent disease after initial treatment (p < 0.001) (▶table 2). No difference in RAI activity was observed between patients with excellent or indeterminate response versus persistent disease. No differences in DRS were observed between the incidental and the nonincidental diagnosis groups. Regarding nodal dissemination, younger age at diagnosis (p < 0.0001), male sex (p < 0.001), and multicentricity (p = 0.048) were associated with higher rates of LN metastasis (table 2 S).

Surgical complications
A total of 20 (7.8 %) patients developed permanent Hypo-PT, and 14 (5.4 %) suffered from PSVD. Any long-term surgical complication, occurred in 31 (12.1 %) patients, and three patients suffered from both permanent Hypo-PT and PSVD. Notably, there was a tendency to more long-term surgical complications in the nonincidental diagnosis than in the incidental diagnosis group (15 (13.5 %) and 5 (5.6 %), respectively, p = 0.062). LN resection data was available for 216 patients. Permanent Hypo-PT was more frequent in patients who underwent LN resection compared to those who did not ▶Fig. 1 Evolution of 224 PTMC patients stratified by ATA risk for recurrence and dynamic response to therapy, both at initial treatment and at last follow-up. Line's trajectory represents patient evolution and thickness represents the number of patients. ATA: American Thyroid Association, DRS: Dynamic risk stratification.

Discussion and Conclusions
We studied a cohort of patients with PTMC who underwent thyroidectomy at the Brazilian public health system. Among the 1091 PTC patients of our cohort, we found a PTMC proportion similar to the literature report [24]. Moreover, we observed that 30.4 % of them presented multicentric disease and 24.5 % had LN metastasis at diagnosis. Another interesting finding is that most of the patients were disease-free after a median follow-up of 5.5 years.
The LN is the main site of metastasis of PTC. In our PTMC cohort we observed in multivariate analysis that nonincidental diagnosis, male sex and younger age were predictors of LN metastasis. This is consistent with previous reports that found male sex, multicentricity, and younger age to be associated with LN metastasis [25]. Younger age is usually associated with more advanced PTMC disease [26,27]. In accordance with this observation, we found an association of younger age with nodal dissemination, but not with worse treatment response. Interestingly, in published AS strategies, younger age is the most significant predictor of tumor growth and development of LN metastasis [27]. All these data point to a pattern of more extensive and progressive disease in younger patients.  Most PTMC patients were classified as excellent response after the initial treatment and at last follow-up. DRS status remained stable over time for most patients, while some demonstrated status improvement and very few progressions of disease. These findings support the long-term excellent prognosis of PTMC tumors in different healthcare scenarios. Both ATA risk classification (p = 0.006) and DRS at initial treatment (p < 0.001) were predictors of excellent response in the last follow-up, reassuring these tools as useful methods for PTMC management. Multifocality and LN metastasis, were associated with persistent disease at last follow-up. Although associated with larger tumors and lymph node dissemination in our cohort, nonincidental diagnosis was not associated with worse response to treatment, a finding that may be due to underpower. Notably, a recent meta-analysis found significative higher risk of recurrence in nonincidental PTMC [28]. In our study, we observed 3.8 % of structural persistent disease at last follow-up (3.4 % cervical) in our cohort, and available reports of structural persistence/ recurrence range from 1 to 6 % [24,29].
We also observed high rates of surgical complications, represented by permanent Hypo-PT and PSVD, totaling 12.1 % of any adverse event in these patients. These surgical complications have been associated with increased treatment costs within the health system [12] and worse quality of life outcomes [13], especially for permanent Hypo-PT regarding the former (due to costs of exams, hospital outpatient and inpatient visits, and pharmacological treatment) and voice disturbance regarding the latter. Since these are key factors in treatment decision-making [30], we further encourage physicians to consider local surgical complication rates to perform patient-shared and patient-centered decisions in PTMC diagnosis, aiming for better quality of life outcomes and healthcare costs.
In this scenario of excellent clinical PTMC outcomes, AS emerges as a safe and effective option for selected cases [10,11]. Miyauchi et al. demonstrated in a low-risk PTMC cohort the good prognosis of these tumors when managed with AS. Tuttle et al. also expanded this finding to PTC with a size up to 1.5 cm. These cohorts and others have shown that a minority of these patients evolve to surgery due to disease progression, and in these cases, despite postponement of the surgical treatment, oncological outcomes were excellent and non-different from immediate surgery management [27,31]. In addition, AS can effectively reduce adverse events related to the management of these generally indolent tumors [32]. This is especially significant in a healthcare scenario containing a large proportion of PTMC cases and a significant risk associated with surgical treatment, such as ours. Patient selection to this approach, however, would demand careful evaluation due to the expressive rates of LN metastasis in this population -an exclusion criterion to this strategy -which is especially important in males and young patients. Since most AS cohorts were conducted in high-resource settings, Latin-American patients' acceptability and engagement on AS strategy should also be considered and clarified during treatment for a shared decision-making [33].
Some limitations of our study should be acknowledged. Due to its retrospective nature, inherent bias may be present. Moreover, since only patients of a single thyroid cancer reference center were studied, selection bias cannot be ruled out. Our institution represents a real-ground clinical practice in Latin America -where data is still needed to understand PTMC patient presentation and evolution. To our knowledge, this is the first Latin American cohort that comprised PTMC patient profile, tumor characteristics, DRS both after initial treatment and long-term, and adverse events associated with treatment. Due to the historic nature of the cohort, which comprises a period from 2000 to 2019, many patients were treated following previous protocols -245 patients underwent TT and only 2 underwent lobectomy, with a substantial portion receiving RAI therapy (60.9 %). These modalities of treatment for PTMC are considered overtreatment in the light of most recent evidence and guidelines for this tumor management. As we are a regional reference center for Thyroid Cancer, many patients are referred to our institution after the initial surgery. Decisions regarding cervical lymph node dissection were made at the discretion of the surgical team at the institution where the procedure was performed, which could be a factor that contributed to overtreatment. Additionally, given that a significant portion of our cohort was operated outside our institution, perioperative clinical, ultrasonographic and pathological information is not consistently available. This especially applies to type of diagnosis -incidental or nonincidental, which was not available for 50 patients of the cohort -perioperative US data, and pathological micro/macroinvasion. Although less aggressive treatment approaches have already been demonstrated as equally effective alternatives in selected cases, aggressive treatment approaches remain common worldwide. TT is still extensively overused, being the treatment modality in up to 80 % of tumors ≤ 2 cm; RAI therapy, with current limited indications by the ATA 2015 guidelines, persists widely used in patients with low-risk disease [34]. Considering that up to 80 % of recurrences occur during 3-5 years after initial treatment, our median follow-up of 5.5 years seems adequate to evaluate long-term disease status [35].
In conclusion, we found a significant proportion of PTMC in a DTC cohort from a tertiary public hospital in Brazil. The PTMC excellent outcomes occurred despite type of diagnosis, and at the expense of high rates of surgical complications. Since nonincidental diagnosis, male sex, and younger age were predictors of LN metastasis (which, in turn, is a predictor of persistent disease) we believe that this group of patients must be evaluated more carefully before AS be considered. Comprehensive consideration of tumor and patient profile, treatment prognosis, and treatment-related adverse effects should, altogether, be considered and conveyed during the shared decision-making process with PTMC patients.