KAHIBARO
13.1 What Are Production Cuts?
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Secondary particle production
In a GATE simulation, particles such as gammas, electrons, and positrons interact with matter and can create new particles, called secondary particles. For example, a high energy gamma can undergo Compton scattering and produce an electron, or pair production can create an electron and a positron. Tracking all these secondaries at very low energies or very short ranges can be extremely expensive computationally, and often unnecessary for the quantities of interest in medical physics.
Production cuts are a way to control which secondary particles are actually created and transported. In Geant4, and therefore in GATE, production cuts are defined as a minimum range in a given material. For each particle type, you specify a range cut in units of length such as millimeters. Geant4 then converts that range into a corresponding kinetic energy threshold for that material and particle. If a physical interaction would produce a secondary below that threshold, the secondary is not created explicitly. Instead, its effects are deposited locally as continuous energy loss.
This is important to understand. A production cut does not mean that energy deposition is lost. The total energy is still conserved. What changes is how the simulation chooses between creating explicit tracks for very low range secondaries and handling their effect as local energy deposition along the track of the parent particle.
In GATE, you usually control production cuts globally, or per region, using distances such as 1 mm, 0.1 mm, or 1 cm. The same numeric cut can have different corresponding energies in different materials, because stopping powers are material dependent. In dense materials such as bone or detector crystals, a 1 mm range corresponds to a higher energy than in air or lung.
The link to transport parameters is direct. A production cut is a transport threshold. Below the threshold, secondaries are not transported as individual tracks. Above the threshold, they are produced and tracked. Typical practice in medical physics simulations is to choose cuts that are small compared to the spatial scale of the scoring or the geometry features that matter to your result. For example, if dose is scored in 2 mm voxels, a gamma or electron range cut of 0.1 mm is usually small enough that local deposition is a good approximation, while still improving performance.
Secondary production cuts can be applied independently for different particle types. In GATE and Geant4 you will most often encounter separate cuts for gammas, electrons, and positrons. Hadronic particles can also be controlled but are usually handled differently, depending on the physics list. By adjusting these cuts, you effectively decide which secondaries are important enough to be treated explicitly.
Whenever you change cuts, you are also changing the microscopic detail of the transport. This can have visible consequences in finely resolved dose distributions, edge effects near material boundaries, or in thin detectors. Therefore, production cuts must be chosen with the physics goal in mind, not just with the desire to run quickly.
Production cuts are defined as a minimum particle range in each material. Secondaries below this range are not created as explicit tracks, but their energy is deposited locally. Energy is not lost, but the microscopic details of low energy transport are simplified.
Computational performance
Every explicit secondary particle requires memory, CPU time, and many tracking steps. In a detailed medical physics simulation with complex geometries, millions or billions of low energy electrons and photons can be generated if no cuts are applied. This can make simulations unreasonably slow or even impossible to complete with practical computing resources.
Production cuts are a key tool to control computational performance. By increasing cuts, you reduce the number of secondaries, shorten particle histories, and simplify transport at small scales. This directly reduces the number of steps, interactions, and geometry boundary checks the simulation must process.
The relation between cuts and performance is typically monotonic but not linear. Small increases in cuts from very fine values, for example from 0.01 mm to 0.1 mm, can already give a noticeable speedup, because many extremely short range electrons are no longer created. Larger increases, such as from 0.1 mm to 5 mm, can bring further gains but at the risk of degrading the accuracy of fine spatial structures in dose or detector response.
In practical GATE work, you balance accuracy and speed by setting different cuts in different regions. For example, in a PET simulation you might use relatively small cuts in the scintillator crystals where interaction details matter for detector response, and larger cuts in surrounding shielding where only attenuation matters. In a dose calculation, you might choose fine cuts in the patient or phantom region, and coarser cuts in parts of the geometry that are far from the scoring volume.
The effect on performance is especially strong for electrons and positrons, because they undergo many small steps and collisions as they slow down. Even a modest increase in the electron range cut can significantly reduce CPU time. Gamma cuts also influence performance, but usually to a lesser extent.
When adjusting cuts, it is common to perform a small study. You run the same simulation with two or three different cut values, record key outputs such as central dose, depth dose, or detector count rates, and compare them. If the differences are within your acceptable uncertainty but the higher cut gives better speed, that cut is a good compromise.
It is important not to confuse production cuts with statistical uncertainty. A simulation with very tight cuts but too few primary events can still be inaccurate due to large Monte Carlo variance. Conversely, a simulation with relaxed cuts and many events can be fast and have low statistical uncertainty, but may have systematic bias in fine spatial details. Both aspects, transport detail and number of histories, must be considered.
Larger production cuts reduce the number of low energy secondaries, which speeds up simulations but can reduce spatial and spectral accuracy. Always balance cuts against the required precision of your results.
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