Comparisons of sectioned micro-TLD dose measurements with predicted dose from 131I-labeled antibody

The dose distribution from radioimmunotherapy is very heterogeneous because of variability in antigen expression, antibody penetration, and tumor architecture. Many models of dose distribution have been constructed but it has been very difficult to confirm these predictions with actual measured doses. The purpose of this study was to determine what degree of resolution could be obtained using mini-thermoluminescent dosimeter(s) (TLD) in a micrometastasis model. TLDs were inserted into 1-mm-diam multicell tumor spheroids that had been treated with 131I-labeled antibody. The spheroids were then sectioned at 30-microns intervals and the TLD sections (measuring 0.14 x 0.1 x 0.03 mm) were removed and read. Calibration of the TLDs was done with whole TLDs using external beam radiation and an 131I-containing gel, and with TLD sections using external beam radiation. Predicted doses were determined by measuring the activity in individual spheroids and the time the TLDs were in the spheroids and incorporating these numbers into a model that assumed either surface binding of 131I or some degree of penetration. There was a correlation between the measured TLD dose and the predicted absorbed dose when comparing the low- to high-dose regions. However, there was considerable variation within any particular dose range, probably due to heterogeneity in TLD grain size.(ABSTRACT TRUNCATED AT 250 WORDS)