THE EFFECT OF 3-DIMENSIONAL ACTIVITY DISTRIBUTION ON THE DOSE PLANNING OF RADIOIMMUNOTHERAPY FOR PATIENTS WITH ADVANCED INTRAPERITONEAL PSEUDOMYXOMA

BACKGROUND. Six patients with histologically proven peritoneal carcino matous pseudomyxomas were treated with radioimmunotherapy. METHODS. Ai l the patients received a tracer dose of iodine-131 (I-131) labeled B7 2.3 anti-TAG-72 monoclonal antibody (MoAb) to test the in vivo affinit y. After informed consent was obtained the therapeutic dose (>3.7 giga becquerels [GBq], 100 mCi) of the I-131 labeled B72.3 anti-TAG-72 MoAb was infused within 60 minutes intraperitoneally using 2 catheters on both sides of the abdomen. The patients were imaged with single photon emission computed tomography (SPECT) at 3, 10, and 24 days after the therapeutic infusion. Treatment-planning software has been developed i n which functional information obtained from SPECT is integrated with anatomic information obtained from computed tomography (CT). The activ ity distribution from SPECT images is converted to absorbed dose distr ibutions using a point source kernel convolution dose calculation. The absorbed dose calculation requires a radionuclide specific dose kerne l. The activity map is divided into equally sized source voxels from w hich the distribution is calculated for the target voxels that cover t he patient volume. The resulting three dimensional (3D) absorbed dose distribution is viewed as isodose contours superimposed on the CT imag es or as 3D isodose surfaces. RESULTS. The measured activity distribut ion shows that the cumulated activity and biologic half-life vary in t he patient's body. The developed planning system provides a method for calculating patient specific absorbed dose distributions. CONCLUSIONS . The variation of biologic clearance indicates that a 3D dose calcula tion method incorporating measured activity distributions is needed to quantify absorbed dose distribution. (C) 1997 American Cancer Society .