Underestimation of Absorbed Dose to Kidney

TO THE EDITOR:

With interest we read the dosimetric analysis of radioimmunotherapy using ⁹⁰Y-labeled ibritumomab by Wiseman et al. (1). We were surprised by the very low absorbed kidney dose. Because there is no actual targeting of the kidney and because the kidney dose can be attributed to activity in the blood; activity in the urine in the tubuli, calices, and pelvis; and radiation by adjacent organs such as the liver, it is curious that the kidney dose is so much lower than that of other organs and even lower than the dose to the urinary bladder wall. Our own radioimmunotherapy data and other studies using radiolabeled monoclonal antibodies for nonmyeloablative radioimmunotherapy report kidney doses of several grays (2–4), whereas the study of Wiseman et al. suggests that kidney doses do not exceed 0.76 Gy (1).

A possible explanation for this observation may be that the region of interest (ROI) for the kidneys was drawn around the right kidney. Because there is significant uptake in the liver, most counts in this kidney ROI can be attributed to the liver. Situating a background region next to the kidney, over the liver, would result in subtraction of background (mainly consisting of liver counts) from kidney (mainly consisting of liver counts), resulting in low numbers or even in the extremely unlikely kidney dose of 0.0003 Gy that was reported in the article.

To prevent underestimation of kidney doses, we usually draw ROIs around the left kidney, representing both kidneys, since no other organs (not even the spleen in most lymphoma patients) project over this kidney.