Radio-immunotherapy dosimetry with special emphasis on SPECT quantification and extracorporeal immuno-adsorption

Chemical radioanalysis of production of positron-emitting radioisotope Gallium-68 via (p,n) and (p,2n) reactions using compact cyclotron for tomography applications

In this study, the proton-induced reactions of 68Zn and 69Ga aimed at generating 68Ga were simulated and modeled using Talys code and neural network software. In the first step, both targets were simulated under different proton energies and at different bombardments times to generate a total of six thousand data. Then, the obtained data from the Talys, including the various cross-sections, contaminations, the main product i.e. 68Ga, and other options were completely saved in the output file. Afterwards, the inputs of the neural network were selected from the output of the Talys by analyzing and considering most of the key features. A total of four inputs, two of which are different energies related to the reaction, the other is the process sequence and the fourth input is the bombardment time, were recognized as suitable inputs and the model was trained differently depending on the type of target. The selected model was a feed-forward neural network with 5 nodes in a middle layer, which was able to estimate the output of Talys code by changing the input parameters with extremely high accuracy. Two different models including the main model for estimating the output of the main sample (product) and the sub-model for estimating process pollution or impurity were trained, and then the trained model was tested on the deduced process data. The implementation results fully demonstrated the high accuracy of the method. The neural network model is much easier to implement than the Talys code, and its execution speed is very high. In addition, it can be used appropriately as a system alternative for optimization and different structures in medical and biological engineering.

Emerging trends for radioimmunotherapy in solid tumors

Due to its ability to target both known and occult lesions, radioimmunotherapy (RIT) is an attractive therapeutic modality for solid tumors. Poor tumor uptake and undesirable pharmacokinetics, however, have precluded the administration of radioimmunoconjugates at therapeutically relevant doses thereby limiting the clinical utility of RIT. In solid tumors, efficacy of RIT is further compromised by heterogeneities in blood flow, tumor stroma, expression of target antigens and radioresistance. As a result significant efforts have been invested toward developing strategies to overcome these impediments. Further, there is an emerging interest in exploiting short-range, high energy α-particle emitting radionuclides for the eradication of minimal residual and micrometastatic disease. As a result several modalities for localized therapy and models of minimal disease have been developed for preclinical evaluation. This review provides a brief update on the recent efforts toward improving the efficacy of RIT for solid tumors, and development of RIT strategies for minimal disease associated with solid tumors. Further, some of promising approaches to improve tumor targeting, which showed promise in the past, but have now been ignored are also discussed.

The Impact of PET and SPECT on Dosimetry for Targeted Radionuclide Therapy

Targeted radionuclide therapy (TRT) is an increasingly used treatment modality for a range of cancers. To date, few treatments have involved the use of dosimetry either to plan treatment or to retrospectively ascertain the absorbed dose delivered during treatment. Also the correlation between absorbed dose and biological effect has been difficult to establish. Tomographic methods permit the determination of the activity volume on a macroscopic scale at different time points. Proper attenuation correction in tomographic imaging requires a patient-specific attenuation map. This can be obtained from scintillation-camera transmission scanning, CT or by using segmented scatter-emission images. Attenuation corrections can be performed either on the projection images, on the reconstructed images, or as part of an iterative reconstruction method. The problem of image quantification for therapy radionuclides, particularly for I-131, is exacerbated by the fact that most cameras are optimised for diagnostic imaging with Tc-99m. In addition, problems may arise when high activities are to be measured due to count losses and mis-positioned events, because of insufficient pile-up and dead time correction methods. Sufficient image quantification, however is only possible if all effects that degrade the quantitative content of the image have been corrected for. Monte Carlo simulations are an appealing tool that can help to model interactions occurring in the patient or in the detector system. This is helpful to develop and test correction techniques, or to help to define detectors better suited to quantitative imaging. PET is probably the most accurate imaging method for the determination of activity concentrations in tissue. PET imaging can be considered for pre-therapeutic treatment planning but ideally requires the use of a radioisotope from the same element as that used for treatment (e.g. I-124 for I-131; Y-86 for Y-90). Problems, however are that--some of the positron emitting isotopes have a shorter half-life--non-standard quantification procedures have to be performed--the availability of the radiopharmaceutical is presently limited; Many 3D-tools and -techniques are now available to the physicist and clinician to enable absorbed dose calculations to both target and critical organs-at-risk. The challenge now facing nuclear medicine is to enable this methodology to be routinely available to the clinic, to ensure common standard operating procedures between centres and in particular to correlate response criteria with absorbed dose estimates.

Ligand liposomes and boron neutron capture therapy

Boron neutron capture therapy (BNCT) has been used both experimentally and clinically for the treatment of gliomas and melanomas, with varying results. However, the therapeutic effects on micro-invasive tumor cells are not clear. The two drugs that have been used clinically, p-boronophenylalanine, (BPA), and the sulfhydryl borane, (BSH), seem to be taken up preferentially in solid tumor areas but it is uncertain whether enough boron is taken up by micro-invasive tumor cells. To increase the selective uptake of boron by such cells, would be to exploit tumor transformation related cellular changes such as over-expression of growth factor receptors. However, the number of receptors varies from small to large and the uptake of large amounts of boron for each receptor interaction is necessary in order to deliver sufficient amounts of boron. Therefore, each targeting moiety must deliver large number of boron atoms. One possible way to meet these requirements would be to use receptor-targeting ligand liposomes, containing large number of boron atoms. This will be the subject of this review and studies of boron containing liposomes, with or without ligand, will be discussed. Two recent examples from the literature are ligand liposomes targeting either folate or epidermal growth factor (EGF) receptors on tumor cells. Other potential receptors on gliomas include PDGFR and EGFRvIII. Besides the appropriate choice of target receptor, it is also important to consider delivery of the ligand liposomes, their pharmacodynamics and pharmacokinetics and cellular processing, subjects that also will be discussed in this review.

Tumour therapy with radionuclides: assessment of progress and problems

Radionuclide therapy is a promising modality for treatment of tumours of haematopoietic origin while the success for treatment of solid tumours so far has been limited. The authors consider radionuclide therapy mainly as a method to eradicate disseminated tumour cells and small metastases while bulky tumours and large metastases have to be treated surgically or by external radiation therapy. The promising therapeutic results for haematological tumours give hope that radionuclide therapy will have a breakthrough also for treatment of disseminated cells from solid tumours. New knowledge related to this is continuously emerging since new molecular target structures are being characterised and the knowledge on pharmacokinetics and cellular processing of different types of targeting agents increases. There is also improved understanding of the factors of importance for the choice of appropriate radionuclides with respect to their decay properties and the therapeutic applications. Furthermore, new methods to modify the uptake of radionuclides in tumour cells and normal tissues are emerging. However, we still need improvements regarding dosimetry and treatment planning as well as an increased knowledge about the tolerance doses for normal tissues and the radiobiological effects on tumour cells. This is especially important in targeted radionuclide therapy where the dose rates often are lower than 1Gy/h.

A Monte-Carlo program converting activity distributions to absorbed dose distributions in a radionuclide treatment planning system

In systemic radiation therapy, the absorbed dose distribution must be calculated from the individual activity distribution. A computer code has been developed for the conversion of an arbitrary activity distribution to a 3-D absorbed dose distribution. The activity distribution can be described either analytically or as a voxel based distribution, which comes from a SPECT acquisition. Decay points are sampled according to the activity map, and particles (photons and electrons) from the decay are followed through the tissue until they either escape the patient or drop below a cut off energy. To verify the calculated results, the mathematically defined MIRD phantom and unity density spheres have been included in the code. Also other published dosimetry data were used for verification. Absorbed fractions and S-values were calculated. A comparison with simulated data from the code with MIRD data shows good agreement. The S values are within 10-20% of published MIRD S values for most organs. Absorbed fractions for photons and electrons in spheres (masses between 1 g and 200 kg) are within 10-15% of those published. Radial absorbed dose distributions in a necrotic tumor show good agreement with published data. The application of the code in a radionuclide therapy dose planning system, based on quantitative SPECT, is discussed.