This study evaluate the possibility of restricting the current image quantification (ROI) from the whole body to a small area in thigh for therapy planning. Whole body planar images were obtained from 13 patients after the administration of 131I tracer dose, and OLINDA/EXM software was used for internal dose calculation. The average value of half-life time was 16.46 ?? 3.45 h in whole body quantification, compared to 14.07 ?? 2.89 h in thigh. The average absorbed dose in bone marrow from the whole body quantification was 0.0519 ?? 0.0250 mGy/MBq, whilst the absorbed dose provided by the thigh quantification represented 87% of this value, with up to 95% in time reduction for dose calculation. Such doses become similar when a correction factor of 1.13 is applied in thigh dose results (p=0.67), which is within the variation coefficient assumed to internal dosimetry (20%). This study shows the possibility of a significant time reduction in internal dose calculation applied to therapy planning.