Internal Radionuclide Dosimetry using Quantitative 3-D Nuclear Medical Imaging

Introduction

In order to estimate the absorbed dose for all significant tissues in nuclear medicine, one must determine for each tissue the quantity of energy absorbed per unit mass. This yields the quantity absorbed dose, and can be extended to the calculation of dose equivalent if desired. This response and the prediction of toxicity is essential to rational the implementation of cancer therapy.

Nevertheless, to state that the absorbed dose alone would predict the radiobiologic response of tissue is an oversimplification that would certainly lead to hypo- or hyper- estimation of the radiation induced effects. It has already been recognized in radiotherapy that the response is affected by a number of parameters such as: the type of radiation (LET), the rate at which absorbed dose is delivered, the radiobiologic characteristics of the tumor or normal tissue etc. Moreover the anatomical characteristic of the patients have to be taken into account, since the presence of different structures affects the distribution of radiation dose.

Presently, nuclear medicine dosimetry is based on the measurement of the biokinetics of the radionuclide by serial gamma camera scans, followed by calculations comprising three steps. First the percentage of administered activity of the radiopharmaceutical must be determined for the accumulating organs for several scan times. Second these biokinetic data must be integrated to obtain the percentage of the number of decays in the source organs, i.e the residence times and third, the radiation absorbed doses of critical organs must be determined.

However, using planar data to quantify the activity in different organs may be severely affected by several factors, such as the inaccurate attenuation and scatter correction as well as the background and organ overlay.

Dosimetry that takes into account quantitative 3-dimensional data is more accurate and obviously allows the so called ‘tailor made’ approach, in terms of the individualization of therapy on each specific patient. Nevertheless there are factors that can potentially degrade the quantitative content of the images, or insert erroneous data, such as the inhomogeneous organ accumulation of the radionuclide, which have to be detected and taken into account.

The research on dosimetry is focused on the development of software tools, which allow the use of tomographic functional data (PET-SPECT) in conjunction with anatomical information from CT or MRI, providing a more accurate and detailed description of the individual patient situation. Firstly anatomical and functional data need to be registered and fused and then manual or automated Regions Of Interest (ROIs) are drawn. Hence dose calculation is based on dose kernel convolution methods, reliant on convolving the radioactivity distribution with a medium specific radionuclide dose-kernel, defined as the adsorbed dose per decay at a point away from the source. The innovation consists in the application of the CT information, which allows the use of different dose kernels depending on the different evaluated structures.