e21528 Background: Thyroid cancer is a common secondary malignancy among childhood cancer survivors who have received radiation to the head, neck and/or upper thorax. The optimal strategy for surveillance for thyroid carcinoma in childhood cancer survivors remains controversial. Current Children’s Oncology Group recommendations are limited to physical exam. The objective of this study was to determine the sensitivity and specificity of thyroid ultrasound versus neck exam by a pediatric endocrinologist in the diagnoses of thyroid cancer in a cohort of high-risk childhood cancer survivors. Methods: Medical records of childhood cancer survivors who received radiotherapy to the head, neck and/or upper thorax were reviewed. These patients were seen in a comprehensive childhood cancer survivorship clinic from 01/01/2010 to 12/31/2017. Patient populations included oncology, bone marrow transplant and brain tumor patients. Results: 226 patients received radiation to the head, neck and/or upper thorax. Of those, 129 patients were male (57%). Sixteen (7.1%) of patients developed a secondary thyroid malignancy including 4 patients previously treated for an oncological malignancy, 9 patients treated with bone marrow transplantation, and 3 patients with a CNS malignancy. Median radiation dose was 1800 cGy (range 400-5940 cGy). Time to thyroid carcinoma diagnosis occurred at a median of 12 years (range 4-19 years) from treatment with radiation. Screening ultrasounds were obtained in 146 (65%) patients while 226 (100%) had a physical exam. Two cases were identified by abnormalities on physical exam. The sensitivity of US was 100% (CI 80.6-100) compared to a sensitivity of 12.5% (CI 3.5-36) using physical exam (P < 0.0001). Screening ultrasound had a specificity of 73% (CI 65.1-80.1) while physical exam yielded a specificity of 100% (CI 98.2-100). Conclusions: Regular screening with ultrasounds provide the greatest sensitivity for detection of secondary thyroid carcinomas after head, neck and upper thorax radiation in childhood cancer survivors. If screening ultrasounds were not routinely utilized in our clinic, 14 of the 16 patients (87.5%) would have had a delay in their diagnosis of a secondary thyroid malignancy. Screening ultrasounds may lead to earlier detection of thyroid carcinomas, with the potential to decrease the need for aggressive surgery, radioiodine therapy and, ultimately, to decrease recurrence risk.