KAHIBARO
36.3. Energy Deposition
Table of Contents
Local energy deposition
In internal dosimetry, radionuclides are distributed inside the body, so radiation is emitted directly inside tissues rather than entering from outside. The basic quantity that links this situation to dose is the energy that particles deposit locally in matter. In a GATE simulation this energy deposition is recorded step by step as particles move and interact in the patient or phantom geometry.
At the microscopic level, charged particles such as electrons and positrons lose energy continuously through ionization and excitation of atoms. Photons deposit energy in a more stochastic way through discrete interactions such as the photoelectric effect and Compton scattering, which then produce secondary electrons. In a Monte Carlo simulation you do not need to implement these details yourself. Once you have defined a suitable physics list and internal sources, GATE tracks all primary and secondary particles and records the energy lost in each voxel or region.
For internal dosimetry you often want to know how much of the emitted energy is deposited close to where it was emitted. This is what we call local energy deposition. If the source is confined to a particular organ or lesion, a large fraction of the emitted energy may be absorbed locally, especially for low energy electrons or short range beta emitters. In GATE you typically attach a dose or energy deposition actor to a voxelized patient geometry or to selected volumes that represent organs. The actor will accumulate the total deposited energy in each voxel or volume over all histories.
You can relate energy deposition to the source activity and the simulation time. For a given radionuclide and activity distribution, the simulation gives you the energy deposited per voxel per simulated decay or per emitted particle. By scaling with the total number of decays in a clinical situation you can estimate the absorbed dose. Although the dose calculation itself is covered in a separate chapter, it is useful to remember that absorbed dose $D$ is defined as energy deposited per unit mass,
$$
D = \frac{E_{\text{dep}}}{m}.
$$
The core internal-dosimetry quantity is the total energy deposited $E_{\text{dep}}$ in a defined mass. Correct local energy deposition requires:
- A realistic activity distribution inside the geometry.
- Accurate physics for the radionuclide emissions and transport.
- Correct material definitions and densities for the voxels or organs.
In practice, when setting up local energy deposition for internal dosimetry in GATE, you must ensure that the activity map and the scoring grid are consistent. If you use a voxelized activity distribution (for example from a SPECT or PET image), the same image grid or a compatible one is usually used for energy or dose scoring. Misalignment between the activity grid and the scoring grid can lead to incorrect local deposition patterns. Visualization tools can help you verify that the activity and materials are correctly assigned to the intended anatomical regions.
Another aspect of local energy deposition is the role of particle range. Short range particles, such as low energy electrons, tend to deposit their energy very close to their emission sites, so the local dose follows the activity distribution quite closely. Long range beta particles or high energy photons can transport energy over larger distances, which reduces the fraction of energy that stays local. In GATE, this behavior is naturally reproduced if your physics and transport parameters are appropriate for internal emitters.
When you analyze local energy deposition results, you often extract voxel energy maps, organ averaged energy, or energy per decay in specific lesions. These quantities then serve as inputs for internal dose calculations, organ specific dose estimates, and eventually for evaluating the effectiveness and safety of radionuclide therapies.
Cross-organ dose
In internal dosimetry, not all energy remains in the source region. Particles can leave the organ or volume where they were emitted and deposit energy in other organs. This is called cross-organ dose or sometimes cross-irradiation. It describes how one region of the body irradiates another due to particle transport.
In a GATE simulation, cross-organ dose occurs naturally whenever particles are allowed to travel from their emission site into surrounding tissues. For example, photons emitted in the liver can escape and deposit energy in nearby organs or even at distant sites, depending on energy and attenuation. Similarly, beta particles with sufficient range can cross boundaries between organs and contribute to dose in neighboring structures.
To study cross-organ dose you need a geometry where individual organs or regions of interest are represented as separate volumes or segmented structures. This can be a stylized phantom or a patient specific voxel phantom derived from CT or MR images with corresponding organ labels. You then attach appropriate dose or energy deposition actors to each target organ, while placing the activity in a source organ or distribution. The same radionuclide and activity may be localized in one organ, but the actors in other organs will still record deposited energy from particles that leave the source region.
From the simulation output you can compute, for each source organ $S$ and target organ $T$, the energy deposited in $T$ per decay in $S$. Once you normalize by the mass of the target organ, you obtain the absorbed dose contribution from that source. Organ level cross-dose relationships are often summarized by quantities such as S values, which are covered elsewhere in the course. What matters here is that GATE can provide these source-to-target energy deposition values directly from particle transport without any simplifying assumptions about geometry or attenuation.
Cross-organ dose arises because energy can be transported from the source organ to other organs. To capture this correctly in GATE, you must:
- Define distinct source and target organs or regions.
- Use a geometry that preserves realistic inter-organ distances and materials.
- Score deposited energy separately in each target region while simulating all relevant emissions.
The importance of cross-organ dose depends strongly on the radionuclide and emission type. Pure beta emitters with short electron ranges may deliver most of their energy locally, making cross-dose relatively small and limited to directly adjacent tissues. Radionuclides with significant photon emissions can irradiate distant organs, so cross-dose becomes essential for accurate whole-body dosimetry. In a GATE internal dosimetry setup this difference appears automatically. If you simulate a photon rich radionuclide, you will see energy deposition in organs far from the main source region.
When interpreting cross-organ dose results from GATE, it is crucial to ensure that the underlying organ segmentation and material assignment are accurate. If an organ is incorrectly labeled or overlaps with another in the voxel data, the recorded cross-dose relationships may not reflect real anatomy. Visual inspection of the voxel phantom and verification of organ boundaries are therefore an important part of internal dosimetry simulations.
Finally, cross-organ dose can be analyzed not only at the organ level but also at a voxel level, where you examine how high activity in one region of a tumor, for example, deposits energy in nearby normal tissues. Although the later steps of converting these energy maps into dose distributions and clinical metrics are handled in other chapters, the key internal dosimetry concept is that GATE allows you to quantify both local and cross-organ energy deposition within a single, consistent patient specific simulation.
Views: 13