Three Distinctly Different Kinds of Papillary Thyroid Microcarcinoma should be Recognized: Our Treatment Strategies and Outcomes
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Papillary microcarcinoma (PMC) of the thyroid generally follows a benign clinical course. However, treatment strategies remain controversial. According to our previous retrospective review of 178 patients with PMC who underwent surgery between 1976 and 1993, the most significant risk factors affecting cancer-specific survival were clinical symptoms at presentation due to invasion or metastasis. Distant metastasis and cancer-specific death were never seen postoperatively for 148 cases (83%) of asymptomatic PMC without clinically apparent (≥1 cm) lymph node metastasis or recurrent nerve palsy. Based on these results, we identified three biologically different types of PMC that should be treated differently. Type I comprises incidentally detected PMC without any symptoms, which is harmless and the lowest-risk cancer. Conservative follow-up with ultrasonography every 6 or 12 months is feasible. Type II involves the early stage of the usual low-risk papillary carcinoma. This can be treated by lobectomy when increasing size is noted during conservative follow-up. Type III comprises clinically symptomatic PMC, representing a high-risk cancer. Immediate wider resection followed by radioiodine treatment and suppression of thyroid-stimulating hormone is recommended.
Since 1995, we have been conducting a prospective clinical trial of nonsurgical observation for asymptomatic PMC. As of 2008, 230 of 244 candidates (94%) have decided to accept this policy, whereas 56 patients underwent surgery for symptomatic PMC between 1976 and 2006.
Nonsurgical observation for a mean of 5 (range, 1–17) years for 300 lesions of asymptomatic PMC revealed that 22 (7%) had increased in size, 269 (90%) were unchanged, and 9 (3%) had decreased. No patients developed extrathyroidal invasion or distant metastasis. Three patients (1%) who developed apparent lymph node metastasis and nine patients (4%) in whom tumor increased in size eventually received surgery after 1–12 years of follow-up. No recurrences have been identified postoperatively. Conversely, 10-year cause-specific survival for symptomatic PMC was 80%. Multivariate analysis identified extrathyroidal invasion, large lymph node metastasis (≥2 cm), and poorly differentiated component as significantly related to adverse outcomes.
Nonsurgical observation seems to represent an attractive alternative to surgery for asymptomatic PMC. Almost 95% of asymptomatic PMC patients are type I, and another 5% are type II and can be treated with conservative surgery. A small number of PMCs with bulky lymph node metastasis or extrathyroidal invasion are high-risk type III and require aggressive treatment.