Comparison of organ residence time estimation methods for radioimmunotherapy dosimetry and treatment planning—patient studies

Abstract

The estimation of organ residence time is essential for high-dose myeloablative regimens in radioimmunotherapy (RIT). Frequently, this estimation is based on a series of simple planar scans and planar processing. The authors previously performed a simulation study which demonstrated that the accuracy of this methodology is limited compared to a hybrid planar/SPECT residence time estimation method. In this work the authors applied this hybrid method to data from a clinical trial of high-dose myeloablative yttrium-90 ibritumomab tiuxetan therapy. Image data acquired from 18 patients were comprised of planar scans at five time points ranging from 1 to 144 h postinjection and abdominal and thoracic SPECT/CT scans obtained at 24 h postinjection. The simple planar processing method used in this work was based on the geometric mean method with energy window based scatter compensation. No explicit background subtraction nor object or source thickness corrections were performed. The SPECT projections were reconstructed using iterative reconstruction with compensations for attenuation, scatter, and full collimator-detector response. Large differences were observed when residence times were estimated using the simple planar method compared to the hybrid method. The differences were not constant but varied in magnitude and sign. For the dose-limiting organ (liver), the average difference was −18% and variation in the difference was 19%, similar to the differences observed in a previously reported simulation study. The authors also looked at the relationship between the weight of the patient and the liver residence time and found that there was no meaningful correlation for either method. This indicates that weight would not be an adequate proxy for an experimental estimate of residence time when choosing the activity to administer for therapy. The authors conclude that methods such as the simple planar method used here are inadequate for RIT treatment planning. More sophisticated methods, such as the hybrid SPECT/planar method investigated here, are likely to be better predictors of organ dose and, as a result, organ toxicities.

Tandem Myeloablative 131 I-rituximab Radioimmunotherapy and High-Dose Chemotherapy in Refractory/Relapsed Non-Hodgkin Lymphoma Patients

“…However, CR was higher in the tandem group compared with RIT alone group (75 vs 33%), but CR were probably underestimated in the present RIT only group (literature reports CR ~50%), and CR were already reached after myeloablative RIT preceding high dose chemotherapy in the tandem approach. 18 F-fluorodeoxyglucose PET was negative in all patients with CR after RIT (14 out of 22); those patients had longer PFS and OS than nonresponders.…”

“…18 F-fluorodeoxyglucose PET was also performed for all patients 6 weeks after RIT. A total of 64% of patients (14 out of 22) showed a complete response (CR) and 23% (five out of 22) showed a partial response.…”

Strengths and Weaknesses of a Planar Whole-Body Method of 153Sm Dosimetry for Patients with Metastatic Osteosarcoma and Comparison with Three-Dimensional Dosimetry

“…For normal organs, the discrepancies may be less or systematic, in which case planar dosimetry adapted to specific organs such as the posterior view-only approach for kidneys may be adequate. 48 Additionally, more sophisticated methods of planar quantification have been developed, which are more accurate 49,50 ; however, they are not widely available, nor do they resolve the issue of tumor nonuniform uptake. Indeed, a caveat to the conclusions regarding the levels of uncertainty in planar dosimetry is that this article made use of a particular approach and generalizations regarding the exact accuracy of planar quantification should be made with caution.…”

Targeted Radionuclide Therapy: Proceedings of a Joint Workshop Hosted by the National Cancer Institute and the Society of Nuclear Medicine and Molecular Imaging

“…Variability in toxicity, response, and organ dose has been identified but not yet entirely explained. Methods to account for these variabilities and accurately estimate delivered activity are under investigation, including the use of 111 In-based organ dosimetry (31).…”