dosimétrie

What happens in a proton therapy session?

Céline Giustranti
12/12/2017
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Due to its precision, proton therapy requires very exact positioning of the patient. Several steps prior to treatment help to achieve this positioning.

The number of proton therapy sessions varies according to the tumor treated. In ophthalmology, treatment comprises four sessions on four consecutive days. For other tumors, treatment may exclusively use protons, particularly in children, or it may employ a proton/photon combination. Treatment through classic radiotherapy will be administered via another machine at Institut Curie. In both cases, there is a preparatory phase prior to treatment.

Preliminary exams

These are usually imaging exams (radiography, ultrasound, angiography, etc.) to precisely define the outlines of the tumor and of nearby organs. The dosimetrists and radiophysicists will then use special dosimetry software (TPS, or treatment planning system) to determine the best ballistics, the number of beams and their characteristics in order to deliver the most appropriate dose for the tumor, while not exceeding the limits of the dose to healthy neighboring organs, as prescribed by the physician.

 

Restraint and placement of beads

To ensure that the patient is positioned very precisely in the same way every day, radio-opaque markers and a personalized restraint system are used at the time of treatment.

  • The restraint: In most cases it is a thermo-formed plastic face mask. The basic plastic template is placed in hot water to soften it. It is then applied to the patient’s face and will mold to fit it. As it cools, the mask will harden and retain the form of the face, thus allowing the head to be replaced in the same position for each treatment session. This restraint is used for so-called “dosimetric” scan imaging exams and MRIs (magnetic resonance imaging), and for all treatment sessions (photons or protons).
  • Metal markers: To check the positioning of the patient each day in proton therapy, radio-opaque markers must be implanted in the cranial cavity. These are gold beads measuring 2 mm by 0.8 mm, or titanium screws. They are implanted by the radiotherapist or the surgeon in the skull bone under local anesthesia, or general anesthesia for children. They are visible on scan sections and on classic skull X-rays.
  • Clips in ophthalmology: The clips are placed, in the operating suite, by an ophthalmologist, under local or general anesthesia.  These are made from tantalum, and are thus visible on standard X-rays. They are about two mm in diameter.

 

A simulation session

Simulation is a preparatory session during which the patient is placed in treatment conditions. It checks that the treatment position determined by dosimetry is feasible, and ensures that the patient can tolerate it. This lasts between one and two hours, and is performed a few days before the start of radiotherapy.

 

The treatment

The treatment comprises several sessions, at the rate of one session per day, five days oer week for several weeks for inter-cranial tumors and in children, and four sessions for ophthalmological tumors. Each session may involve one to three beams per day.

The treatment session is identical in every way to that of the simulation, and starts with a series of radiology images to check and adjust the patient’s position.

The average duration of this placement is 35 minutes per beam.

The handlers leave the room to sit at the beam control desk. Several surveillance cameras monitor inside the room. The handlers can communicate with the patient at any time using an interphone.


The radiation lasts one minute on average, during which the patient feels nothing (neither heat, cold, pressure nor pain).