Background  

Papillary carcinoma with clinically apparent node metastasis but lacking a primary carcinoma lesion in the thyroid is designated as occult papillary carcinoma. In the era of routine ultrasonographic examination, occult papillary carcinoma is defined as papillary carcinoma with clinically apparent node metastasis but showing a primary lesion that is microscopic or overlooked by ultrasonography. In this study we investigated the prevalence and clinicopathologic features, including prognosis, of occult papillary carcinoma.

Methods  

This is a retrospective series study of all patients with occult papillary thyroid carcinoma who underwent initial surgery at a single institution over 14 years.

Results  

Between 1990 and 2004, 5400 patients underwent surgery for papillary thyroid carcinoma at Kuma Hospital, Japan. Seventeen (0.3%) were regarded as having occult papillary carcinoma and were enrolled in the study. Clinically apparent node metastasis was detected in the lateral compartment in 16 patients and in the mediastinal compartment in 1 patient. Multiple metastatic nodes were detected in 5 patients (29%). Primary lesions of papillary carcinoma were intraoperatively detected in 3 of 14 patients (21%) who underwent thyroidectomy, but there were no apparent carcinoma lesions in the thyroid in 5 patients (36%), even on pathologic examination. Six patients (35%) showed extranodal tumor extension to adjacent organs and two of these patients showed recurrence. None of the patients showed distant metastasis or died of carcinoma over the study period.

Conclusions  

Patients with occult papillary thyroid carcinoma were found to have a favorable overall prognosis. However, occult papillary carcinoma is automatically classified as N1b in the UICC classification, and in our series it is also likely to show other aggressive clinicopathologic features. In particular, extranodal tumor extension portends a worse prognosis for patients with occult papillary carcinoma. Therefore, careful total thyroidectomy with lymph node dissection is recommended except for elderly or high-risk patients.