IMAGE-BASED PATIENT-SPECIFIC DOSIMETRY FOR RADIONUCLIDE THERAPY Michael Ljungberg, PhD, Katarina Sjögreen-Gleisner, PhD International Symposium on Standards, Applications and Quality Assurance in Medical Radiation Dosimetry 9-12 November 2010 Vienna, Austria Medical Radiation Physics, Clinical Sciences Lund, Lund University
BASIC DOSIMETRY Absorbed dose is mean imparted energy in a mass element! Conditions: A radiation source volume somewhere � A target volume somewhere � The intensity and characteristics of the radiation will be changed on its way � toward the target volume! Common equation for dosimetry in Nuclear Medicine % D = A S ~ A is the total number of disintegrations (cumulated activity) � S describes the energy, emitted from the source volume, absorbed in the � target volume per mass unit and disintegration. Medical Radiation Physics, Clinical Sciences Lund, Lund University
THE ”MIRD” EQUATION φ % % % = ⋅ = ⋅Δ⋅Φ = ⋅ ⋅ ⋅ D A S A A n E m { 1 4 2 4 3 S S ( ) ( ) φ ← φ ← r r r r ∑ ∑ ( ) % % ← = ⋅ Δ = ⋅ i T S i T S D r r A A n E k h h i h i i m m i i T T Mean energy per disintegration Absorbed Fraction ∑ ∑ Photons Δ = Δ = n E ( ) ≤ φ ← ≤ 0 r r 1 i i i i T S i i Electrons n is the number of particles emitted per transition ( ) φ ← = r r 1 E is the mean energy per particle np T S ( ) φ ← = ≠ S denotes the source r r 0 ; T S np T S T denotes the target Medical Radiation Physics, Clinical Sciences Lund, Lund University
APPLICATIONS OF DOSIMETRY IN NUCLEAR MEDICINE Dosimetry for Diagnostic Nuclear Medicine Estimate risk for cancerogenic effects and hereditary changes � Low activities / gamma-radiation � Individuals is not in focus � Populations � Specific for the study but not for the individual patient � Dosimetry for therapy with radionuclides Primary aim is to treat a disease with radiation � High activities / charged particles � The individual is in focus � Study specific as well as patient specific � Medical Radiation Physics, Clinical Sciences Lund, Lund University
DOSIMETRY FOR TREATMENT Activity (A) Measurement with a ’diagnostic tracer amount ’ for kinetic and dosimetry � calculations Preferably made with a scintillations camera or SPECT/PET � Use Gy/MBq to predict activity needed to delived an prescribed absorbed � dose to the target Geometry (S) The more accurate the geometry is - the better. � Patient-specific geometry from a CT study � Sometimes difficult to segment target volumes in 3D � General Goal: To determine absorbed dose for individuals Medical Radiation Physics, Clinical Sciences Lund, Lund University
WHY IS RADIONUCLIDE DOSIMETRY DIFFICULT? External Therapy Well-defined source and intensity � Turn-on and off � Energy usually evenly distributed within a volume element � High dose-rate � Radionuclide therapy Injection of the source � Cannot turn the source on and off! � Need to measure the activity distribution in time and space � Imaging systems have limitations (spatial resolution, noise,attenuation ..) � Localization in the tissues and cells generally heterogeneous � Biokinetic may vary with patients � Low dose-rate � Medical Radiation Physics, Clinical Sciences Lund, Lund University
THE DIFFICULTIES IN ACTIVITY MEASUREMENTS A B C D E F No patient motion and perfect camera resolution A. Patient respiration and heart beating B. Normal system resolution and patient movements C. Photon attenuation D. Photon attenuation and scatter E. Realistic noise level F. Monte Carlo simulated images Medical Radiation Physics, Clinical Sciences Lund, Lund University
2D DOSIMETRY – PRINCIPLES A S Dosimetry based on tools developed for diagnostic dosimetry Activity from Planar WB measurements � (Geometrical-Mean) S-values calculated from analytical � phantoms Assumes homogenous activity i organ � Calculate mean absorbed dose in � organs Correction for differences in organ � masses relative to reference phantom! Medical Radiation Physics, Clinical Sciences Lund, Lund University
BIOKINETICS TO OBTAIN CUMULATED ACTIVITY Time [h] Intensity [counts] AUC Medical Radiation Physics, Clinical Sciences Lund, Lund University
DEVELOPMENT OF MORE REALISTIC PHANTOMS May lead to more ’patient specific’ dosimetry Example: The NCAT phantom by P Segars, Duke University Medical Radiation Physics, Clinical Sciences Lund, Lund University
WHY IMAGE ‐ BASED 3D DOSIMETRY Dosimetry based on two-dimensional (2D) whole-body imaging has known limitations. Contribution from overlapping structures � Attenuation correction is 2D � Scatter correction � Source thickness correction � Background/overlap correction � 3D images provides information of the absorbed dose on a voxel level Heterogenity � Corrections more accurate � Patient-specific anatomy � SPECT/CT on different time points – biokinetics on voxel level � Hybrid SPECT/WB method common compromise! One Quantitative SPECT measurement to nomalise a kinetic curve obtained � from multiple WB measurements Medical Radiation Physics, Clinical Sciences Lund, Lund University
3D DOSIMETRY Multiple registered SPECT/CT or PET/CT studies Correction for Image Function Imaging Anatomical Imaging Image Photon attenuation Function Imaging Anatomical Imaging � Registration SPECT/PET CT Registration SPECT/PET CT Scattered radiation � Collimator resolution Image � Image Reconstruction Reconstruction Septal penetration � Partial-Volume Effect � Correction for Correction for Attenuation and Scatter Attenuation and Scatter 3D dose calculation from Collimator Response Collimator Response Septal Penetration Dose kernels Septal Penetration � Partial-Volume Effects Partial-Volume Effects Monte Carlo method � Dose Calculation by Obtain Segmentation Evaluated as Dose Calculation by Obtain Segmentation Dose Kernels biokinetics DV Histogram Dose Kernels biokinetics DV Histogram Dose/volume histograms Monte Carlo � Monte Carlo Relate to biological effect � Medical Radiation Physics, Clinical Sciences Lund, Lund University
3D DOSIMETRY Modern SPECT/CT (PET/CT) systems makes life easier Correction for Hybrid Hybrid Photon attenuation � SPECT/CT SPECT/CT Scattered radiation � Collimator resolution Image � Image Reconstruction Reconstruction Septal penetration � Partial-Volume Effect � Correction for Correction for Attenuation and Scatter Attenuation and Scatter 3D dose calculation from Collimator Response Collimator Response Septal Penetration Dose kernels Septal Penetration � Partial-Volume Effects Partial-Volume Effects Monte Carlo method � Dose Calculation by Obtain Segmentation Evaluated as Dose Calculation by Obtain Segmentation Dose Kernels biokinetics DV Histogram Dose Kernels biokinetics DV Histogram Dose/volume histogram Monte Carlo � Monte Carlo Relate to biological effect � Medical Radiation Physics, Clinical Sciences Lund, Lund University
3D DOSIMETRY Today Quantification is made by iterative methods Include correction for Hybrid Photon attenuation Hybrid � SPECT/CT SPECT/CT Scattered radiation � Collimator resolution � Iterative Methods Septal penetration Iterative Methods � preferrable preferrable since they allow for Partial-Volume Effect since they allow for � correction of correction of Attenuation and Scatter 3D dose calculation from Attenuation and Scatter Collimator Response Collimator Response Septal Penetration Dose kernels Septal Penetration � Backscatter Backscatter Monte Carlo method � Dose Calculation by Obtain Segmentation Evaluated as Dose Calculation by Obtain Segmentation Dose Kernels biokinetics DV Histogram Dose Kernels biokinetics DV Histogram Dose/volume histogram Monte Carlo � Monte Carlo Relate to biological effect � Medical Radiation Physics, Clinical Sciences Lund, Lund University
PRINCIPLES OF THE ML-EM ALGORITHM Initial Image Estimate Measured Projections New Forward Estimated Image Projection Projections Estimate Ratio yes More Exit Comparing Iterations step Update step yes More Backproject Error angles? Error projection Image space Projection space Medical Radiation Physics, Clinical Sciences Lund, Lund University
ABSORBED DOSE CALCULATIONS FROM SPECT/PET IMAGES Source is one voxel in the SPECT/PET image set Same ”MIRD” Equation!!! Target is one voxel in ( ) φ ← the density image set r r ⋅ ∑ ( ) ← = i T S D r r A n E T S S i i S values not pre-tabulated but m i calculated when needed T Density Patient-specific geometry Absorbed Dose Rate Activity Dose Calculation Medical Radiation Physics, Clinical Sciences Lund, Lund University
POINT ‐ DOSE KERNELS – 3D Describes specific absorbed fraction as function of radial distance from a point source. Derived for homogeneous media (H 2 O) using Monte Carlo calculations. photons � mono-energetic electrons � β -particles � Radionuclides � Medical Radiation Physics, Clinical Sciences Lund, Lund University
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