Value of Post-therapy Whole Body Scintigraphy in Predicting the Need for Subsequent Radioactive Iodine Therapy in Patients with Well-differentiated Thyroid Carcinoma

Abstract

Introduction. Patients with well-differentiated thyroid carcinoma (WDTC) may require more than one session of radioactive iodine therapy (RAIT). This study was conducted to determine if posttherapy whole body scintigraphy (PTWBS) can identify patients who will require repeat RAIT due to persistent disease.

Method. The records of patients with WDTC who were referred to the Philippine General Hospital for RAIT from 2005-2007 were reviewed in this retrospective cohort study. PTWBS results (number of remnants, area of remnants, and presence of metastasis) of patients who had repeat RAIT (based on laboratory and clinical evidence of persistent disease) were compared with those of patients who did not have repeat RAIT. Multiple logistic regression analysis was done.

Results. Forty-five of 99 patients in the study [45%, 95% Confidence Interval (CI): 35 – 55] had repeat RAIT. Thirty-six of 38 patients (95%) with metastases on PTWBS required repeat RAIT, while only 9 of 61 patients (15%) with no scan evidence of metastases required subsequent therapy. Controlling for age
group (age?57), the odds ratio was 102 (95% CI: 20 – 507). Using the model to predict repeat RAIT, the sensitivity was 80 (95% CI: 68-92); specificity was 94 (95% CI: 85 – 99). Positive and negative predictive values were 95 and 85% respectively. The same results were obtained using metastases alone as a predictor. Twenty-six patients had extra-cervical metastases by PTWBS and all required repeat RAIT. The number and aggregate size of thyroid remnants by PTWBS, however, were not found to be predictive of the need for repeat therapy.

Conclusion. The presence of functioning metastases seen on PTWBS was highly predictive of the need for repeat RAIT.