William G. Hawkins

Dosimetry and treatment planning for 90Y-labelled antiferritin in hepatoma

Radiation absorbed-dose estimates and treatment planning are reported for 11 patients with hepatoma who were administered 90Y-labeled polyclonal antiferritin IgG for therapy in a Phase 1-2 trial. Dosimetric studies included quantitation of the localization and clearance of 111In-labeled antiferritin IgG in tumor and normal tissues and computer-assisted tumor and normal liver volumetrics from X ray CT scans. For the group of patients studied, hepatoma volumes at the time of treatment ranged from 135 to 3442 cm3. Quantitative 111In antiferritin imaging prior to and following 600 or 900 cGy of external-beam irradiation of the primary tumor demonstrated that tumor uptake increased 1.1 to 5.8-fold (mean 2.8) following external beam. In contrast, changes in uptake of radiolabeled antiferritin in normal liver ranged from 0.35 to 2.1-fold (mean 0.93) after external irradiation. Administered activities of 90Y antiferritin ranged from 8 to 37 mCi and were dependent on tumor volume and tumor localization of radiolabeled antiferritin. Following external-beam irradiation, tumor dose rates achieved with 90Y antiferritin ranged from 10 to 20 cGy/hr and normal liver dose rates from 1.1 to 5.7 cGy/h. The corresponding absorbed dose in hepatomas ranged from 900 to 2150 cGy and in normal liver from 80 to 650 cGy. After external-beam irradiation, tumor and normal liver uptake of 90Y antiferritin was consistent with that of 131I antiferritin.

Effective atomic numbers for low‐energy total photon interactions in human tissues

A new method is introduced in which the total photon interaction cross sections per electron of human tissues are used to define effective atomic numbers for blood, bone, brain, fat, heart, kidney, liver, lung, muscle, ovary, pancreas, spleen, and water. These effective atomic numbers are equal within 4% from 10 to 200 keV in each soft tissue, whereas for bones of different chemical compositions the variation ranges from 2.86% to 5.03%. This effective atomic number definition is less energy dependent than a previous definition based on the total photon interaction cross section per atom averaged over all elements in the tissue, from which the computed effective atomic numbers varied by as much as 50% (in bone) as a function of photon energy over the energy range from 10 to 200 keV.

CT volumetrics of primary liver cancers.

A new computer algorithm is described for liver and tumor volume determinations for patients with hepatoma and primary hepatic cholangiocarcinoma. The algorithm is based on global histograms of CT numbers of the liver and primary liver cancers. The algorithm includes computer-assisted definition of the liver boundary in each CT slice. Liver and tumor volumes of 10 patients calculated by the histogram method were compared with volumes obtained from CT slices that were manually contoured by experienced observers. A correlation coefficient of 0.995 was determined for these two methods of volume computations. Mean values of the differences in volumes obtained by the two methods were 6.7 and 8.0% for the liver and tumor, respectively. The computer algorithm was tested on CT scans for an additional 46 patients by highlighting regions corresponding to normal liver and tumor tissues in each CT slice and determined to be accurate by experienced observers. The computer software is being used clinically to assess tumor response in a new treatment program for primary liver cancers that includes radiolabeled antibodies.

An intrinsic 3D Wiener filter for the deconvolution of spatially varying collimator blur

The authors have developed distance-dependent deconvolution of collimator blur for 3D SPECT reconstruction. The collimator model is based on the frequency-distance principle (FDP), and the authors have applied it to the circular harmonic transform (CHT) SPECT reconstruction algorithm. The filter model used is a 3D Wiener (FT stochastic) filter. Since the filter is applied between the premultiply and backprojection phases of reconstruction, it is an intrinsic filter, rather than a pre or post filter. The authors compare the new 3D filter to (i) the 3D post filter followed by attenuation correction as implemented in a commercial nuclear medical imaging system, (ii) the 3D collimator model and a standard Wiener filter formulation, and (iii) the gold standard or quantitative CHT SPECT protocol that utilizes a 2D Wiener prefilter. In all cases, the new 3D filter yielded the best images with minimal noise amplification.<<ETX>>