TSH Assisted I-131 SPECT and FDG PET-CT and Dosimetry in Metastatic Thyroid Cancer

1280 Objectives Illustration of a case of metastatic thyroid cancer with improved uptake and clearance of I-131 from synthetic TSH stimulation versus hormone withdrawal. Methods 68-year old male presenting with hoarseness found to have bilateral thyroid nodules and bilateral cervical lymphadenopathy. The patient was found to have Stage IVA (pT3 pN1b Mx) papillary thyroid cancer status post total thyroidectomy, bilateral neck dissection, and mediastinal lymph node dissection. The patient underwent an I-131 neck and chest scan with dosimetry by serial body and blood sample counting. I-131 scan demonstrated metastatic disease to the mediastinum and lung with thyroid withdrawal and synthetic TSH injections but the clearance of I-131 from the body and blood or marrow was markedly enhanced by synthetic TSG. A subsequent 18F- FDG PET/CT showed additional and concurrent FDG avid lymph nodes and pulmonary nodule. Due to the overwhelming more FDG avid but I-131 non-avid tissue, a second mediastinal dissection was done for metastatic disease to a left paratracheal lymph node that was 18F-FDG and/or I-131avid. Laboratory work include renal function, LS-MS/MS assay for thyroglobulin (Tg) due to anti-antibodies and a genetic test for BRAF mutation. Results The synthetic TSH stimulated I-131 neck and chest scan and 18F-FDG PET/CT showed significant improvement in positive uptake and the clearance of the I-131 was better in dosimetry study especially in view of mild renal impairment in this BRAF/V600E mutation positive metastatic case. Conclusions The synthetic TSH assisted I-131 SPECT-CT and 18F-FDG PET-CT will enable us to identify the different clones of thyroid metastasis and help the clinical decision for salvage surgical resection before the maximal I-131 was given per dosimetry calculations of bone marrow tolerance. With synthetic TSH injections, these patients can avoid the lengthy discomfort of thyroid hormone withdrawal yet the patient would be maintained at maximal thyroid suppressive therapy. This method would improve I-131 clearance to give the maximal tolerable I-131 dose to treat the metastatic disease. RESEARCH SUPPORT: none