Aortic wall modeling/dosimetry in lymphoma patients treated with radioimmunotherapy

Objectives Periaortic and periarterial tumors in the abdomen in Non-Hodgkin’s lymphoma (NHL) patients are relatively common. In external radiotherapy, large blood vessel irradiation is considered a long-term risk factor for arteriosclerosis. Current SPECT/CT resolution is insufficient for patient-specific, imaging-based dosimetry of these areas. We have estimated the absorbed dose (AD) to these regions using geometric modeling.

Methods Four NHL patients with periaortic tumors received ~200 MBq ¹³¹I-tositumomab Bexxar followed by SPECT/CT and whole body planar (WB) imaging at 1, 72, and 144 h. Activity concentrations for tumor and blood were obtained from SPECT and WB images, respectively. Using these data and a GEANT4 model of the aorta and femoral artery, Monte Carlo (MC) simulation for all time points were performed for ⁹⁰Y and ¹³¹I. The dose rate results were fitted and the absorbed dose to the aortic wall, tumor and blood were obtained for both isotopes.

Results AD ranged from 0.09 – 0.6 Gy/GBq for ¹³¹I and 0.25 – 1.6 Gy/GBq for ⁹⁰Y for the aorta wall and from 0.35 – 1.3 Gy/GBq for ¹³¹I and 0.9 – 1.7 Gy/GBq for ⁹⁰Y for the deep femoral artery wall. The tumor results from the model (1.4 – 10.2 Gy) were comparable to the 3D-RD results for the same patients.

Conclusions AD from radioimmunotherapies to the arterial walls were lower (0.5 – 1.8 Gy for ¹³¹I-tositumomab) than those typical from external beam therapy (20 Gy). Patient-specific, 3D-RD dosimetry, combined with MC modeling can provide important and more accurate dosimetry for anatomical features smaller than the imaging resolution.