Initial results for hybrid SPECT-conjugate-view tumor dosimetry in I-131-anti-B-1 antibody therapy of previously untreated patients with lymphoma

A study of the use of I-131-labeled anti-B-1 monoclonal antibody, proceeded by an unlabeled predose, for therapy of previously untreated non-Hodgkin's lymphoma patients has recently been completed at the University of Michiga n, Ann Arbor. More than half of the patients treated were imaged intrathera py with SPECT to separate apparently large tumors, unresolved by conjugate views, into individual ones specified by CT scan. The dosimetry of these tu mors is reported here. Methods: The activity-quantification procedure used 3-dimensional CT-to-SPECT fusion so that attenuation maps could be computed from CT and that volumes of interest could be drawn on the CT slices and t ransferred to the SPECT images. Daily conjugate-view images after a tracer dose of labeled anti-B-1 antibody followed by an unlabeled predose provided the shape of the time-activity curve for the calculation of therapy dosime try. Reconstructed SPECT counts that were within a volume of interest were converted to activity by using a background-and-radius-adaptive conversion factor. Activities were increased for tumors less than 200 g using a recove ry-coefficient factor derived from activity measurements for a set of spher es with volumes ranging from 1.6 to 200 cm(3). The calculated tumor radiati on absorbed dose was based, in part, on the CT volume and on the intrathera py-SPECT activity. Results: The mean of the radiation dose values for 131 a bdominal or pelvic tumors in 31 patients was 616 cGy with a standard deviat ion of +/-50 cGy. The largest dose was 40 Gy and the smallest dose was 73 c Gy. The mean volume for the tumors was 59.2 +/- 11.2 cm(3). The correlation coefficient between absorbed dose and tumor volume was small (r(2) = 0.007 ), and the slope of the least-squares fit represented a decrease of only 36 .4 cGy per 100 cm(3) increase in volume. This small slope may reflect a cha racteristic of anti-B-1 antibody therapy that is important for its success. The mean absorbed dose per unit administered activity was 1.83 +/- 0.145 G y/GBq. The largest Value was 12.6 Gy/GBq, and the smallest value was 0.149 Gy/GBq. The mean dose for 9 axillary tumors in 5 patients was significantly lower than the average dose for abdominal and pelvic tumors (P = 0.01). Th erefore, axillary tumors should be grouped separately in assessing dose-res ponse relationships. Anecdotal patient results tended to verify the validit y of using the shape of the conjugate-view time-activity curve for the aver age SPECT-intratherapy curve. However, there was also an indication that th e shape Varies somewhat for individual tumors with respect to time to peak. Conclusion: Hybrid SPECT-conjugate-view dosimetry provided radiation absor bed dose estimates for the individual patient tumors that were resolved by CT.