Activity quantification based on scintillation camera imaging - Application to 111In/90Y radioimmunotherapy

Sammanfattning: Radionuclide therapy (RNT), is important for the treatment of certain benign and malignant diseases. 90Y-Zevalin™ therapy has become an established method of treating patients with non-Hodgkin B-cell lymphoma. Several observ¬ations regarding the outcome of therapy have prompted the use of higher absorbed doses by administering larger amounts of activity than those indicated in standard protocols. This type of treatment, which generally requires bone marrow support, increases the risk of deterministic effects on healthy organs and, therefore, requires the use of reliable patient-specific internal dosimetry. An important part of patient-specific dosimetry is the quantification of the activity of the radiopharmaceutical by means of scintillation camera imaging. The aim of the work presented in this thesis was thus to develop quantitative scintillation camera imaging methods in 2 and 3 dimensions. A method for planar activity quantification and absorbed dose assess¬ment is presented and evaluated (Paper I). The planar quantification method requires a narrow-beam, patient-specific attenuation map, which can be obtained through a transmission study using an external radionuclide source. This method has clear drawbacks, which prompted investigations of the use of a whole-body X-ray image for attenuation correction. The method is based on acquiring a whole-body X-ray image using the CT unit on a hybrid SPECT/CT system (Paper II). 90Y only emits β-particles, and has previously been considered difficult to image. The radionuclide 111Indium has therefore been used as a substitute tracer for imaging purposes. However, free unbound 111In and 90Y behave differently in vivo. It would thus be of interest to be able to image 90Y alone, without an admixture of 111In. This could be done by detecting the bremsstrahlung that is produced when β-particles interact with tissue. The feasibility of quantitative bremsstrahlung SPECT (paper III) and quantitative planar whole-body imaging (paper IV) have been investigated by means of simulations and measure¬ments using phantoms. The feasibility of these 90Y bremsstrahlung imaging procedures has also been investigated using data from patients undergoing 90Y-Zevalin treatment for B-cell lymphoma (Paper V).