Discussion

Several studies have reported that clinically unsuspected differentiated thyroid cancer may occur in 1–10 per cent of the population.Reference Nishiyama, Ludwig, Thompson, De Groot, Frohman, Kaplan and Refetoff ^{1 –} Reference Autelitano, Spagnoli, Santeusanio, Villaschi and Autelitano ⁴ This is consistent with both the estimates of differentiated thyroid carcinoma occurrence in the general population,Reference Bisi, Fernandes, Asato de Camargo, Koch, Abdo and de Brito ³ and the most recent evidence on incidental thyroid cancer, as previously described by Miccoli et al.Reference Miccoli, Minuto, Galleri, D'Agostino, Basolo and Antonangeli ¹⁵ Nodal metastasis from differentiated thyroid cancer can also be incidentally found in specimens from neck dissections performed for other head and neck malignancies. The incidence of this finding is lower than the reported incidence of occult thyroid carcinoma. For example, in 2003, Ansari-Lari and Westra reported that 0.7 per cent of patients who underwent neck dissections for pathological staging and treatment of a primary head and neck malignancy showed unsuspected nodal metastases from a papillary thyroid carcinoma.Reference Ansari-Lari and Westra ¹⁴ This finding might reflect the rate of incidental metastases from papillary thyroid carcinoma in the general population.

Owing to the indolent nature of most papillary thyroid carcinomas, there is no consensus on the management of patients with incident thyroid cancer (found after thyroidectomy for another reason)Reference Minuto, Miccoli, Viola, Ugolini, Giannini and Torregrossa ¹⁶ or those with thyroid cancer metastases in the lymph nodes discovered after extensive neck dissection performed for another (generally, more aggressive) head and neck neoplasm.Reference Vassilopoulou-Sellin and Weber ^{7 ,} Reference Fliegelman, Genden, Brandwein, Mechanic and Urken ^{13 ,} Reference De Jong, Demeter, Jarosz, Lawrence and Paloyan ^{17 ,} Reference Clark, Hickey, Butler, Ibanez and Ballantyne ¹⁸

Previously, there was considerable controversy about whether these lesions truly represent nodal metastases from differentiated thyroid carcinoma or are merely benign inclusions.Reference Block, Wylie, Patton and Miller ¹¹ In 1964, Gerard-Marchant examined 16 649 cervical lymph nodes after radical neck dissections in 647 patients with head and neck SSC and identified 4 patients with benign thyroid follicle inclusions.Reference Gerard-Marchant ¹⁹ No further investigation or surgical management was performed, and all patients were free of disease at follow up. Rosai et al. suggested several criteria that might be useful for discriminating metastatic foci from these benign thyroid inclusions.Reference Rosai, Carcangiu, DeLellis, Rosai, Carcangiu and DeLellis ²⁰ Intranodal thyroid tissue might be regarded as a possible benign inclusion when it: (1) is limited to a few small follicles in a single lymph node; (2) is located in or immediately beneath the nodal capsule; or (3) does not demonstrate any cytoarchitectural features of papillary thyroid carcinoma.

Lateral aberrant thyroid tissue is currently believed to represent metastatic cervical lymphadenopathy from a primary thyroid carcinoma.Reference Nussbaum and Bukachevsky ^{8 ,} Reference Butler, Tulinius, Ibanez, Ballantyne and Clark ^{9 ,} Reference De Jong, Demeter, Jarosz, Lawrence and Paloyan ^{17 ,} Reference Park and Min ²¹ In 1944, Clay and Blackman contended that lateral aberrant thyroid tissue represents cervical lymph node metastases from a primary thyroid carcinoma despite the presence of a normal thyroid gland.Reference Clay and Blackman ²² This was later confirmed in 1966 by Clark and colleagues.Reference Clark, Hickey, Butler, Ibanez and Ballantyne ¹⁸ When treating the thyroid tumour, several studies suggest that the ultimate outcome for patients with coexisting head and neck carcinomas is determined by the biology of the more aggressive primary tumour and not by that of the less aggressive thyroid carcinoma.Reference Vassilopoulou-Sellin and Weber ^{7 ,} Reference Fliegelman, Genden, Brandwein, Mechanic and Urken ^{13 ,} Reference Pacheco-Ojeda, Micheau, Luboinski, Richard, Travagli and Schwaab ^{23 –} Reference Yamamoto, Tatemoto, Hibi, Ohno and Osaki ²⁵

In the present series, 10 patients (3.3 per cent) had nodal metastases from occult papillary thyroid cancer, with bilateral neck involvement in 1. Therefore, 11 out of 457 neck dissections (2.4 per cent) were positive for metastases from papillary thyroid cancer. These values are higher than those reported for other series.Reference Ansari-Lari and Westra ^{14 ,} Reference Leòn, Sancho, Garcìa, Sañudo, Orús and Quer ²⁴ All patients in the present study underwent total thyroidectomy, and a primary thyroid tumour was found in eight patients. No patient developed a thyroid tumour after surgery. These results confirm that, despite the metastatic spread, the thyroid cancer does not affect the overall prognosis of patients who are being treated for a more aggressive malignancy. However, in otherwise healthy patients, it is worth treating this second malignancy to avoid potential future complications related to local disease or metastatic thyroid cancer. Total thyroidectomy and dissection of the central lymph node compartment followed by ¹³¹iodine therapy is a safe, effective treatment.

In the present series, the primary thyroid tumour could not be identified in two patients. Potential explanations were previously proposed by Fliegelman et al. as (1) possible malignant transformation of the lateral aberrant thyroid tissue; (2) failure to detect an occult primary tumour; or (3) spontaneous regression of a primary malignancy.Reference Fliegelman, Genden, Brandwein, Mechanic and Urken ¹³

Potential drawbacks of our study are its retrospective nature and out-of-date case series. Indeed, management protocols for thyroid cancer have evolved over the last few years and are now more conservative. Moreover, the length of follow up was adequate for head and neck SCCs, but not for papillary thyroid carcinomas.