Three methods assessing red marrow dosimetry in lymphoma patients treated with radioimmunotherapy.

Abstract

Maximum injected activity in radioimmunotherapy (RIT) is limited by bone marrow toxicity. Many dosimetric approaches have been proposed, leading to high variability in the results and elusive absorbed dose-effect relations. This study presents the results of red marrow (RM) absorbed dose estimates performed with 3 methods. Five patients received 2 co-infusions of (90)Y-labeled (370 MBq/m2) and (111)In- labeled (120 MBq) epratuzumab (1.5 mg/kg) 1 week apart. RM-absorbed dose was estimated by 3 methodologies. The first approach (M1) used L(2)-L(4) lumbar vertebrae imaging. M2 and M3 methods used different red marrow to blood ratios (RMBLR) to assess RM-absorbed dose. RMBLR was set to a fixed value of 0.36 in M2 or assessed according to each patient's hematocrit in M3. Median RM-absorbed doses were 4.1 (2.9-8.4), 2.3 (2.0-2.7), and 2.3 (1.6-2.5) mGy/MBq for M1, M2, and M3, respectively. No trend could be found between total RM-absorbed dose and toxicity for M2 and M3. Conversely, M1 seemed to provide the best absorbed dose-effect relation. The 4 patients with the highest RM-absorbed doses exhibited grade 4 toxicity. The fifth patient, with the lowest RB absorbed dose, exhibited only a mild (grade 2) toxicity. Image-based methodology (M1) seems to better predict hematological toxicity as compared with blood-based methods. Only this method provides for bone marrow involvement.