Two-orthogonal-view method for quantification of rad dose to neck lesions in thyroid cancer therapy patients.

Abstract

A new technique is described for the determination of the dose in rad to metastatic lesions in the neck following administration of 131I for the treatment of thyroid carcinoma. The technique utilizes a high‐energy pinhole collimator mounted on an Anger camera which can be rotated to record lateral and anterior views from known positions relative to the patient. A new algorithm is derived for the determination of lesion location in three dimensions, utilizing the distance of closest approach between back‐projected rays from the two views. In addition, a new derivation is presented for the estimation of ellipsoid volume from two pinhole projections. The method makes use of a computer edge detection program to delineate the projected lesion boundaries. The output is either a mean estimate from one view or a more definitive upper bound derived from both views. The uptake in counts is determined from daily anterior images which are corrected for pinhole efficiency falloff with distance. This uptake is converted into microcuries by use of an in‐air calibration curve. An attenuation correction is applied using the calculated lesion depth and an assumed attenuation coefficient. The dose rate is calculated using standard assumptions and is integrated graphically over the first few days when the rate peaks and by utilizing a single‐decaying‐exponential fit to the data over the remaining days. Illustrative patient data are presented in which doses are calculated for three neck lesions after an administration of 175 mCi of 131I. These doses include a lower bound of 2400 rad and estimates of 5300 and 9500 rad.