A case of low-risk PTMC eligible for AS but caused multiple lung metastases without being identified on imaging was reported.
Although distant metastasis can occur even in microcarcinoma, it is thought to be associated with high-risk PTMC with clinically evident lymph node metastasis or extrathyroidal extension [1, 2]. Since there have been no reports of lung metastases at the diagnosis of low-risk PTMC, the JAES recommends that chest CT is not mandatory to search for distant metastases at the diagnosis of low-risk PTMC and during AS [5,6,7,8]. There are also no reports of the appearance of new distant metastasis in low-risk PTMC during AS [3, 4, 9]. On the other hand, there are many reports of occult PTC. Most of them were first detected with neck lymph node metastasis . The occult PTC cases diagnosed after lung metastasis are extremely rare [11,12,13]. The previous papers have also reported cases of occult carcinoma with distant metastasis, including the case of pulmonary metastasis . However, the lung metastases were more than 1 cm in diameter in all the previous reports and were easy to recognize. On the other hand, our case had no gross-visualized lung metastases, just frosted glass shadows which could not be diagnosed as malignant. To the best of our knowledge, distant metastasis after surgery for low-risk PTMC is extremely rare, with only one case of metastasis to skeletal muscle . The present case may be the first reported case of multiple “occult” or “micro” lung metastases from low-risk PTMC that was eligible for AS.
If PTC had been detected first, it would have been difficult to diagnose the lung metastases on imaging, and this case would have likely been subjected to AS as cT1a N0 M0. By chance, metastases of PTC were found in the lung cancer lesion, which led to the diagnosis of high-risk PTC and allowed us to administer appropriate treatment, total thyroidectomy followed by radioactive iodine (RAI) treatment, in contrast to AS. Although the clinical practice guideline of the JAES and the Japan Thyroid Association task force defines cT1aN0M0 cases as low-risk PTMC, they do not mention a method to confirm M0 [1, 2].
The clinical practice guidelines of the JAES recommend postoperative RAI therapy for high-risk PTC, especially for lung metastases . RAI is effective when it accumulates in microscopic foci that are not visible on imaging [17,18,19,20,21]. In the present case, diffuse accumulation of I-131 in the lung field was expected before therapy, considering the detection of lung metastases. Unexpectedly, no accumulation was observed.
As a treatment strategy for this case, several options were considered before thyroid surgery. One option was to limit the procedure to a right lobectomy of the thyroid gland, considering that the lung metastases might not affect the prognosis, since the lesion was too small to be visualized, and the patient was elderly. As another option, it was thought that AS could have been performed, since it was a small cancer, and the lung metastasis was too small to visualize. In reality, however, the patient chose total thyroidectomy plus RAI treatment. Considering the patient’s anxiety due to the discovery of lung metastases, we believe that this choice was appropriate.