High-dose (131)I-tositumomab (anti-CD20) radioimmunotherapy for non-Hodgkin's lymphoma: adjusting radiation absorbed dose to actual organ volumes.
Radioimmunotherapy (RIT) using (131)I-tositumomab has been used successfully to treat relapsed or refractory B-cell non-Hodgkin's lymphoma (NHL). Our approach to treatment planning has been to determine limits on radiation absorbed dose to critical nonhematopoietic organs. This study demonstrates the feasibility of using CT to adjust for actual organ volumes in calculating organ-specific absorbed dose estimates.


METHODS
Records of 84 patients who underwent biodistribution studies after a trace-labeled infusion of (131)I-tositumomab for RIT (January 1990 and April 2003) were reviewed. Serial planar gamma-camera images and whole-body NaI probe counts were obtained to estimate (131)I-antibody source-organ residence times as recommended by the MIRD Committee. The source-organ residence times for standard man or woman were adjusted by the ratio of the MIRD phantom organ mass to the CT-derived organ mass.


RESULTS
The mean radiation absorbed doses (in mGy/MBq) for our data using the MIRD model were lungs = 1.67; liver = 1.03; kidneys = 1.08; spleen = 2.67; and whole body = 0.3; and for CT volume-adjusted organ volumes (in mGy/MBq) were lungs = 1.30; liver = 0.92; kidneys = 0.76; spleen = 1.40; and whole body = 0.22. We determined the following correlation coefficients between the 2 methods for the various organs: lungs, 0.49 (P = 0.0001); liver, 0.64 (P = 0.004); kidneys, 0.45 (P = 0.0004); spleen, 0.22 (P = 0.0001); and whole body, 0.78 (P = 0.0001), for the residence times. For therapy, patients received mean (131)I administered activities of 19.2 GBq (520 mCi) after adjustment for CT-derived organ mass compared with 16.0 GBq (433 mCi) that would otherwise have been given had therapy been based only using standard MIRD organ volumes-a statistically significant difference (P = 0.0001).


CONCLUSIONS
We observed large variations in organ masses among our patients. Our treatments were planned to deliver the maximally tolerated radiation dose to the dose-limiting normal organ. This work provides a simplified method for calculating patient-specific radiation doses by adjusting for the actual organ mass and shows the value of this approach in treatment planning for RIT.
Rajendran
Joseph G
JG
Department of Radiology, University of Washington, Seattle, Washington 98195, USA. rajan@u.washington.edu
Fisher
Darrell R
DR
Gopal
Ajay K
AK
Durack
Lawrence D
LD
Press
Oliver W
OW
Eary
Janet F
JF
D000328:Adult; D000368:Aged; D000911:Antibodies, Monoclonal / Q000008:administration & dosage / Q000493:pharmacokinetics* / Q000627:therapeutic use*; D001822:Body Burden; D004307:Dose-Response Relationship, Radiation; D005260:Female; D006801:Humans; D007668:Kidney / Q000378:metabolism; D008099:Liver / Q000378:metabolism; D008168:Lung / Q000378:metabolism; D008228:Lymphoma, Non-Hodgkin / Q000378:metabolism* / Q000532:radiotherapy*; D008297:Male; D008875:Middle Aged; D009929:Organ Size; D009928:Organ Specificity; D016499:Radioimmunotherapy / Q000379:methods*; D011874:Radiometry / Q000379:methods*; D019275:Radiopharmaceuticals / Q000008:administration & dosage / Q000493:pharmacokinetics / Q000627:therapeutic use; D011879:Radiotherapy Dosage; D011880:Radiotherapy Planning, Computer-Assisted / Q000379:methods*; D012062:Relative Biological Effectiveness; D013154:Spleen / Q000378:metabolism; D014018:Tissue Distribution
D000911:Antibodies, Monoclonal; D019275:Radiopharmaceuticals; C119496:tositumomab I-131