Comparisons of sectioned micro‐TLD dose measurements with predicted dose from 131 I‐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 131 I‐labeled antibody. The spheroids were then sectioned at 30‐μm intervals and the TLD sections (measuring 0.14×0.1×0.03 mm) were removed and read. Calibration of the TLDs was done with whole TLDs using external beam radiation and an 131 I‐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 131 I 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. TLD sections of this size, originating from standard chips available from the manufacturer, are useful in measuring cumulative absorbed doses from 131 I‐labeled antibody within 4% of the calculated dose for doses in the range of 150 Gy, within 6% to 20% for the 40‐Gy range, and within a factor of two for doses less than 25 Gy.