The retrospective analyzis of the patients with thyroid cancer followed in our department and the importance of I-131 Whole Body Scanning and F-18 FDG PET/CT in clinical follow-up

Totally 217 patients diagnosed as differentiated thyroid cancer in between 2006-2009 were included into the study and the patients were investigated retrospectively. After surgery as total thyroidectomy, all patients were referred to our department for the purposes of ablation and therapy for metastasis.Aim: The treatment succes and the factors affecting treatment and effectivity of I-131 whole body scanning and F-18 FDG PET/CT were assessed.Materials and Methods: Radioactive Iodine (RAI) treatment was applied to the patients whose remnant thyroid gland had been diminished in size suffiently. Empiric costant dose application was carried out. All patients were prepared for I-131 treatment by withdrawal of laevothyroxin (T4) for 4 weeks. Patients were instructed to follow a low-iodine diet for ten days before the treatment. All patients provided written informed consent approved by instituional review board. After that, these patients were followed systematically. Follow-up included; I-131 whole body scanning on 7th day; serum TSH, Tg, antiTg measurements and neck USG on 3-6th months. After that in accordance with the patients risk condition and serum Tg level, in 6 month or one year period I-131 whole body scanning was performed. Some patients who had high serum Tg levels and a group of patient with negative I-131 and high Tg levels were searched with F-18 FDG PET/CT. An absence of pathologic uptake in I-131 whole body scanning and an unremarkable Tg level of <2 ng/ml by stimulated TSH were the criteria for successful treatment. Relationship between histopathologic classification of thyroid tumor and their response to treatment and their second dose taking state were also investigated.Results and Conclusions: In our group the factors affecting the treatment were determined as tumor size, number of focus, invasion state and stage. The success rate of I-131 treatment with one dose was 70 %, with two dose was 5 %. 40 % of papillary carcinoma was microcarcinoma. 34 % of these were multifocal located. Tretment success were highly great for both single focused and multifocused location. During the follow up, postablative diagnostic I-131 WBS sensitivity and specificity were 59 % and 99 % respectively. In the postablative period, while persentage of positive diagnostic I-131 whole body scanning imaging was 40 %, rate of positive I-131 whole body scanning imaging on the 7th day after treatment increased to 88 %. It was determined that most of these cases were the patients with diffuse hepatic uptake. When the F-18 FDG PET/CT was performed to the patients with diffuse hepatic uptake by post-therapeutic I-131 whole body scanning, the method could reveal metastatic focus in 30 % of patients.It was observed that the sensitivity of high dose I-131 whole body scanning was not statistically different from that of low dose whole body scanning. The method with both doses revealed increased sensitivity with the increase in serum Tg level. However, when diffuse hepatic uptake of I-131 was considered as a finding, the sensitivity difference between different doses of I-131 whole body scanning become significant. There was no statistical difference between intensity of diffuse hepatic uptake and response to the treatment. With on other perspective based on patients individually, while 67 % of these patients showed stable findings and 22 % showed regression, progression was observed in only 11 % of them. These findings encourage the continuety of radioiodine treatment unless any contraindication exists.When two methods were compared with the same patient population, the sensitivity and specificity values of F-18 FDG PET/CT and high dose I-131 whole body scan were 58 % -71 % and 62 % - 100 %, respectively. Among the patients with serum Tg levels under 10 ng/ml, F-18 FDG PET/CT couldn?t reveal any correct focus. For the patients with serum Tg levels between 10-100 ng/ml, both methods were capable of showing pathologic sites correctly which had not been showed by the other method in two patients. Among the patients with serum Tg level >100 ng/ml rest of the revealed positive both methods showed concordant positivity with a ratio of 71 %. For the other part of these patients, both methods played a complementary role in a ratio of 50 % in detecting the pathologic sites. According to our findings, any of the two methods did not show any superiority. In the differentiated thyroid cancer patients with a significant high serum Tg level, F-18 FDG PET/CT could play a role in detecting the pathologic site as the same sensitivity as I-131 whole body scan. And it also can be used as a complementary imaging method.