The effect of kidney volume estimation on dosimetry in lutetium-177 DOTATATE therapy

The kidneys are the dose-limiting organ in lutetium-177 DOTATATE therapy. Therefore, it is advisable to perform critical organ dosimetry focussed on renal dose in treated patients. A key uncertainty in such dose estimates is the use of standard phantoms to represent the individual patient. The primary aim of this study was to investigate the accuracy of methods for estimating kidney size, and hence absorbed kidney dose, by comparison with individual measurements from computed tomography (CT) imaging.

Kidney volume was measured using diagnostic CT images for 57 patients who underwent lutetium-177 DOTATATE therapy. Kidney mass was also estimated in two ways: using the standard adult phantoms, as well as through the application of a weight scaling factor to these phantoms and their organs. Dose calculations were performed for each of the three methods using OLINDA/EXM software.

Scaling of the phantom by patient weight gave a more accurate result when compared with the CT gold standard than the standard phantom. The dose difference from the CT method had mean values of 1.4% (SD=22.6%) and 8.4% (SD=21.5%) for scaled and unscaled, respectively. Patient weight was not found to be a good predictor of kidney mass in these patients (r of 0.12 from linear regression analysis).

The most accurate method of organ volume estimation would be individual measurements from CT imaging; however, where this is not possible, scaling of organ masses by weight ratio is more accurate than the use of the standard phantom.