An Approach to a Rare Case of Collision Tumor of Follicular and Papillary Carcinomas of the Thyroid

DISCUSSION

Thyroid malignancies constitute the most common type of endocrine cancer. Of all the thyroid malignancies, papillary carcinoma was encountered to be the commonest and accounted for 89.8% of all thyroid malignancies.⁷ The main histologic types of thyroid carcinoma are (1) differentiated thyroid carcinoma (including papillary, follicular, and Hurthle cell carcinoma); (2) medullary thyroid carcinoma; and (3) anaplastic thyroid carcinoma.⁸ Solitary tumors of the thyroid are common, but it is rare for the gland to harbor two distinct types of malignancies at the same time.

Multiple challenges may be encountered while dealing with a follicular neoplasm of undetermined significance. FNAC cannot distinguish between a follicular adenoma, follicular carcinoma, and follicular variant of papillary carcinoma.⁹ Further, follicular neoplasm cannot be defined accurately on frozen section assessment either; hence, the role of frozen section in the diagnosis of follicular carcinoma intraoperatively is minimal.¹⁰ Only a paraffin section of the excised specimen showing vascular and/or capsular invasion can confirm the diagnosis of a follicular carcinoma.¹¹

Hence in our case, even at the time of completion thyroidectomy, frozen section assessment was not done considering it was a follicular carcinoma on the right side. Also, considering the size of the tumor which was more than four centimeters, a clearance of the central compartment of the neck lymph nodes was performed.¹² However in spite of a completely normal and non-suspicious appearance at the time of surgery, the left thyroid lobe was reported as having a papillary carcinoma of thyroid with thyroiditis. No metastasis was reported in the central compartment nodes.

Although both follicular and papillary carcinomas carry a good prognosis, they have the potential to spread hematogenously and by lymphatic means, respectively.¹³ This scenario increases the potential risk of lymphovascular metastasis, and hence, the surgeon should bear in mind the possibility of the same.

Whole body iodine scan with I-131 done 2 months after the completion thyroidectomy showed traces of residual thyroid tissue in the neck and no distant metastasis, thus requiring the need for radioactive I-131 ablation. Repeat whole body iodine scans are advisable in all cases annually for a period of at least three years post-surgery.

To summarize, collision tumors of the thyroid are rare entities as such, while the coexistence of papillary and follicular carcinomas together is even rarer. One or even both components of the tumor may be silent and slow-growing, and the patient may be completely asymptomatic for a significant amount of time in spite of harboring two histologically distinct primary tumors in the thyroid. The site of the two tumors may be in the same lobe or two separate lobes as was seen in this case.

Usually, one of the two tumors will increase in size earlier than the other and produce symptoms, which may or may not indicate a malignancy. Thorough clinical examination and a complete array of investigations may not be sufficient to establish a diagnosis, especially in a case of follicular neoplasm. However, they are extremely useful in diagnosing a papillary or medullary carcinoma or any other benign thyroid lesion and hence must be performed in every case of a palpable thyroid swelling.

Treatment options will vary depending upon the diagnosis of the primary tumor. Frozen section is only useful in case the primary tumor is a papillary carcinoma and may provide the road map for a completion thyroidectomy and/or lymphatic clearance at the same setting. However, it may not provide the adequate details as regards the diagnosis of a follicular carcinoma. Only paraffin sections can diagnose the latter and only then can further treatment be planned.

Size of the tumor, risk factors, and the histological type of the primary malignancy will decide the further management of these patients at this stage.¹² All management options including a completion thyroidectomy, central and lateral lymphatic clearance, I-131 ablation therapy, and radiotherapy must be kept in mind, and the surgeon must follow the established protocol according to the type of the primary tumor.

Histological type of the second tumor may further complicate things, and it is important to state here that the surgeon may be completely unaware of a second malignancy at this stage, especially if it presents with no symptoms and is reported as a normal-looking thyroid gland on investigations. This headache to the primary surgeon may be prevented if the second malignancy gets presented in the same thyroid lobe as the first, which was unfortunately not the case in our patient. So, diagnosis of a second primary malignancy of the thyroid can occur as a surprise to the surgeon but more importantly a big shock to the patient, who may already be under a considerable amount of stress due to multiple investigations, surgery, and knowledge of primary malignancy.

It is therefore extremely important to counsel the patient thoroughly before performing any kind of secondary procedure, once the diagnosis of primary malignancy is made. This counseling should include a complete explanation of the chances of a second primary tumor, however meager they may be. Once the diagnosis of both the tumors is established, a whole body iodine scan is mandatory and further management will depend on the histological types of both the tumors and nuclear medicine reports. Here again, the patient should be counseled well as diagnosis of multiple malignancies can be confusing as well as demoralizing, while the head and neck surgery team should always keep their colleagues in endocrinology, nuclear medicine, and radiotherapy in the loop and seek their help and advice whenever necessary.

Finally, good follow-up and repeated scanning are essential parts in successful management of collision tumors of the thyroid, just like any other primary thyroid malignancy.