Room: Exhibit Hall | Forum 4
Purpose: To assess the accuracy of predicting post-131I-mIBG therapy initial external exposure rate (RT(0)) and clearance effective half-life (T1/2T) from dosimetry-study mR/h (RD) and whole body count rate measurements, for pre-planning patient release from radiation isolation.
Methods: RD and anterior (CA) and geometric mean (CGM) whole body count rates were measured at 0, 24 and 48 h, and RT measurements were obtained at 0, 24, 48 and 72 h (time of release), in fourteen patients. RT(0) was predicted (RP(0)) by scaling RD(0) by the therapy-to-dosimetry activity ratio. T1/2T was predicted (T1/2P) by fitting RD(t), CA(t) and CGM(t) to mono-exponential functions. T1/2T was derived from a mono-exponential fit of RT(t). First-treatment RT(0) and T1/2T predictive accuracy for nine patients who received a second treatment were also assessed. Accuracy was quantified using Student's paired t-test and linear correlation analyses. P < 0.05 was considered a statistically significant difference or correlation r-value.
Results: RP(0) was significantly lower than RT(0) (54.8Â±13.1 vs. 63.74Â±19.3, p=0.04), and the correlation was not significant (r=0.33, p=0.25). T1/2P vs. T1/2T t test p values were: 0.80 (RD), 0.03 (CA) and 0.09 (CGM); and correlations were significant (y=0.71x, r=0.74; y=0.94x, r=0.98; and y=0.94x, r=0.98; all p<<0.05), although T1/2P appeared systematically lower. First- and second-treatment RT(0) (65.9Â±13.1 vs. 64.33Â±10.9, p=0.47; y=1.03x, r=0.89) and T1/2T (27.7Â±2.8 vs. 29.4Â±4.5 h p=0.16; y=0.93x, r=0.65) were statistically identical and correlated significantly. However, first-treatment T1/2T appeared to be systematically about 7% lower.
Conclusion: Scaled RD(0) was not predictive of RT(0)(difference=-10.4%Â±18.2%). T1/2P was predictive of T1/2T, but systematically lower. These results suggest the feasibility of pre-planning post-131I-mIBG therapy patient release, by combining immediate post-therapy measured exposure rate with bias- or slope-corrected pre-therapy T1/2T estimated from anterior or geometric mean whole body count rate versus time.