Radionuclide therapy: current status and prospects for internal dosimetry in individualized therapeutic planning

Abstract

The efficacy and toxicity of radionuclide therapy are believed to be directly related to the radiation doses received by target tissues; however, nuclear medicine therapy continues to be based primarily on the administration of empirical activities to patients and less frequently on the use of internal dosimetry for individual therapeutic planning. This review aimed to critically describe the techniques and clinical evidence of dosimetry as a tool for therapeutic planning and the main limitations to its implementation in clinical practice. The present article is a nonsystematic review of voxel-based dosimetry. Clinical evidence pointing to a correlation between the radiation dose and therapeutic response in various diseases, such as thyroid carcinoma, neuroendocrine tumors and prostate cancer, is reviewed. Its limitations include technical aspects related to image acquisition and processing and the lack of randomized clinical trials demonstrating the impact of dosimetry on patient therapy. A more widespread use of dosimetry in therapeutic planning involves the development of user-friendly dosimetric protocols and confirmation that dose estimation implies good efficacy and low treatment-related toxicity.

Figure 1

Time-activity curve: activity was quantified in consecutive images after drawing a region of interest (ROI) in the organ. The area under the curve corresponds to the accumulated activity (Ã), reflecting the total number of atoms that disintegrated in the region.

Figure 2

Sequential PET or SPECT images are coregistered so that a voxel of the same xyz coordinate always corresponds to the same structure. The calibration factor allows the transformation of counts in radioactivity (MBq). A parametric image of accumulated activity à (MBq.h) is obtained from the integral of activity through time in each voxel. The energy deposited in each voxel is calculated based on the local energy deposition (LED) or dose point kernel (DPK) and results in the radiation absorbed dose parametric image (Gy).