Proton therapy

Proton Therapy: Advanced infrastructure for precise tumor treatment

Proton therapy requires highly specialized infrastructure, including a system for producing and delivering proton beams, precise patient positioning devices, advanced treatment planning tools, dose monitoring systems, safety control mechanisms, and sophisticated software for data management and clinical workflow.

1. Interaction with matter: the Bragg Peak

Unlike photons (X-rays) that gradually lose energy, protons deposit most of their dose at the end of their path, creating the "Bragg peak." This physical phenomenon allows radiation to be concentrated in the tumor while sparing tissues located beyond it.

Protons penetrate tissues with minimal energy loss initially, then slow down and transfer their energy massively just before stopping. The result: maximum dose in the tumor area and virtually no dose beyond the tumor.

2. Depth control: longitudinal ballistics

The penetration depth of protons directly depends on their initial energy. By modulating this energy between 65 and 250 MeV, physicians can target tumors located from a few centimeters to more than 30 cm deep.

This longitudinal precision reaches a few millimeters, allowing preservation of adjacent critical organs.

3. Lateral control: transverse ballistics

Protons undergo lateral scattering as they pass through tissues, creating progressive beam widening. This scattering generates lateral penumbra - a gradual transition zone between maximum dose and zero dose - which varies according to the technique used:

• Passive Scattering (see below):
  • Single scattering: very reduced penumbra (1 to 3 mm), optimal for precision
  • Double scattering: larger penumbra (5 to 10 mm) due to complex scattering
• Scanning techniques (US/PBS): intermediate penumbra (3 to 8 mm) depending on beam size

This lateral penumbra remains significantly lower than that of other particles, ensuring good geometric precision. Collimation systems allow adaptation of the beam shape to the exact tumor contour, optimizing treatment selectivity.

Application to ocular tumors

Single scattering passive diffusion with fixed horizontal beamlines remains the gold standard for ocular tumors because:

• Appropriate simplicity: the eye (approximatively 24 mm diameter) and ocular tumors do not require complex systems
• Maximum stability: no dose fluctuation, crucial for preserving sensitive structures (optic nerve, macula, lens)
• Optimal positioning: horizontal configuration facilitates immobilization and reproducible positioning
• Rapid execution: irradiation times of a few seconds, minimizing the impact of ocular movements, particularly important in children

This approach ensures optimal precision for treating ocular tumors while preserving vision when possible.

With an energy of 65 MeV, the MEDICYC isochronous cyclotron in Nice is particularly well-suited for the treatment of ocular tumors. This relatively low energy allows for precise targeting of superficial structures such as the eye without requiring significant beam degradation, thereby minimizing unwanted multiple scattering effects in tissue.

Thanks to this optimized configuration, MEDICYC offers some of the highest ballistic precision in the world, featuring:

• a sharp distal dose fall-off
• an extremely narrow lateral penumbra

1. Initial step: surgical procedure

Before starting proton therapy, a preparatory surgical procedure is usually necessary. The type of surgery depends on the location and nature of the tumor.

• For conjunctival tumors:
  A surgical excision is performed to remove all or part of the tumor. In some cases, small tantalum markers may be sutured onto the sclera (the white outer layer of the eye) to allow for highly precise targeting during proton therapy. In this context, proton therapy serves as an adjuvant treatment, complementing the surgical removal of the tumor.
• For intraocular tumors:
  There is no tumor excision in this case. Instead, surgery is performed to suture tantalum markers to the sclera. These small metallic (non-ferromagnetic) clips—compatible with MRI imaging—serve as radiological reference points that are essential for accurate treatment planning.

After this procedure, a healing period of two to four weeks is typically required before proceeding to the next steps of the treatment process.

2. Preparation phase: Planning and Simulation

Before arriving at the proton therapy center, all relevant medical data—such as reports and imaging studies—are sent ahead to the team responsible for creating your personalized treatment plan. This team includes radiation oncologists, dosimetrists, and medical physicists.

Once at the proton therapy center, several key steps will take place:

• A CT scan is performed to assess the size and shape of the eye.
• A custom mask and dental impression are made to ensure remaining completely still during treatment.
• X-rays are taken to precisely locate any surgical markers (clips) placed during the preparatory procedure.
• The treatment plan is then designed and calculated, tailored to tumor’s location and depth.
• A simulation session is carried out to verify that the treatment is feasible and accurate.
• A final verification session is conducted to ensure all treatment parameters are correct before beginning therapy.

Detailed simulation process

3. Treatment phase: proton therapy

Proton therapy is delivered in multiple sessions, called fractions. The number of sessions and the total dose vary depending on the type of tumor, typically ranging from 4 to 8 sessions, spread out over one to two weeks. However, for most ocular tumors, the total time spent at the proton therapy center usually does not exceed two weeks.

During each treatment session, you will be positioned exactly as you were during the simulation. Once the alignment is confirmed with high precision, the treatment staff will leave the room to begin the irradiation. You will be alone in the room for less than one minute.

While the treatment is being delivered, you will be continuously monitored by two cameras: one focused on your entire body and chair, and the other zoomed in on your eye. This setup allows the team to immediately stop the treatment if your eye moves out of position. If this happens, the staff will re-enter the room and carefully reposition you before resuming the session.

The actual delivery of the proton beam lasts about ten seconds (in Nice) but may vary slightly depending on the center. The beam is activated from a control console outside the room. You may hear a loud but tolerable noise as the beam is turned on, and you’ll notice several lights operating around you during the session. The treatment is painless.