DISCUSSION
The 10-year survival rate of PTC patients is usually over ~80% to 95%; however, if it is accompanied by distant metastasis, the 10-year survival rate drops to ~24% [
4]. However, the overall prognosis is relatively good; patients or their families tend to have high expectations for cure of PTC. Such expectations may make it difficult to understand that the prognosis can worsen over long-term follow-up, due to recurrence or metastases to other organs. In addition, in terms of treatment, PTC progresses slowly and its malignant degeneration is lower than other malignant tumors that develop in other organs, so that treatment is provided on the basis of surgical removal of PTC, high-dose RAI therapy and internal medicine therapy to maintain a low thyroid stimulating hormone level.
Recently, as the detection rate of thyroid nodules is on the rise, diagnosis of PTC is also increasing in a substantially younger age group. Young patients in particular have a longer life expectancy, so that the need for long-term follow-up becomes more prominent, requiring more accuracy and detailed precision in examinations for the recurrence and aggravation of cancer through follow-up.
To this end, the authors reported four cases whose PTC had poor prognosis. All of these patients had been diagnosed with PTC and received total thyroidectomy, but had shown findings of recurrence and metastases after substantial time had passed. These included patients who had recurrence of PTC in structures adjacent to the thyroid, concurrent development of other organ cancers, anaplastic transformation or dedifferentiation, and disease refractory to typically curative treatments. Therefore, clinicians should pay careful attention when performing follow-up in patients with PTC even if they have shown good prognosis.
The first case had findings of tumor thrombus in superior vena cava, compression of the trachea, superior vena cava syndrome, and metastasis of PTC to the brain that are considered to be the end stage findings of cancer metastases. These findings show serious and various substantial thyroid metastases, and as the development and progression of PTC itself would have been significantly slower than other types of cancer. When the follow-up examination was performed at adequate time, the complications of thyroid cancer metastases would be minimized than this case. Wada et al. [
5] reported two very rare cases that exhibited PTC, wide-ranging tumor thrombi in the superior vena cava, and metastases to the adjacent region of the superior vena cava and to the mediastinal lymph node. Both patients underwent wide resection of the superior vena cava and reconstructive procedures sufficiently early. Brain metastasis of PTC is very rare, and its prognosis is quite poor, but several cases with relatively good prognoses have also been reported, as a result of early diagnosis and adequate treatment [
6,
7]. This case emphasize the importance of early detection in the treatment of recurrent PTC and its metastasis through periodic sonographic follow-up examination of structures typically adjacent to the thyroid, despite total thyroidectomy. The second case reported reconfirmed that thyroid carcinoma can metastasize to the lung and can be accompanied by primary lung cancer in 15% to 47% of PTC patients. Roscoe et al. [
8] reported a case of PTC which metastasized to the right upper lobe of the lung in a patient who was diagnosed with cicatrical primary adenocarcinoma and presented with moderate differentiation on postpneumonectomy biopsy, with findings of metastatic PTC cells present in mixed multiplicity. Therefore, when performing long-term follow-up of patients with PTC, clinicians should be aware that metastasis to other organs is possible in addition to recurrence and metastasis of PTC, and lesions in other organs should not be overlooked no matter how small they are. In addition, although metastasis of cancer to cancer is quite rare, but in case of PTC, cautions should be made for the fact that metastasis to the primary lung cancer can be happened. The third case reported in the present study was a patient who was diagnosed with metastasis of PTC to the pleura accompanied by pleural fluid, who had died within 3 weeks of diagnosis. In this case, the prognosis of the patient did not become poorer due to the progression of PTC, which has relatively slow progression and low aggressiveness of malignancy. Therefore, the authors hypothesized that the patient in this case was likely to have experienced dedifferentiation of PTC into an aggressive tumor with rapid progression, or to be transformed into malignancy of other organ rather than the issue of PTC itself only. Ogawa et al. [
9] observed degradation of PTC into highly malignant cancer cells. In this case, the patient was diagnosed with PTC and metastasis to the lung and had been treated by lymph node dissection and total thyroidectomy and had completed RAI therapy. Nevertheless, the patient died suddenly due to seeding of cancer cells. The fourth case demonstrates that it is possible for this disease to be refractory to treatment when there are findings of recurrence and metastasis of PTC at the pleura with accompanying pleural fluid. If the disease is refractory to treatment, the prognosis, of course, is not good. Therefore, in cases of showing signs of recurrence and metastasis of PTC, but demonstrating little or no response to adequate treatment, particularly in patients exhibiting symptoms of pleura metastasis accompanied by pleural fluid, preemptive and immediate treatment will be required, even if the current condition and prognosis of the patient appear to be good.
As the cases presented here, careful long-term follow-up should be undertaken even though thyroid cancer patients have a good prognosis. Furthermore, for patients who respond poorly to current therapeutic tools, more developed management modalities should be investigated.
In summary, PTC is known to have a good prognosis if treated properly at an early stage, however, we should keep in mind the possibility of facing the unexpected aggravated patients. To this end, the authors aimed to inform clinicians by presenting several rare cases representative of those likely to have poor prognoses during long-term follow-up in patients with PTC.