This training scenario has been developed to help you understand the principles and clinical applications of proton therapy, with a particular focus on how imaging, physics, and treatment planning come together in practice. By following Michaelโs case, you will see how MRI and CT imaging are integrated into the workflow, how the Bragg peak and spread-out Bragg peak are applied, and why proton therapy can offer significant advantages over conventional photon treatments.
The scenario highlights concepts such as pencil beam scanning, passive scattering, proton range uncertainties, verification strategies, and biological effectiveness. These details are presented in a clinical context to reinforce both technical understanding and clinical relevance.
After reviewing the scenario, you will be asked to complete the knowledge check quiz at the bottom of the page. The questions are directly related to the information presented here, so take note of the details as you read through the case. This will help you apply the key principles to both the quiz and to future clinical practice.
Scenario: Michaelโs Proton Therapy
Michael, a 45-year-old man, was recently diagnosed with a tumour located at the base of his skull. This area presents a unique challenge for radiotherapy: the tumour lies close to critical neurological structures, including the brainstem, optic nerves, and parts of the cranial base. Conventional photon radiotherapy, delivered with techniques such as IMRT or VMAT, could provide good tumour coverage but would also expose nearby tissues to unnecessary radiation.
After careful review in a multidisciplinary team meeting, Michaelโs oncologist recommended proton therapy. The decision was based on the known advantage of protons: their ability to reduce the integral dose delivered to healthy tissue. This reduction is especially valuable in regions where even a small excess dose can lead to significant long-term side effects.
Imaging for Treatment Planning
The first step in Michaelโs preparation was imaging. He underwent both MRI and CT scans.
- MRI was used as the standard basis for defining the tumour boundaries. Its excellent soft tissue contrast made it possible to distinguish between tumour tissue and surrounding critical structures.
- CT was acquired to provide the electron density map required for dose calculations. Proton interactions in the body depend heavily on tissue density, so CT data are essential for calculating how far the protons will travel and where they will stop.
The integration of MRI and CT ensured that the treatment planning system had both anatomical precision and physical accuracy.
The Bragg Peak
Michaelโs oncologist explained to him how proton therapy differs from photon therapy. Photons deposit dose continuously as they pass through the body, which means that both entrance and exit doses affect normal tissues. Protons behave differently: as they slow down in matter, they deposit a relatively small dose initially, then release most of their energy at a defined depth, known as the Bragg peak. Beyond this point, the dose falls off rapidly.
By adjusting the beamโs initial energy, clinicians can control the depth at which the Bragg peak occurs. For example, the maximum clinical proton energy used in treatment is typically around 250 MeV, which corresponds to an approximate range of 38 cm in water. This makes it possible to treat both superficial and deep-seated tumours effectively.
For Michaelโs tumour at the skull base, the Bragg peak could be positioned to coincide with the target volume while sparing critical brain structures located just a few millimetres away.
Spread-Out Bragg Peak (SOBP)
One limitation of a single Bragg peak is that it only covers a very narrow depth. Tumours are rarely this thin. To cover the entire target, multiple Bragg peaks of different energies are combined, creating a spread-out Bragg peak (SOBP).
This composite distribution provides a uniform dose across the tumour depth while still maintaining the sharp fall-off beyond the distal edge. In Michaelโs case, the SOBP was shaped to fully encompass the skull base tumour while minimising dose to the optic nerves and other sensitive tissues.
Techniques: Pencil Beam Scanning vs Passive Scattering
Michaelโs treatment was delivered using pencil beam scanning (PBS), a modern form of proton therapy that allows for high precision. In PBS:
- The proton beam is โpaintedโ across the tumour in a series of narrow pencil beams.
- Lateral coverage is achieved by magnetically scanning the pencil beam across the tumour cross-section.
- Depth coverage is achieved by adjusting the proton energy layer by layer, delivering dose in a three-dimensional pattern known as โlayered painting.โ
For more superficial tumours, a range shifter can be added to adjust the effective beam depth.
By contrast, an older method called passive scattering achieves coverage by spreading the proton beam with physical devices such as scatterers and range modulators. While effective, passive scattering lacks the fine precision of PBS and introduces more secondary radiation.
Managing Uncertainties
Proton therapy offers precision, but it is not free from uncertainty. Small changes in patient setup, anatomy, or even daily physiology (such as sinus filling or tumour shrinkage) can alter how far the protons travel.
To manage this, clinicians apply a margin. A common rule-of-thumb is to allow for 3.5% of the proton range plus 1โ2 mm. This ensures that, despite uncertainties, the prescribed dose adequately covers the tumour.
A major contributor to range uncertainty is tissue heterogeneity. Structures such as bone, air cavities, and soft tissue all affect proton stopping power differently. This is why accurate CT calibration and quality assurance are essential parts of treatment planning.
Verification Strategies
Michaelโs team also discussed how they would monitor the accuracy of his treatment. One promising method is PET-based range verification, which takes advantage of positron-emitting isotopes created when protons interact with tissues. By imaging these isotopes after treatment, clinicians can confirm whether the beam stopped where it was intended.
In addition, the team explained that while PET imaging can be useful for verification, the principal type of secondary radiation produced in proton therapy is neutrons. These are generated when protons interact with beamline components or patient tissues and must be accounted for in radiation protection protocols.
Biological Considerations
Finally, Michaelโs oncologist explained the biological assumptions used in planning. Proton therapy is generally planned with an assumed relative biological effectiveness (RBE) of 1.1. This means that, on average, protons are considered about 10% more biologically effective than photons at producing the same biological effect for a given physical dose.
While this is a simplification, it provides a consistent framework for clinical practice. Researchers continue to investigate whether RBE varies with tissue type, depth, or dose per fraction, but 1.1 remains the standard value used in treatment planning.
Putting It All Together
Michaelโs treatment plan brought together multiple elements:
- Imaging with MRI and CT to define tumour and normal tissues.
- Physics in the form of Bragg peak positioning, SOBP creation, and PBS delivery.
- Uncertainty management through range margins and patient-specific QA.
- Verification using PET-based techniques.
- Biology through the assumed RBE.
The result was a plan that delivered high-dose coverage to the tumour while sparing nearby structures. For Michael, this meant a better chance of long-term tumour control with reduced risk of radiation-induced side effects.
Transition to Quiz
You have now reviewed a clinical scenario that introduces the essential principles of proton therapy. The following knowledge check quiz is based on the information presented above. Please refer back to the scenario if needed and use it to guide your answers.