Macroscopic dosimetry for radioimmunotherapy: Nonuniform activity distributions in solid tumors

TLDR

Radioimmunotherapy dosimetry currently assumes uniform radionuclide distribution in tumors, yet experimental evidence shows nonuniformity, highlighting the need for improved dosimetric models. The study develops a model that incorporates nonuniform radioactivity distribution within tumors. The model uses spherically symmetric linear and exponential radial distributions to calculate dose‑rate profiles for beta‑emitting radionuclides (32P, 67Cu, 90Y, 111Ag, 131I, 188Re) and the conversion‑electron emitter 193mPt. High‑energy beta emitters such as 90Y are most effective for tumors ≥1 cm, medium‑energy emitters like 67Cu are better for ~1 mm tumors, and low‑energy electron emitters such as 193mPt are optimal for sub‑millimeter micrometastases.

Abstract

In the context of radioimmunotherapy of cancer, there is a need for continued improvement of dosimetry of radionuclides localized in tumors. Current methods assume uniform distribution of radionuclides in the tumor despite experimental evidence indicating nonuniformity. We have developed a model in which nonuniform distribution of radioactivity in the tumor is taken into account. Spherically symmetric radionuclide distributions, depending linearly and exponentially on the radial position, are considered. Dose rate profiles in the tumor are calculated for potentially useful beta‐emitting radionuclides, including 3 2 P, 6 7 Cu, 9 0 Y, 1 1 1 Ag, 1 3 1 I, and 1 8 8 Re, and for 193 m Pt, an emitter of conversion electrons and low‐energy Auger electrons. For the radionuclide distributions investigated, high‐energy beta emitters, such as 9 0 Y, are most effective in treating large tumors (diameter, d ≳1 cm), whereas for small tumors ( d ∼1 mm), medium energy beta emitters such as 6 7 Cu are better suited. Very small tumors ( d <1 mm), and micrometastases are best handled with low‐energy electron emitters such as 193 m Pt.