AMES prognostic index and extent of thyroidectomy for well-differentiated thyroid cancer in the United States

AMES prognostic index and extent of thyroidectomy for well-differentiated thyroid cancer in the United States Philip I. Haigh, MD, FRCSC, FACS, David R. Urbach, MD, MSc, FRCS, FACS, and Lorne E. Rotstein, MD, FRCSC, FACS, Los Angeles, Calif, and Toronto, Ontario, Canada

Background. The optimal extent of thyroidectomy for well-differentiated thyroid cancer is controversial. It has been suggested that surgeons use the Age, Metastases, Extent and Size (AMES) risk classification to decide between partial thyroidectomy (PT) or total thyroidectomy (TT). Methods. The Surveillance, Epidemiology and End Results database was used to identify patients who underwent PT or TT for well-differentiated thyroid cancer between 1992 and 1997. Age, distant metastases, extrathyroidal extension, tumor size, AMES risk group, gender, histologic subtype, and lymph node metastases were analyzed by using logistic regression models to evaluate whether surgeons use these factors to determine extent of thyroidectomy. Results. Of 9,226 patients, most patients had small cancers confined to the thyroid gland. Of these, 79.9% were AMES low-risk, and 83% underwent TT. Age 40 years or older (40 to 49 years: odds ratio [OR], 0.75, 95% confidence interval [CI], 0.65 to 0.86; 50 years or older: OR, 0.66; 95% CI, 0.58 to 0.75) was associated with a lower likelihood of TT, as were female gender (OR, 0.80; 95% CI, 0.70 to 0.92) and follicular histology (OR, 0.65; 95% CI, 0.55 to 0.78). Extrathyroidal extension (OR, 3.85; 95% CI, 3.09 to 4.80), regional lymph node metastases (OR, 6.98; 95% CI, 5.45 to 8.93), distant metastases (OR, 7.29; 95% CI, 2.69 to 19.8), AMES high-risk group (OR, 2.82; 95% CI, 2.36 to 3.38), and larger tumor size (OR, 1.27; 95% CI, 1.01 to 1.59) were associated with greater likelihood of TT. In multivariable analyses, only age, extrathyroidal extension, and regional and distant metastases were associated with extent of thyroidectomy; AMES risk group and tumor size were no longer significant. Conclusions. Most patients undergo TT for well-differentiated thyroid cancer regardless of AMES risk grouping. In contrast to AMES risk-group classification, younger patients are more likely to undergo TT compared with older patients. Although extrathyroidal extension and distant metastases were associated with TT in accordance with AMES criteria, tumor size had no independent influence. Regional lymph node metastasis, not an AMES criterion, increased the likelihood of TT. Some components of AMES riskgroup classification are used by surgeons to choose the extent of thyroidectomy. (Surgery 2004; 136:609-16.) From the Department of Surgical Oncology, Kaiser Permanente Los Angeles Medical Center, Los Angeles, Calif; and Department of Surgery, University of Toronto, Toronto, Ontario, Canada

VARIOUS PROGNOSTIC INDICES HAVE BEEN FORMULATED that stratify patients with well-differentiated thyroid cancer into low- to high-risk prognostic groups. Several prognostic risk-stratification systems have been proposed for papillary thyroid cancer1,2 and for papillary or follicular thyroid cancer,3-8 all of which accurately predict survival in patients with

Accepted for publication December 20, 2003. Reprint requests: Philip I. Haigh, MD, FRCSC, FACS, Department of Surgical Oncology, Kaiser Permanente Los Angeles Medical Center, 4760 Sunset Blvd, Los Angeles, CA 90027. 0039-6060/$ - see front matter Ó 2004 Elsevier Inc. All rights reserved. doi:10.1016/j.surg.2003.12.009

well-differentiated thyroid cancer. The Age, Metastases, Extent and Size (AMES) prognostic index categorizes patients into low-risk and high-risk groups on the basis of age, distant metastases, extent of primary tumor, and tumor size.3 Low-risk patients have an excellent prognosis, with an approximate overall survival of 98% at 20 years, as compared to 54% for high-risk patients.3 AMES is widely accepted, easy to remember, and can be used readily at operation to help with therapeutic decision making. Molecular prognostic factors, which include DNA content9,10 and Ret-PTC oncogene rearrangements, 11,12 have been evaluated as adjuncts to clinicopathologic classification systems, but they are not yet in widespread use because molecular analysis is usually not readily available, and the data SURGERY 609

610 Haigh, Urbach, and Rotstein

regarding the direction of their prognostic effect are conflicting.11,12 The survival of patients with low-risk welldifferentiated thyroid cancer is excellent regardless of the extent of thyroidectomy. Therefore, it has been suggested that a less extensive thyroidectomy, such as thyroid lobectomy and isthmusectomy, could be used for patients in the AMES low-risk group, rather than routinely performing total thyroidectomy on all patients.1,3,5,10,13-16 Unfortunately, there are no randomized trials that compare the effectiveness of total thyroidectomy with a less extensive operation. Surgeons are left to choose the extent of thyroidectomy on the basis of expert opinion and evidence from retrospective studies. This population-based study was performed to determine whether AMES risk-group criteria govern the extent of thyroidectomy in the United States. Our hypothesis was that surgeons in the United States do not use the AMES classification for the purpose of choosing the extent of thyroidectomy. METHODS Data sources and study subjects. The Surveillance, Epidemiology and End Results (SEER) database17 was used to identify persons in the United States diagnosed with thyroid cancer (International Classification of Diseases for Oncology [ICD-O-2] codes C73.9) in 11 SEER regions between 1992 and 1997. The SEER population is quite similar to the general US population in terms of educational and poverty levels, but it is somewhat more urban than the general US population. Specific SEER regions can be found on the World Wide Web at www.seer.cancer.gov/registries. Only those with histologic subtypes of papillary thyroid cancer (consisting of papillary adenocarcinoma [ICD-O-2 histology code 8260], follicular variant of papillary carcinoma [code 8340], mixed papillary/ follicular [code 8340], and papillary carcinoma [code 8050]) or follicular thyroid carcinoma (consisting of follicular carcinoma [code 8330], moderately differentiated follicular carcinoma [code 8332], and well-differentiated follicular carcinoma [code 8331]) were included. Excluded were patients younger than the age of 18 years and patients for whom data on tumor size or presence of distant metastases were missing. Surgical procedures. Surgical therapy was determined according to the site-specific thyroid surgery codes, indicating the most extensive surgical procedure performed. In SEER, patients who have more than 1 thyroid operation within 4 months are classified as having had a single

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operation as an aggregate of the 2; for instance, 2 unilateral thyroid lobectomies are classified as a single total thyroidectomy. We dichotomized surgical therapy into (1) total thyroidectomy, which included patients who had a total or neartotal thyroidectomy, and (2) partial thyroidectomy, which included all operations of lesser extent than total or near-total thyroidectomy, including partial lobectomy, lobectomy, or lobectomy with isthmusectomy. Patients who had only a needle aspiration or an incisional biopsy without additional surgery and those with an unspecified or unknown type of thyroidectomy were excluded. AMES risk stratification. The AMES risk classification categorizes patients according to their risk of death into low- and high-risk groups. The lowrisk group includes all younger (40 years or younger for men, 50 years or younger for women) patients with intrathyroidal cancers and all older patients with intrathyroidal cancers less than 5 cm without distant metastases.3 The high-risk group includes all younger patients who have cancers with extrathyroidal extension, or all older patients who have cancers at least 5 cm in size, any cancer with extrathyroidal extension, or who have intrathyroidal follicular cancers with major tumor capsular involvement, or any patient with distant metastases.3 With the above criteria, the variables used for AMES risk-group assignment (age, presence or absence of distant metastases, extrathyroidal extension, and tumor size) were determined from the SEER database.3 Age in SEER is an interval variable in 5-year intervals and therefore was categorized into 3 subgroups to allow assignment of an AMES classification for different genders. A SEER extent of disease code specifies the local and distant extent of tumor in a hierarchical and mutually exclusive manner. Extrathyroidal extension was dichotomized into localized or contiguous spread into adjacent structures, which included hierarchical coding of pericapsular soft connective tissue, extension to major blood vessels, sternocleidomastoid, esophagus, larynx, trachea, skeletal muscle, bone, or further contiguous spread. The SEER database does not contain enough detail on tumor extent to determine the presence of major tumor capsular involvement for follicular tumors. Distant metastasis was recorded as present or absent. Only one extent of disease code is used per patient in the SEER database, which does not allow determination of local extent if the patient has synchronous distant disease. Because the presence of distant metastases is a high-risk AMES criterion regardless of local extent, this did not

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affect our ability to classify subjects into low- and high-risk categories. Tumor size was dichotomized with a 5-cm cutoff, mirroring the AMES size category. By using age, distant metastases, extent, and tumor size, each patient was assigned AMES low- or high-risk group. We also analyzed the effect of gender, histologic subtype (papillary vs follicular), and the presence or absence of metastatic thyroid cancer in regional or distant lymph nodes on extent of thyroidectomy. Regional nodes include ipsilateral, bilateral, contralateral, midline, cervical nodes, tracheoesophageal, upper anterior mediastinal, submandibular, or submental nodes, or regional nodes not otherwise specified. Nodes other than regional or if unknown were grouped into distant/unknown nodes. Statistical analysis. Associations between predictor variables and the extent of thyroidectomy were assessed by using logistic regression modeling. Variables analyzed for their influence on total thyroidectomy were age (in categories of 39 years or younger, 40 to 49 years, and 50 years or older), gender, histologic type (papillary or follicular), extrathyroidal extension (none or adjacent spread), distant metastases (none, present), tumor size (less than 5 cm, 5 cm or greater), lymph node metastases (none, regional, distant, or unknown), and AMES risk grouping (low-risk, high-risk). The strength of associations was expressed as odds ratios (ORs) and 95% confidence intervals (CIs) for having a total thyroidectomy. An OR of 1 indicates that a variable is not associated with the extent of thyroidectomy. For each variable, an OR of less than 1 indicates that the presence of a variable is less common among those having total thyroidectomy, and an OR of greater than 1 indicates that the presence of a variable is more common among those having total thyroidectomy. We determined the independent effect of potential predictor variables by using multiple logistic regression, adjusting for the same variables used in the univariate analysis: age, gender, histologic type, extrathyroidal extension, distant metastases, tumor size, lymph node metastases, and AMES risk groups. To assess the possibility that our results were influenced by misclassification of AMES risk of follicular tumors because of the uncertainty regarding tumor capsular extension in the SEER database, we performed sensitivity analyses for the association of extent of thyroidectomy with AMES risk. We performed 3 sensitivity analyses: (1) omitting all follicular tumors, (2) classifying all follicular tumors as low-risk, and (3) classifying all follicular tumors as high-risk. Estimates were

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considered statistically significant if the 95% CI did not overlap 1.0, or if P values were less than .05. All P values reported were two-tailed. SEER*Stat (National Cancer Institute, Cancer Statistics Branch, Bethesda, Md) was used to extract the data from the SEER database (available as a publicuse file). The data were then exported to SAS Version 8 (SAS Institute, Inc, Cary, NC) statistical software for subsequent analyses. RESULTS Patient demographics. There were 11,384 patients initially identified with well-differentiated thyroid cancer. Of these, 751 (6.6%) were excluded because they had needle aspiration or an incisional biopsy alone or an unspecified or unknown type of thyroidectomy. One thousand one hundred ninety (10.5%) were excluded because of missing data on distant metastases, 76 (0.7%) were excluded because of missing data on tumor size, and 141 (1.2%) were excluded because of missing data on both, leaving 9226 (81.0%) for analysis. The majority (59.1%) of patients were 40 years of age or older, and most (77.2%) were women (Table I). Papillary thyroid cancer accounted for 91% of cases; 92.7% of tumors were less than 5 cm in size, 83.6% were confined to the thyroid gland, 23.4% had regional lymph node metastases, and 1.3% had distant spread within 4 months of diagnosis. The majority of patients (79.9%) were low-risk according to AMES criteria. There were 7649 (82.9%) patients who underwent total thyroidectomy and 1577 (17.1%) who underwent partial thyroidectomy (Table II). As compared with persons having partial thyroidectomy, those undergoing total thyroidectomy were more likely to be young, male, have papillary histology, and have local, regional, or distant disease. The majority of the patients in both the lowrisk and the high-risk AMES groups had a total thyroidectomy (Table III). In the low-risk group, 5938 of 7369 (80.6%) had a total thyroidectomy as compared with 1711 of 1857 (92.1%) in the highrisk group. Older age was associated with lower likelihood of total thyroidectomy as compared with age younger than 40 years, in contrast to the suggested use of partial thyroidectomy in younger patients in the AMES low-risk group (Table IV). Female gender, follicular histology, and distant/unknown lymph node metastases (as compared with no lymphatic spread) were associated with lower likelihood of total thyroidectomy. Patients with extrathyroidal extension were more likely to have had a total thyroidectomy as compared with those whose

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Table I. Characteristics of patients residing in SEER areas who underwent thyroidectomy for well-differentiated thyroid cancer Characteristic Age (y) # 39 40-49 $ 50 Gender Female Male Histology Papillary Follicular Extrathyroidal extension None Adjacent spread* Distant metastasis No Yes Size < 5 cm $ 5 cm Lymph node metastasis None Regionaly Distant or unknownz AMES group Low-risk High-risk

Number (%), N = 9226 3777 (40.9) 2204 (23.9) 3245 (35.2) 7126 (77.2) 2100 (22.8) 8408 (91.1) 818 (8.9) 7711 (83.6) 1515 (16.4) 9102 (98.7) 124 (1.3) 8549 (92.7) 677 (7.3) 5247 (56.9) 2160 (23.4) 1819 (19.7) 7369 (79.9) 1857 (20.1)

*Adjacent spread includes pericapsular soft connective tissue, extension to major blood vessels, sternocleidomastoid, esophagus, larynx, trachea, skeletal muscle, bone, further contiguous spread. yRegional nodes include ipsilateral, bilateral, contralateral, midline, cervical nodes, tracheoesophageal, upper anterior mediastinal, submandibular, or submental nodes, or regional nodes not otherwise specified. zNodes other than regional but distant, or unknown.

tumors were confined to the thyroid gland. The presence of regional or distant lymph node metastases, AMES high-risk group, and larger tumors were associated with total thyroidectomy. In multivariable analyses, age older than 39 years (OR, 0.81, 95% CI, 0.70 to 0.94 for age 40 to 49 years; OR, 0.67, 95% CI, 0.59 to 0.77 for age older than 49 years) and the presence of distant or unknown lymph node metastases (OR, 0.60, 95% CI, 0.53 to 0.68) were independently associated with a lower likelihood of undergoing total thyroidectomy (Table IV). Extrathyroidal extension (OR, 3.53, 95% CI, 2.16 to 5.75), the presence of distant metastases (OR, 7.72, 95% CI, 2.61 to 22.9), and the presence of regional lymph node metastases (OR, 5.31, 95% CI, 4.13 to 6.82) were associated with an increased likelihood of total thyroidectomy. After adjustment for age, gender, histology, tumor

Table II. Characteristics of patients residing in SEER areas who underwent either partial or total thyroidectomy for well-differentiated thyroid cancer

Characteristic Age (y) # 39 40-49 $ 50 Gender Female Male Histology Papillary Follicular Extrathyroidal extension None Adjacent spread* Distant metastasis Yes No Size < 5 cm $ 5 cm Lymph node metastasis None Regionaly Distant or unknownz

Partial thyroidectomy, number (%) (N = 1577)

Total thyroidectomy, number (%) (N = 7649)

533 (33.8) 397 (25.2) 647 (41.0)

3244 (42.4) 1807 (23.6) 2598 (34.0)

1266 (80.3) 311 (19.7)

5860 (76.6) 1789 (23.4)

1387 (88.0) 190 (12.1)

7021 (91.8) 628 (8.2)

1487 (94.3) 90 (5.7)

6524 (85.3) 1125 (14.7)

4 (0.3) 1573 (99.7)

120 (1.6) 7529 (98.4)

1481 (93.9) 96 (6.1)

7068 (92.4) 581 (7.6)

1006 (63.8) 71 (4.5) 500 (31.7)

4241 (55.5) 2089 (27.3) 1319 (17.2)

*Adjacent spread includes pericapsular soft connective tissue, extension to major blood vessels, sternocleidomastoid, esophagus, larynx, trachea, skeletal muscle, bone, further contiguous spread. yRegional nodes include ipsilateral, bilateral, contralateral, midline, cervical nodes, tracheoesophageal, upper anterior mediastinal, submandibular, or submental nodes, or regional nodes not otherwise specified. zNodes other than regional but distant, or unknown.

extent, size and the presence of lymph node metastasis, large tumor size (OR, 1.19; 95% CI, 0.85 to 1.66), and AMES high-risk group (OR, 0.84, 95% CI, 0.53 to 1.35) were not statistically significant predictors of undergoing total thyroidectomy. Sensitivity analyses of the extent of thyroidectomy in relation to AMES risk suggested that follicular tumor risk-category misclassification was unlikely to have substantially influenced our findings. The OR for total thyroidectomy associated with AMES high-risk status was 3.43 (95% CI, 2.79 to 4.22) when all 818 patients with follicular tumors were omitted. When all follicular tumors were classified as low-risk, the OR was 3.40 (95% CI, 2.80 to 4.14), and when all follicular tumors were

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classified as high-risk, the OR was 1.72 (95% CI, 1.50 to 1.97). DISCUSSION This large population-based study provides insight into the factors used by surgeons in choosing the extent of thyroidectomy for patients with welldifferentiated thyroid cancer. In the United States, total or near-total thyroidectomy is the preferred choice of operation for patients with well-differentiated thyroid cancer, regardless of whether low or high AMES risk group. Patients who were high-risk according to AMES criteria were more likely to have a near-total or total thyroidectomy than those who were low-risk, but it appeared that AMES risk-group classification was inconsistently used to determine the extent of thyroidectomy. Although many patients who are low-risk according to the AMES criteria might do well with a partial thyroidectomy, a near-total or total thyroidectomy was done in the majority of these patients. Although valuable for prognosticating, strict AMES risk-group assignment does not independently determine the choice of thyroidectomy by surgeons in the United States. Interestingly, although surgeons use age as a factor in determining the extent of thyroidectomy, they appear to do so in a direction opposite to that intended by the AMES classification. The likelihood of near-total or total thyroidectomy decreased with increasing age, which is the opposite of AMES and other age-specific classification systems when used as tools to select high-risk patients who should undergo near-total or total thyroidectomy.1-7 Age is one of the most important prognostic factors in many of the prognostic indices, and most endocrine surgeons would recommend more aggressive surgery in the older patients. In a recent study of patients 70 years or older with well-differentiated thyroid cancer, the authors recommended total thyroidectomy with radioactive iodine in these patients because of improved survival.18 Only 41% underwent total thyroidectomy, and this was attributed to comorbidities or advanced tumor stage. Another prospective study of patients with high-risk papillary thyroid cancer participating in the National Thyroid Cancer Treatment Cooperative Study Registry in United States and Canada concluded that those who are treated with near-total or total thyroidectomy and postoperative radioactive iodine have improved overall survival.19 The preferred operation of the National Thyroid Association and the Royal College of Physicians in the United Kingdom for older patients, recognizing they have more

Table III. AMES risk-group classification and extent of thyroidectomy in patients undergoing surgery for well-differentiated thyroid cancer AMES category

Partial thyroidectomy

Total thyroidectomy

Total

Low-risk (%) High-risk (%) Total (%)

1431 (19.4) 146 (7.9) 1577 (17.1)

5938 (80.6) 1711 (92.1) 7649 (82.9)

7369 1857 9226

aggressive tumors, is more extensive thyroidectomy.20 There is a treatment paradox, as in the data from this SEER study, that older patients who usually have more aggressive tumors and are at higher risk of recurrence are less likely to undergo total thyroidectomy compared with younger, lowerrisk patients. Other factors probably affect the use of AMES criteria to select surgical therapy. In this study, nodal status was identified as a determinant of the extent of thyroidectomy. Most surgeons would perform near-total or total thyroidectomy for patients with gross nodal disease so that radioactive iodine can be used as a postoperative adjuvant. Unmeasured variables in SEER, such as surgeon or patient preference and the request by referring endocrinologists, are surely also responsible for influencing the likelihood of undergoing near-total or total thyroidectomy. Surgeons might be choosing near-total or total thyroidectomy because of the advantages of measuring thyroglobulin reliably, removing multicentric disease, and the ability to use radioactive iodine. In addition, retrospective studies suggest that near-total or total thyroidectomy, even for low-risk patients, is associated with a decreased recurrence rate8,21-24 and possibly an improved overall survival.8,21-23 Other retrospective studies have shown no survival benefit from more extensive thyroidectomy in low-risk patients.15,25,26 No randomized controlled trials comparing survival after partial and total thyroidectomy have been done and probably ever will be because of the large sample size necessary to show a small difference in survival.27 If total thyroidectomy is performed on low-risk group patients, it is imperative that the operation be done safely to minimize the risks of hypoparathyroidism and laryngeal nerve injury, complications that are less common with partial thyroidectomy. The majority of patients in this study (83%) underwent near-total or total thyroidectomy, and most were in the AMES low-risk group. In another large population-based study using the National Cancer Data Base between 1985 and 1995, 47.9% of

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Table IV. Univariate and multivariable analysis of association between AMES criteria, histology, or lymph node status and the likelihood of undergoing total thyroidectomy Univariate analysis Variable Age (y) # 39y 40-49 $ 50 Gender Maley Female Histology Papillaryy Follicular Extrathyroidal extension Noney Adjacent spread Distant metastasis Noy Yes Size < 5 cmy $ 5 cm Lymph node metastasis Noney Regional Distant or unknown AMES group Low-risky High-risk

Multivariable analysis

OR*

95% CI

P value

OR*

95% CI

P value

1.0 0.75 0.66

— (0.65-0.86) (0.58-0.75)

— < .0001 < .0001

1.0 0.81 0.67

— (0.70-0.94) (0.59-0.77)

— .005 < .0001

1.0 0.80

— (0.70-0.92)

— .0016

1.0 0.90

— (0.78-1.04)

— .15

1.0 0.65

— (0.55-0.78)

— < .0001

1.0 0.85

— (0.71-1.02)

— .08

1.0 3.85

— (3.09-4.80)

— < .0001

1.0 3.53

— (2.16-5.75)

— < .0001

1.0 7.29

— (2.69-19.8)

— < .0001

1.0 7.72

— (2.61-22.9)

— .0002

1.0 1.27

— (1.01-1.59)

— .04

1.0 1.19

— (0.85-1.66)

— .22

1.0 6.98 0.63

— (5.45-8.93) (0.55-0.71)

— < .0001 < .0001

1.0 5.31 0.60

— (4.13-6.82) (0.53-0.68)

— < .0001 < .0001

1.0 2.82

— (2.36-3.38)

— < .0001

1.0 0.84

— (0.53-1.35)

— .48

*An OR of 1 indicates no difference compared to the referent group; an OR greater than 1 indicates a higher likelihood of undergoing a total thyroidectomy compared to the referent group, and an OR less than 1 indicates a higher likelihood of undergoing a partial thyroidectomy compared to the referent group. yReferent group.

patients with papillary thyroid cancer and 39.3% with follicular thyroid cancer underwent total thyroidectomy.26 A study linking the National Cancer Data Base with data from the American College of Surgeons Commission on Cancer Patient Care Evaluation Thyroid Cancer Study in 1996 for 5105 patients (the majority of whom were in a low-risk group) revealed that 61.9% of patients with papillary cancer and 53.6% of patients with follicular cancer had a total thyroidectomy, similar to results of our study.28 It appears that most patients in the United States undergo near-total or total thyroidectomy regardless of risk of recurrence, rather than using total thyroidectomy selectively in high-risk patients. The preponderance of total thyroidectomy for well-differentiated thyroid cancer in the United States seems to show that surgeons prefer the advantages of more extensive thyroidectomy. What do our findings say about how surgeons incorporate medical evidence into their surgical

practice? One interpretation is that surgeons do not make extensive use of prognostic information when choosing the extent of thyroid surgery in patients with well-differentiated thyroid cancer. Although the available evidence suggests that younger adult patients are less likely than older patients to experience distant metastases or death after surgery for thyroid cancer, surgeons appear to treat younger patients more aggressively. This interpretation appears to contradict previous research suggesting that surgeons are responsive to published studies when choosing the extent of surgery in the case of breast cancer.29 However, this interpretation of our findings might be misleading, because the evidence pertaining to the extent of surgery for thyroid cancer is not as authoritative as that for breast cancer. There are no large, randomized trials comparing total and partial thyroidectomy. Furthermore, surgeons might be exercising other considerations when using total thyroidectomy for patients at low risk, such as the

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ability to treat with radioactive iodine and follow serum thyroglobulin levels and the influence of referring endocrinologists. Although risk-group assignment might be done for prognosis, many other factors are probably influencing the decision about extent of thyroidectomy. A few weaknesses are inherent with this study. The presence of distant metastases in SEER is gleaned from information available within 4 months of diagnosis; therefore it is not reported whether distant metastases were identified preoperatively or postoperatively.30 In clinical practice, distant metastases are often only identified postoperatively with radioactive iodine scanning performed after near-total or total thyroidectomy. Therefore, the association between distant metastases and near-total or total thyroidectomy observed in this study might be related to the use of radioactive iodine scanning performed after total thyroidectomy. Similarly, there is no way to distinguish lymph node metastases that presented clinically from those identified pathologically; or whether they were removed at the primary procedure or at a second operation within 4 months after the initial thyroid cancer diagnosis, because they were identified by using postoperative radioactive iodine scanning. The association between the use of total thyroidectomy and the presence of regional or distant metastases might have been overestimated if patients with metastases were identified with the use of radioactive iodine or presented clinically with involved regional nodes. In summary, surgeons in the United States usually perform near-total or total thyroidectomy for well-differentiated thyroid cancer, regardless of AMES risk grouping. Younger age is associated with higher likelihood of total thyroidectomy, in contrast to the AMES risk stratification. Distant metastases and extrathyroidal extension are associated with total thyroidectomy, but tumor size is not independently used when choosing between partial and total thyroidectomy. Metastatic spread to regional nodes, not a prognostic criterion in AMES, is associated with the use of total thyroidectomy. Some components of the AMES prognostic index are used by surgeons in the United States as a guide to determine the extent of thyroidectomy.

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