Characteristics of patients
The main characteristics of the study patients are listed in Table 1. The study patients consisted of 28 females (52.8%) and 25 males (47.2%) with a median age of 34 years (range: 7-69 years). Total or near-total thyroidectomy was performed in all patients. PTC was present in all patients. The median tumor size was 2.3 cm (range, 0.6-10.0 cm). The most frequently observed T classification was T4a (35.8%). The median number of positive LNs was 11, and the median LN ratio was 0.57. 46 patients (86.8%) had LNM in the central and lateral compartments (N1a and N1b), the median number with LNM of N1a and N1b compartment was 7.0 (range 0-31) and 7.0 (range 1-18) respectively. Distant metastases confined to the lungs were identified in 12 patients (22.6%). The risk of recurrence was intermediate and high in 45.3% and 54.7% of the patients, respectively. All patients underwent 131I therapy with a median activity administered at 5.5 GBq (range, 3.7-7.4 GBq).
Number and region of metastatic lymph nodes
In the 53 patients who underwent the 131I SPECT/CT scans, a total of 95 neck iodine-avid LNMs were ascertained. After a secondary LN dissection, the number of LNMs was 212, which was significantly higher than that detected on the 131I SPECT/CT scans (Z=-2.714, p=0.004). Twenty-six LNMs detected by 131I SPECT/CT in 17 patients were totally consistent with those found by reoperation; 29 patients were found to have more LNMs by reoperation, increasing from 55 to 180; eight false positive LNs were confirmed in 14 iodine-avid LNs of 7 patients; among these 7 patients, 2 patients had no metastasis. Therefore, a total of 51/53 (96.2%) patients had suspected cervical LNMs confirmed by postoperative pathology (Fig. 1). One false-positive patient’s sTg level was 0.3 ng/mL (TgAb <10 IU/mL), the other ones’ was 124.3 ng/mL because combination with lung metastases. The FNA cytologic findings of both were indeterminate which means hard to distinguish benign or malignant. The sensitivity and specificity of 131I SPECT/CT lesion-based were 44.8% (95/212) and 91.6% (87/95), respectively.
The location of the total iodine-avid nodal metastases detected on the 131I SPECT/CT scans and confirmed by postoperative pathology were classified based on the cervical lymph node levels as shown in Table 2. Based on 95 LNMs found by 131I SPECT/CT, 46 (48.42%) LNMs were found in level Ⅳ, making it the level with the highest frequency of nodal metastases. The next most frequent regions of nodal metastases were found in the following: level Ⅲ, 16 (16.84%); Ⅵ, 12 (12.63%); Ⅶ (superior mediastinal lymph nodes), 12 (12.63%); and Ⅱ, 7 (7.37%). Additionally, 1(1.05%) was detected in level Ⅴ, and 0 (0%) were found in level Ⅰ. Furthermore, 1 (1.05%) lymph node was found in the axilla (Fig. 2).
Reoperation found 212 LNMs, cervical level Ⅳ had the greatest frequency of LN metastasis with 69 (32.55%), followed by cervical levels Ⅲ, 48 (22.64%), and Ⅵ, 41 (19.34%), which is the same ranking as that of the 131I SPECT/CT scans. Of note, 17 (8.02%) LNMs were located at level Ⅶ, a lower frequency of nodal metastasis than at level Ⅱ, 31 (14.62%). There were 4 (1.89%) LNMs found in level Ⅴ and 1 (0.47%) in level Ⅰ. Similar to 131I SPECT/CT, one (0.47%) lymph node was found in the axilla. There was no significant difference in the distribution of lymph node metastases between the two groups (χ2=12.58, p=0.057).
Efficacy of 131I therapy and reoperation on the treatment of LNMs
All patients were still in SIR after the initial 131I therapy, excluding the two patients with indeterminate FNA. Thirty-two patients received repeated 131I therapy after reoperation, among these, there were 6 patients with lung metastases (Fig. 3). A comparison of the treatment response to 131I therapy with that of reoperation was performed in the 16 patients with TgAb<40 IU/L and without distant metastasis. Four patients (23.5%) were classified as SIR (one patient’s sTg level was less than 0.04 ng/mL, Fig. 4); 3 patients (17.6%) had achieved ER; 4 patients (23.5%) became BIR, the remaining 5 patients (29.4%) were in IR. There was a statistically significant difference in the therapeutic effect between two groups (χ2=33.019, p=0.000).