The effect of three dimensional activity distribution on the dose planning of radioimmunotherapy for patients with advanced intraperitoneal pseudomyxoma

Six patients with histologically proven peritoneal carcinomatous pseudomyxomas were treated with radioimmunotherapy.

All the patients received a tracer dose of iodine-131 (131I) labeled B72.3 anti-TAG-72 monoclonal antibody (MoAb) to test the in vivo affinity. After informed consent was obtained the therapeutic dose (>3.7 gigabecquerels [GBq], 100 mCi) of the 131I 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 in which functional information obtained from SPECT is integrated with anatomic information obtained from computed tomography (CT). The activity distribution from SPECT images is converted to absorbed dose distributions using a point source kernel convolution dose calculation. The absorbed dose calculation requires a radionuclide specific dose kernel. The activity map is divided into equally sized source voxels from which the distribution is calculated for the target voxels that cover the patient volume. The resulting three dimensional (3D) absorbed dose distribution is viewed as isodose contours superimposed on the CT images or as 3D isodose surfaces.

The measured activity distribution shows that the cumulated activity and biologic half-life vary in the patient's body. The developed planning system provides a method for calculating patient specific absorbed dose distributions.

The variation of biologic clearance indicates that a 3D dose calculation method incorporating measured activity distributions is needed to quantify absorbed dose distribution.