I-131 anti-B1 therapy/tracer uptake ratio using a new procedure for fusionof tracer images to computed tomography images

In patients with non-Hodgkin's lymphoma being treated by I-131-radiolabeled anti-B1 monoclonal antibody, we test the hypothesis that the activity take n up in tumors during therapy is the same as that observed during tracer ev aluation, except for scaling by the ratio of administered activities. Chemo therapy-relapsed patients are imaged only with planar conjugate views, wher eas previously untreated patients are imaged with planar conjugate views an d with single-photon emission computed tomography (SPECT), The SPECT tracer activity quantification requires computed tomography (CT) to SPECT image f usion, for which we devised a new procedure: first, the tracer SPECT images are fused to the therapy SPECT images. Then, that transformation is combin ed with the therapy SPECT-to-CT transformation. We also use (a) the same vo lumes of interest defined on CT for both tracer and therapy image sets, and (b) a SPECT counts-to-activity conversion factor that adapts to background and rotation radius. We define R as the ratio of therapy activity percenta ge of infused dose over tracer activity percentage of infused dose at 2-3 d ays after monoclonal antibody infusion. For 31 chemotherapy-relapsed patien ts, the R ratio for 60 solitary or composite tumors averages 0.931 +/- 0.03 1. The hypothesis of R being 1 is rejected with greater than 95% confidence . However, the difference from 1 is only 7.4%. The range of R is 0.43-1.55. For seven previously untreated patients, R averages 1.050 +/- 0.050 for 24 solitary tumors evaluated by SPECT, For six of these patients, R averages 0.946 +/- 0.098 for one of these solitary tumors and for five composite tum ors, evaluated by conjugate views. Both results agree with the hypothesis t hat R is 1, The range of R for the SPECT tumors is 0.71 +/- 0.03 to 1.82 +/ - 0.53, and for the conjugate view tumors, it is 0.70-1.35. Plots of R vers us tumor volume yield small correlation coefficients. That from SPECT appro aches a statistically significant difference from zero correlation (P = 0.0 6). In summary, on average, the tumor percentage of infused dose following tracer administration is predictive of therapeutic percentage of infused do se within 8%. For greater accuracy with individual tumors, however, an intr atherapy evaluation is probably necessary because the range of R is large.