Estimation of Dose Distribution for Lu-177 Therapies in Nuclear Medicine

In nuclear medicine, two frequent applications of 177-Lu therapy exist: DOTATOC therapy for patients with a neuroendocrine tumor and PSMA thearpy for prostate cancer. During the therapy a pharmaceutical is injected intravenously, which attaches to tumor cells due to its molecular composition. Since the pharmaceutical contains a radioactive 177Lu isotope, tumor cells are destroyed through irradiation. Afterwards the substance is excreted via the kidneys. Since the latter are very sensitive to high energy radiation, it is necessary to compute exactly how much radioactivity can be administered to the patient without endangering healthy organs. This calculation is called dosimetry and currently is made according to the state of the art MIRD method. At the beginning of this work, an error assessment of the established method is presented, which has determined an overall error of 25% in the renal dose value. The presented study improves and personalizes the MIRD method in several respects and reduces individual error estimates considerably. In order to be able to estimate of the amount of activity, first a test dose is injected to the patient. Subsequently, after 4h, 24h, 48h and 72h SPECT images are taken. From these images the activity at each voxel can be obtained a specified time points, i. e. the physical decline and physiological metabolization of the pharmaceutical can be followed in time. To calculate the amount of decay in each voxel from the four SPECT registrations, a time activity curve must be integrated. In this work, a statistical method was developed to estimate the time dependent activity and then integrate a voxel-by-voxel time-activity curve. This procedure results in a decay map for all available 26 patients (13 PSMA/13 DOTATOC). After the decay map has been estimated, a full Monte Carlo simulation has been carried out on the basis of these decay maps to determine a related dose distribution. The simulation results are taken as reference (“Gold Standard”) and compared with methods for an approximate but faster estimation of the dose distribution. Recently, a convolution with Dose Voxel Kernels (DVK) has been established as a standard dose estimation method (Soft Tissue Scaling STS). Thereby a radioactive Lutetium isotope is placed in a cube consisting of soft tissue. Then radiation interactions are simulated for a number of 10^10 decays. The resulting Dose Voxel Kernel is then convolved with the estimated decay map. The result is a dose distribution, which, however, does not take into account any tissue density differences. To take tissue inhomogeneities into account, three methods are described in the literature, namely Center Scaling (CS), Density Scaling (DS), and Percentage Scaling (PS). However, their application did not improve the results of the STS method as is demonstrated in this study. Consequently, a neural network was trained finally to estimate DVKs adapted to the respective individual tissue density distribution. During the convolution process, it uses for each voxel an adapted DVK that was deduced from the corresponding tissue density kernel. This method outperformed the MIRD method, which resulted in an uncertainty of the renal dose between -42.37-10.22% an achieve a reduction in the uncertainty to a range between -26.00%-7.93%. These dose deviations were calculated for 26 patients and relate to the mean renal dose compared with the respective result of the Monte Carlo simulation. In order to improve the estimates of dose distribution even further, a 3D 2D neural network was trained in the second part of the work. This network predicts the dose distribution of an entire patient. In combination with an Empirical Mode Decomposition, this method achieved deviations of only -12.21%-2.13% . The mean deviation of the dose estimates is in the range of the statistical error of the Monte Carlo simulation. In the third part of the work, a neural network was used to automatically segment the kidney, spleen and tumors. Compared to an established segmentation algorithm, the method developed in this work can segment tumors because it uses not only the CT image as input, but also the SPECT image

EMT for HDR-BT

The current study develops an automatic analysis tool of EMT data sets recorded with a solenoid sensor to assure concordance of the source move- ment with the treatment plan. The tool combines machine learning tech- niques such as multi-dimensional scaling (MDS), ensemble empirical mode decomposition (EEMD), singular spectrum analysis (SSA) and particle filter (PF) to precisely detect and quantify any mismatch between the treatment plan and actual EMT measurements

Estimation of [177Lu]PSMA-617 tumor uptake based on voxel-wise 3D Monte Carlo tumor dosimetry in patients with metastasized castration resistant prostate cancer

Zusammenfassun