KAHIBARO
34.2. Photon Beams
Table of Contents
Megavoltage photons
In external beam radiotherapy, photon beams are typically megavoltage beams, with nominal energies such as 4 MV, 6 MV, 10 MV, or higher. In a Monte Carlo simulation with GATE, you never transport the full linac head physics by specifying “MV” directly. Instead, you represent the megavoltage beam either with a phase space that already contains particles leaving the treatment head, or with a primary source model that approximates the beam emerging from the linac, usually described in MeV.
Clinically, a “6 MV” photon beam corresponds to a complex bremsstrahlung spectrum produced when high energy electrons hit a target. The mean photon energy at the patient surface is much lower than 6 MeV, often around 1 to 2 MeV. In GATE, you model this spectrum explicitly. A simple model uses a monoenergetic photon source with an energy in the MeV range, for example 2 MeV, to approximate the spectral average. A more realistic model uses an energy distribution that follows a bremsstrahlung-like spectrum, implemented through a histogram or analytical function.
To create a primary megavoltage photon source in GATE, you define a gamma particle type and choose an energy description that matches the linac beam model you want to simulate. The source is then positioned at or near the linac head exit, upstream from the patient or phantom geometry in your simulation. You can either specify a parallel beam with a small angular divergence or a point-like source with a given field size at the isocenter.
An important property of megavoltage photons is their depth dose behavior in water or tissue. Unlike kilovoltage beams, megavoltage beams exhibit build-up. The dose increases with depth from the surface to a maximum at depth, then decreases approximately exponentially. This has a direct impact on how you interpret dose results from GATE. When you use a dose actor in a water phantom, the resulting depth dose curve should show a surface build-up region followed by a fall-off that depends on the nominal beam energy.
Another key aspect is the field size. In GATE, you can represent different square or rectangular fields by limiting the spatial extent of the source or by using collimating geometry volumes between the source and the phantom. For example, a 10 cm by 10 cm field at the isocenter can be produced by defining a source position and angular distribution that illuminate exactly that region, or by adding collimator blocks that physically shape the beam.
Because you work in MeV and cm or mm, correct use of units is essential. A typical configuration might use photon energies given in MeV, positions in mm, and a source intensity expressed as a number of primary histories rather than dose rate. You then scale the absolute dose after the simulation, for example by relating the number of histories to monitor units or to an absolute dose measurement.
Megavoltage photon simulations are also sensitive to physics settings. For radiotherapy, you choose physics lists and production cuts that give accurate Compton scattering, pair production, and secondary electron transport in the MeV range. This directly affects the accuracy of dose distributions such as percent depth dose and lateral profiles.
For realistic megavoltage photon beams in GATE, always:
- Use photon energies in MeV that match your beam model.
- Include secondary electron transport with appropriate production cuts.
- Verify that the simulated depth dose in water shows the expected build-up and fall-off for the nominal linac energy.
If you use a phase space instead of a primary source, then the megavoltage spectrum, angular spread, and field shape are already encoded in the phase space file. In that case, your GATE configuration focuses on importing the phase space, defining the patient or phantom geometry, and scoring dose. This approach is common when you want to reuse a detailed linac head simulation or a vendor-provided beam model.
Beam transport
Once you have defined a megavoltage photon source or imported a phase space, beam transport describes how photons and their secondary particles travel from the source through the linac head components, air, patient geometry, and finally deposit energy in tissues. In the context of this chapter, we focus on transport from the treatment head exit to the patient or phantom, since detailed modeling of the head itself is usually treated elsewhere.
In GATE, photon beam transport follows the physics models you select in your physics list. Primary photons propagate through space until they undergo interactions such as Compton scattering, photoelectric absorption at lower energies, or pair production at higher energies. These interactions produce secondary electrons and sometimes secondary photons. The secondary charged particles are then transported and lose energy through ionization and scattering, which is ultimately responsible for dose deposition.
To obtain accurate dose distributions, especially in heterogeneous geometries, you need appropriate transport parameters. The most important are production cuts, which define the minimum range or energy above which secondary particles are generated and transported. In high density regions, such as bone, or in low density regions, such as lung, overly large cuts can underestimate the contribution from secondary electrons, especially near interfaces. For radiotherapy beams, you usually choose cuts expressed as small spatial ranges, for example fractions of a millimeter, so that dose gradients and build-up regions are well resolved.
Beam transport is also influenced by the geometry through which the photons travel. In a simple water phantom, photons propagate in a uniform medium. In a patient CT model, the beam crosses bone, lung, soft tissue, and possibly air cavities. Each material has different attenuation and scattering properties. In GATE, this is naturally handled through the material assignments in the geometry. The beam automatically experiences more attenuation in bone and less in lung, and the resulting dose distribution reflects these heterogeneities.
A typical radiotherapy transport problem involves a collimated field passing through air, then entering the patient. The field edges produce penumbra regions, where the dose changes rapidly with lateral position. Penumbra shape is governed by geometrical penumbra from finite source size and collimator geometry, as well as by lateral scattering of secondary electrons and photons. In Monte Carlo transport, these features appear naturally if the source model and geometry are correctly defined. In GATE, you examine lateral dose profiles at specific depths to verify that the simulated penumbra width matches reference data.
You also control beam transport through the number of primary histories and multithreading. More primary photons lead to lower statistical uncertainty in dose. However, transport of MeV photons and their secondaries is computationally expensive. You address this by using multithreading and by choosing a simulation region of interest that is as small as possible while still capturing the clinically relevant volume. For example, you may simulate only a region around the target volume instead of an entire body.
In some workflows, beam transport is split into two stages using phase space. In the first stage, you simulate transport through the linac head to a scoring plane, recording a phase space file that contains the energy, position, direction, and particle type for all particles crossing that plane. In the second stage, you create a phase space source that emits those particles into the patient geometry. This method reduces computation when you need to reuse the same linac head model for many different patient setups or field configurations, because the expensive head transport is done only once.
Mathematically, dose is related to the energy fluence and mass energy absorption coefficients, but in Monte Carlo you obtain it directly from the tracked energy deposited in each voxel. Conceptually, photon transport determines the spatial distribution of energy fluence, while electron transport determines how that fluence turns into absorbed dose at small scales. If you disable electron transport or use too high production cuts, the depth dose curve and lateral profiles will not match experimental measurements.
For reliable photon beam transport in radiotherapy simulations:
- Use physics lists appropriate for MeV photon and electron transport.
- Set production cuts small enough to resolve build-up regions and penumbra.
- Validate transport by comparing depth dose curves and lateral dose profiles in water against reference or measured data.
Finally, beam transport in GATE must be interpreted together with the beam geometry and dose scoring configuration. The same source and physics can produce very different dose statistics if you change voxel size or scoring volume. Smaller voxels give better spatial resolution of transport effects but higher statistical noise, while larger voxels average transport effects over a broader region. For photon beams in clinical geometries, you often balance voxel size and number of histories so that both transport physics and clinically relevant dose metrics, such as depth-dose curves and profiles at specific depths, are represented with acceptable uncertainty.
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