Tumor dosimetry using 177Lu: influence of background activity, measurement method and reconstruction algorithm

Abstract

Background Image-based tumor dosimetry after radionuclide therapy, using the isotope 177Lu, finds application e.g., for tumor-to-organ dose comparison and for dose response evaluation. When the tumor extent is not much larger than the image resolution, and when 177Lu is found in nearby organs or other tumors, an accurate determination of tumor dose is particularly challenging. Here a quantitative evaluation of three different methods for determining the 177Lu activity concentration in a phantom is performed, and the dependence on a variety of parameters is described. The phantom (NEMA IEC body phantom) has spheres of different size in a background volume, and sphere-to-background 177Lu activity concentration ratios of infinity, 9.5, 5.0 and 2.7 are applied. The methods are simple to implement and well-known from the literature. They are based on (1) a large VOI encompassing the whole sphere, without background activity and with volume information from other sources, (2) a small VOI located in the sphere center, and (3) a VOI consisting of voxels with voxel value above a certain percentage of the maximum voxel value. Results The determined activity concentration varies significantly with sphere size, sphere-to-background ratio, SPECT reconstruction method and method for determining the concentration. Based on the phantom study, criteria are identified under which the activity concentration can be determined with a maximal error of 40% even in the presence of background activity. Conclusions Tumor dosimetry is feasible in the presence of background activity using the above-mentioned methods, provided appropriate SPECT reconstructions are applied and tumors are selected for dosimetry analysis according to the following criteria for the three methods: (1) solitary tumor with diameter > 15 mm, (2) tumor diameter > 30 mm and tumor-to-background ratio > 2, and (3) tumor diameter > 30 mm and tumor-to-background ratio > 3.

A single-institution experience with 177Lu RPT workflow improvements and qualifying the SPECT/CT imaging for dosimetry

“…Different studies quantitatively evaluate the SPECT/CT imaging camera for performance using different reconstruction algorithms. Some assess recovery coefficients and spatial resolution in a NEMA IEC phantom with different sphere-to-background ratios for determining 177 Lu activity concentration ( 59 , 60 ). Another study proposes incorporating patient dosimetry into clinical practice with recovery coefficient determination with various phantoms ( 61 ).…”

Position dependence of recovery coefficients in 177Lu-SPECT/CT reconstructions – phantom simulations and measurements

“…For more accurate dose calculations, PVC should take into account tumor location and surrounding activity, which would require a wide range of phantom measurements or accurate RC modeling. A further option is to account for the background activity by modifying the RC to a contrast recovery coefficient [ 21 ], which is consistent with the experimental findings of Staanum [ 33 ]. In practice, it might be necessary to modify the linear relationship [ 21 ] between background-to-object ratio and RC [ 34 ].…”