TY - JOUR T1 - SPECT thresholding to determine tumor volumes for dose estimation in radionuclide therapy JF - Journal of Nuclear Medicine JO - J Nucl Med SP - 2082 LP - 2082 VL - 52 IS - supplement 1 AU - David Howard AU - Anca Avram AU - James Sisson AU - Peter Roberson AU - Yuni Dewaraja Y1 - 2011/05/01 UR - http://jnm.snmjournals.org/content/52/supplement_1/2082.abstract N2 - 2082 Objectives CT images are often not sufficient to determine tumor volumes for estimating the radiation-absorbed dose. Our goal was to determine the concordance between anatomical tumor volumes from CT and functional volumes from SPECT using thresholds determined by phantom studies. Methods Phantoms with spheres of varying sizes (1 to 100 mL) and sphere to background activity concentration ratio, b, (1:0 to 5:1) were examined to determine optimum SPECT thresholds, which give perfect agreement between SPECT and CT volumes. SPECT/CT images of 8 patients, undergoing I-131 radionuclide therapy for thyroid cancer or non-Hodgkin’s lymphoma, with tumors that were well defined on both modalities were examined. SPECT reconstructions were performed using 3D OSEM with scatter, attenuation and detector response compensation. Patient’s CT-defined tumor volumes (range 1.2 to 242 mL) and tumor-to-background SPECT count ratio were used to determine optimum SPECT thresholds based on phantom relationships. Enough data was available to perform dosimetry calculations (using OLINDA) on 4 patients. Results In the phantom studies, the optimum threshold rose rapidly with decreasing volume for small volumes (< 20 ml), but approached a constant value (33% for b of 6:1) for large volumes. For patient tumors, the median difference between CT volumes and SPECT volumes using the phantom-predicted threshold was 36% (range -100 to 48%). The median difference between absorbed doses estimated using the CT volumes and the SPECT volumes was 33% (range -74 to 43%). Conclusions Studies with uniform activity spheres reveal a clear relationship between the optimum SPECT threshold and both volume and contrast. When these thresholds were applied to patients, large differences were observed between CT and SPECT volumes and estimated doses, due to the inhomogeneous radionuclide uptake in tumor, and differences in anatomical and functional volumes. Research Support 2R01 EB001994 awarded by the NIH, USA ER -