A simplified approach to beta dosimetry for small spheres labelled on the surface

Absorbed doses for internal radiotherapy from 22 beta-emitting radionuclides: beta dosimetry of small spheres

We calculated the mean absorbed fractions, specific absorbed fractions and mean doses per unit of cumulated activity in source spheres 10 microm-2 cm in radius for 22 beta-emitting radionuclides potentially useful in radioimmunotherapy. We considered two models of radionuclide distribution, either uniform at the surface of the source or throughout its volume. For each model, we calculated both the absorbed fractions in the spherical segments composing the source and the mean absorbed fractions. For surface distribution, we calculated the mean dose per unit of cumulated activity for a concentric sphere with a small radius (5 microm) in order to determine the minimal dose delivered to the target. Calculations were performed using point kernels for monoenergetic emissions and then integrated into the beta spectra of the different emitters (32p, 33p, 47Sc, 67Cu, 77As, 90Y, 105Rh, 109Pd, 111Ag, 121Sn, 131I, 142Pr, 143Pr, 149Pm, 153Sm, 159Gd, 166Ho, 177Lu, 186Re, 188Re, 194Ir and 199Au). Monoenergetic emissions were taken into account. Results are reported in the form of tables to facilitate use during dosimetric studies for radioimmunotherapy. An application is presented showing the potential utility of associating emitters with different energies in order to sterilize a range of tumour targets of variable size.

Beta dose-rate distributions in microscopic spherical tumors for intraperitoneal radioimmunotherapy

PURPOSE

This work was designed to calculate the radial beta dose-rate profiles through microscopic spherical tumors. Its application is in the treatment of micrometastases in the peritoneal cavity by the intraperitoneal administration of radiolabeled immunoliposomes.

METHODS AND MATERIALS

Using previously published data for the dose-rate as a function of distance from a point source of activity, dose-rate profiles through five sizes of tumors (radii: 10 microm, 50 microm, 100 microm, 500 microm, 1 mm) for six different radionuclides ((188)Re, (186)Re, (32)P, (90)Y, (67)Cu, (131)I) were calculated. Dose-rate profiles were calculated for two source geometries: (1) a large bath of radioactivity in which the tumor is submerged, and (2) surface-bound radioactivity that results from tumor targeting.

RESULTS

The bath geometry produced profiles that were uniform for sufficiently small tumors. For high-energy emitters (i.e., (90)Y and (188)Re), uniformity was maintained up to a tumor radius of 100 microm. For lower energy emitters (i.e., (67)Cu and (131)I) deviations from uniformity start to appear at a tumor radius of 50 microm. Surface-bound radioactivity produced a much greater range of dose-rates within tumors of all sizes. Lower energy emitters bound to the surface of tumors produce higher dose-rates for very small micrometastases compared with high-energy emitters. Upon consideration of the simultaneous contributions from both source geometries, we believe that liposome-mediated radioimmunotherapy would benefit from the inclusion of a high-energy beta emitter, possibly as a component of a cocktail of radionuclides.

CONCLUSIONS

The calculated dose-rate profiles provide a tool for making tumor control probability estimations for micrometastases and for assessing the potential benefit offered by a targeted approach over a nontargeted approach. These calculations also suggest that the inclusion of a high-energy beta emitter is appropriate for this treatment modality.

Computational methods in radionuclide dosimetry

The various approaches in radionuclide dosimetry depend on the size and spatial relation of the sources and targets considered in conjunction with the emission range of the radionuclide used. We present some of the frequently reported computational techniques on the basis of the source/target size. For whole organs, or for sources of targets bigger than some centimetres, the acknowledged standard was introduced 30 years ago by the MIRD committee and is still being updated. That approach, based on the absorbed fraction concept, is mainly used for radioprotection purposes but has been updated to take into account the dosimetric challenge raised by therapeutic use of vectored radiopharmaceuticals. At this level, the most important computational effort is in the field of photon dosimetry. On the millimetre scale, photons can often be disregarded, and beta or electron dosimetry is generally reported. Heterogeneities at this level are mainly above the cell level, involving groups of cell or a part of an organ. The dose distribution pattern is often calculated by generalizing a point source dose distribution, but direct calculation by Monte Carlo techniques is also frequently reported because it allows media of inhomogeneous density to be considered. At the cell level, alpha and electron (low-range beta or Auger) are the predominant emissions examined. Heterogeneities in the dose distribution are taken into account, mainly to determine the mean dose at the nucleus. At the DNA level, Auger electrons or alpha-particles are considered from a microdosimetric point of view. These studies are often connected with radiobiological experiments on radionuclide toxicity.

Absorbed fraction to the cell nucleus for low energy electrons

The absorbed fraction phi to the cell nucleus for low energy electrons (0.5 keV-50 keV) was evaluated. Distributions of radioactivity within either the nucleus (nu) and the cytoplasm (cy), or on the cell membrane (mem) were considered. phi was computed as a function of the cell sizes and of the electron energy E. For a strictly intranuclear distribution, phi(nu) is close to 1 for very low energy values (E < 4 keV), i.e. the energy is totally absorbed in the cell nucleus itself. The absorbed fraction decreases when the energy increases and phi(nu) becomes less than 0.1 for E > or = 40 keV. For a cell membrane distribution, the absorbed fraction remains always less than 0.2. For very low energy electrons (E < 6 keV), phi(mem) = 0, due to the fact that the electron falls short to the nucleus target. The absorbed fraction is maximum for E ranging from 12 keV to 20 keV. For higher values of E, phi(mem) decreases when E increases. When considering a cytoplasmic distribution, the maximum absorbed fraction phi(cy) is obtained for E values ranging from 10 keV to 25 keV (phi(cy max) = 0.27). Dosimetric computations at the cellular level show that the absorbed fraction to the cell nucleus may have values ranging from 0 to 1, depending on the dimensions of the cell, the energy of the emitted electron and on the intracellular localization of the Auger emitter.

Targeted immunotherapy. An update with special emphasis on ovarian cancer

An overview of antibody-guided immunotherapy for neoplasia is presented. The diversity of the antibody molecule is highlighted, through the many sophisticated strategies proposed and employed, to overcome a number of problems impeding successful targeting. An update of radioimmunotherapy of ovarian cancer is presented and the exciting concepts that are introduced to the field of targeted cancer therapy by molecular biology and genetic engineering are addressed.

Use of multi-cell spheroids of ovarian carcinoma as an intraperitoneal radio-immunotherapy model: uptake, retention kinetics and dosimetric evaluation

The purpose of this study, using multi-cell spheroids as an in vitro model of micrometastases of ovarian carcinoma for i.p. radio-immunotherapy, was to measure the uptake and retention kinetics of 111In-labeled F(ab')2 fragments of OC125 monoclonal antibody (MAb) in spheroids of the NIH:OVCAR-3 cell line and to estimate absorbed doses with beta-emitting radionuclides (kinetics was assumed to be similar to that of indium-111). With 0.2-mm-diameter spheroids at different MAb concentrations the highest binding value was determined. Retention kinetics showed a biological half-life of 50 hr. These data were used to calculate absorbed doses by integration of Berger's latest-point kernels. Mean absorbed doses when spatial distribution was considered to be uniform at the surface (no penetration) or throughout the spheroid (total penetration) were, respectively, 247 and 417 Gy with 153Sm, and 90 and 135 Gy with 90Y. Thus, the use of a similar MAb concentration and specific activity in patients should lead to high absorbed doses in i.p. radio-immunotherapy of micrometastases.