I-124-PET/CT based tumor dosimetry for I-131 therapy of metastatic differentiated thyroid cancer (DTC) - a comparison of recombinant human thyroid-stimulating hormone vs thyroid hormone withdrawal patient preparation methods

394 Objectives In this study we compare two patient preparation methods, recombinant human TSH (rhTSH) and thyroid hormone withdrawal (THW), in terms of the calculated tumor absorbed dose (AD) using 124I-PET/CT based dosimetry for 131I therapy of metastatic DTC, where the same patients were prepared and imaged following both methods. Methods Three DTC patients at MedStar Wash Hosp Ctr were first prepared using the rhTSH method, and PET/CT scans were obtained at 2, 24, 48, 72 and 96 hrs post administration of about 0.8 mCi Na124I. This was repeated 1-2 months later using the THW method for each patient. A total of 22 lesions, with volumes ranging from 0.4-47 cm3 were identified. The contours were drawn on each PET image of each study (rhTSH and THW), by applying a threshold above background to the PET values, using the imaging software Velocity™. The background was then subtracted from the measured tumor activity in each contour. Tumor residence times were calculated and used as input in OLINDA/EXM software to obtain tumor AD in mGy/MBq for 131I therapy of metastatic DTC. Results With the exception of 2 tumors (1.7 and 4.3 cm3), the uptake was higher in the THW study compared to the rhTSH study, resulting in a higher calculated tumor AD per administered activity of 131I in the THW study, consistently for all patients. The average ratio of tumor AD in the THW study to the AD in the rhTSH study was about 4. Conclusions Significantly higher uptake was observed in the tumors in the THW study than in the rhTSH study, resulting in a higher calculated tumor dose for 131I therapy in the THW case. Additional comparison for both tumor and normal organ AD are needed before definitive conclusions may be drawn regarding rhTSH vs THW patient preparation for 131I therapy of metastatic DTC.