Hybrid Imaging for Patient-Specific Dosimetry in Radionuclide Therapy

Abstract

Radionuclide therapy aims to treat malignant diseases by systemic administration of radiopharmaceuticals, often using carrier molecules such as peptides and antibodies. The radionuclides used emit electrons or alpha particles as a consequence of radioactive decay, thus leading to local energy deposition. Administration to individual patients can be tailored with regards to the risk of toxicity in normal organs by using absorbed dose planning. The scintillation camera, employed in planar imaging or single-photon emission computed tomography (SPECT), generates images of the spatially and temporally varying activity distribution. Recent commercially available combined SPECT and computed tomography (CT) systems have dramatically increased the possibility of performing accurate dose planning by using the CT information in several steps of the dose-planning calculation chain. This paper discusses the dosimetry chain used for individual absorbed-dose planning and highlights the areas where hybrid imaging makes significant contributions.

Keywords: CT; Monte Carlo; SPECT; absorbed dose; activity; dosimetry; hybrid; quantitation; reconstruction; therapy.

Figure 1

A schematic illustration of the different physical effects explained in the above text.

Figure 2

Flow-chart describing the different steps in the dosimetry chain and where information from 2D planar X-ray scout images or 3D tomographic CT images can be useful.

Figure 3

Patient whole-body images acquired during a ¹⁷⁷Lu-Dotatate therapy. (A) is scintillation-camera image, (B) is an X-ray scout image, and (C) is an overlay of the scintillation-camera image on an X-ray scout image. The information from the scout image is useful for attenuation and scatter corrections, and for localization of the patient border and the lungs.

Figure 4

SPECT/CT image acquired during a ¹⁷⁷Lu-Dotatate therapy, with activity localized in the kidneys, intestines and tumour. (A) is a SPECT image, while (B) shows SPECT overlaid on CT image. The CT image is useful for localization but also for attenuation and scatter corrections and for determination of the mass of the tissues where activity is localized.

Figure 5

The principles for voxel-based absorbed dose calculations from a set of quantitative SPECT images in units of Bq and a set of anatomical CT images in units of g·cm⁻³. The Monte Carlo method calculate the radiation transport from each source voxel (r_S) to all other target voxels (r_T) from the principles described in Equation (4) by explicit simulate the particle tracks of photons and charge-particles.

Figure 6

An example of a pre-clinical mouse study shown as fused images obtained from a combined μSPECT/μCT system. The mouse was injected with a ^99mTc-labelled peptide mainly excreted through the kidneys. (Courtesy—Thuy Tran, Lund Biomedical Imaging Center, Lund, Sweden).