Papillary thyroid carcinoma (PTC) is the most prevalent type of thyroid cancer, accounting for 80–85% of all thyroid malignancies globally. Recent data indicate that the incidence of thyroid cancer, particularly PTC, has been increasing worldwide, with a significant rise observed, where the age-adjusted incidence rate increased from 4.9 per 100,000 in 1975 to 14.5 per 100,000 in 2013. This rise is attributed largely to the increased detection of small, indolent tumors through advanced imaging techniques (1,2). PTC is characterized by a typically indolent clinical behavior, a good prognosis and high long-term survival rates, especially when diagnosed early (2,3). The most common clinical presentation is a solitary thyroid nodule. However, in rare cases, PTC can present with unusual symptoms, such as a painful neck mass (4,5). The spectrum of PTC presentations is not limited solely to these conventional patterns. In rare instances, PTC may deviate from the expected clinical course, posing diagnostic challenges to clinicians. One such atypical presentation reported in only a handful of cases is the development of a metastatic cystic lesion within the neck, representing an uncommon manifestation of PTC (6,7).

The primary objective of the present study is to present a unique case of a young female patient with a longstanding swelling on the left side of the neck, initially diagnosed as a cystic lesion, which was later revealed to be metastatic PTC. This report adheres to ethical publishing standards by avoiding the referencing of predatory journals (8).

The rarity of metastatic PTC presenting as a cystic lesion emphasizes the importance of a comprehensive diagnostic approach. In the present case, the initial misinterpretation of the cystic lesion as a likely hemangioma or branchial cleft cyst underscores the challenges in diagnosing unusual presentations of thyroid malignancies. The incorporation of FNAC and subsequent imaging studies led to the correct identification of metastatic disease. Initially, FNAC performed in 2019 did not detect malignancy. However, with the recurrence of symptoms and new imaging studies in 2022, a repeat FNAC was conducted, which, along with updated imaging findings, confirmed the presence of metastatic PTC. This highlights the importance of repeated diagnostic evaluations and imaging in cases where initial results may not fully capture the nature of the disease.

Thyroid carcinoma is a relatively rare malignancy in the United States, constituting <1% of all cancer cases. Within this category, PTC is the most prevalent subtype (10,11). PTC primarily affects individuals under the age of 40 years, with a notable predilection for females (2.3:1 ratio) (12). PTC typically manifests between the ages of 20 and 50 years, and is characterized by indolent clinical behavior (13). Cystic neck masses are a common clinical entity, often attributed to benign conditions, such as dermoid cysts, branchial cleft cysts, epidermoid cysts, teratomas and cystic hygromas. In the context of a para-median cystic lesion, additional differential diagnoses should include conditions such as second branchial cleft cysts and lymphangiomas, which can present as lateral neck masses. In recent years, the rise in oropharyngeal carcinoma incidence has raised the possibility that cystic neck masses may also represent metastases from tumors in the oropharyngeal or tonsillar region (1,11,13). Notably, PTC rarely presents as a cystic neck mass without any palpable lesions in the thyroid gland, making its diagnosis particularly challenging (14).

Sonographically, PTC typically appears as a hypoechoic solid lesion with a smooth or irregular contour, intrinsic vascularity and a lack of a hypoechoic halo. However, it is important to note that only a small fraction of PTC cases (5%) present as purely cystic lesions, with the majority exhibiting features of intrinsic hypervascularity (69%) and perinodal hypervascularity (20%) (15,16). The sonographic features of cystic PTC or metastatic cystic nodes include the presence of multiple or solitary cysts with internal septations, calcifications and internal nodules (13). In the current case, a large cystic lesion (8.5×5.5×2.6 cm) was observed on the left side of the neck, accompanied by an echogenic focus, internal vascularity and two papillary components, A CT scan of the neck showed a thin-walled cystic lesion lateral to the carotid vessels, measuring ~6.6×3.5 cm.

FNAC is a common diagnostic tool; however, it carries a false-negative rate of 45% in diagnosing cystic PTC. Sampling error is the primary culprit for this high level of false-negatives, rather than cytological misdiagnosis. The accuracy of FNAC can be enhanced by obtaining material from the wall and solid components of the cyst under ultrasound guidance (17). In the present case, FNAC was performed on both the thyroid lobe and the neck mass, both yielding evidence of PTC. It is important to note that the initial FNAC conducted in 2019 did not identify malignancy, and the diagnosis of PTC was only confirmed upon repeat FNAC during the subsequent evaluation in 2022. The role of thyroglobulin analysis in aspirated fluid should not be underestimated. Studies have shown that elevated thyroglobulin levels in aspirated fluid can be indicative of cystic PTC. Furthermore, intranodal concentrations of thyroglobulin may correlate with high attenuation on CT and inhomogeneous intracystic signal changes on magnetic resonance imaging (18,19). For patients with a negative FNAC, excisional biopsy is a crucial consideration to detect PTC. Suspicious clinical or radiological features of malignancy warrant frozen section analysis, which can guide decisions on total thyroidectomy with modified radical neck dissection (13).

Following confirmation of cystic PTC, the definitive treatment involves the removal of the cyst along with all thyroid tissue and accessible lymph nodes to improve the prognosis. Postoperative radioiodine ablation of the thyroid remnant, coupled with suppressive thyroxine dosing, helps reduce the risk of recurrences (13,20). The long-term prognosis for patients with metastatic cystic lymph nodes from PTC is generally favorable, especially when the disease is localized and amenable to surgical resection and radioactive iodine therapy. However, careful and vigilant follow-up is crucial to detect and manage recurrences promptly. PTC generally exhibits a favorable prognosis, boasting a survival rate of >90% at the 20-year mark. Nonetheless, when extra-thyroidal extension occurs, survival rates notably decline to 54% at 15 years and further to 29% at 30 years (13).

The present case report underscores the clinical significance of recognizing metastatic PTC presenting as a cystic neck lesion, highlighting the importance of a comprehensive diagnostic approach. The rarity of this manifestation adds a distinctive dimension to the understanding of thyroid malignancies. The successful treatment outcome, involving a total thyroidectomy with a left lateral neck dissection followed by radioactive iodine treatment, reinforces the efficacy of the chosen therapeutic strategy. This case underscores the importance of considering metastatic disease in the differential diagnosis of cystic neck lesions in patients with a history of PTC.

One limitation of the present study was the decision not to pursue scintigraphic analysis for the suspected cystic lesion, which was based on established clinical protocols in the Smart Health Tower. Another limitation in this case is that specific images cannot be provided from the scans and ultrasound due to data storage and access limitations within the institutional database. However, the findings and diagnostic outcomes have been detailed comprehensively. In conclusion, the rarity of metastatic PTC presenting as a cystic lesion emphasizes the importance of a comprehensive diagnostic approach.