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Main stages of the treatment course

The Medico-Technical Complex of JINR has been the first centre in Russia to implement and apply the method of 3D conformal irradiation of deep-seated tumours with a proton beam in therapy sessions.

These are the following basic methodological and technological stages of pre-irradiation arrangements and the exposure procedure:
  • Immobilization of the region to be irradiated.
  • X-ray and magnetic resonance tomographic imaging and incorporating of slices into the treatment planning software.
  • 3D computer planning of irradiation.
  • Manufacture of individual devices to form the beam – profiled collimators and compensative boluses.
  • Daily measurement of the proton beam parameters.
  • Implementation and verification of the exposure plan.
Immobilization and tomographic imaging

 Due to a strictly localized path and small side scattering, protons allow delivery of dose fields with sharp gradients. It becomes possible to irradiate neoformations that are localized directly near the critical radiosensitive structures and organs of the patient. However, to use these opportunities to the full extent it is necessary beforehand to organize a thorough planning of exposure. For this purpose, firstly, it is necessary to get the data on the 3D distribution of the patient’s tissue density in the region of the target location. It can be done with an X-ray computer tomograph (CT).

 Another basic requirement is to obtain full coincidence of the irradiated region position during diagnostics and in each subsequent session of the fractionated irradiation of the patient. In case targets are localized in the region of the head or neck, for each patient an individual immobilizing mask, made of perforated thermoplastic, is manufactured for safe fixation during a tomographic session or in the therapeutic armchair.

Tomographic imaging is taken at a spiral X-ray tomograph, with the patient lying and the fixation mask on. Usually, up to a hundred slices are measured with the step of 1-2 mm. Then the data in the digital form are incorporated into the 3D computer system of exposure planning. As an additional measure to adjust the spread borders of the neoformation, the magnetic resonance tomography, angiography and other techniques are applied.

 For a proton therapy session, the patient is fixed in a special therapeutic armchair, sitting. The armchair has a very flexible system of fixing and regulation devices that allow a high degree of the patient’s position reproducibility from session to session, as well as the patient’s immobility during the session, not only in the region of head and neck, but also in the chest.  

In a short time (less than 5 minutes) the armchair can be transformed into a deck with a support assembly for patients’ irradiation in supine position (for example, for prostate irradiation).

The design of the armchair also provides its easy adaptation for installing the majority of the standard certified individual facilities for the patient’s immobilization.

3D computer planning of the exposure

 The conformal ray therapy is impossible without computer simulation of the irradiation process. As a result of cooperation with the first in the world hospital centre of proton therapy in Loma-Linda, USA, the 3D computer system for proton irradiation planning “TPN” designed at the centre was adapted to the equipment and proton beams of the LNP JINR Phasotron. After a cycle of dosimetry experiments that verify the dose calculation algorithm, the system is used in clinical practice.

 A 3D package of topometric data obtained in computer tomography is incorporated into the treatment planning system in the digital form. A physician-radiologist outlines (segments) the irradiation target in each CT slice (blue colour – an intracranial tumour) and critical structures-organs (green colour – optic nerves).

 The planning system generates 3D models of outlined structures according to these data.

The physician-radiologist chooses the value and the number of irradiation directions-fields (usually, from 2 to 7). Using the programme function «beams-eye-view» and digital simulated X-ray radiograms for each field of irradiation, the proton beam of a certain form in the cross section is determined and outlined.

Fabrication of profiled collimators and boluses

 The therapeutic proton beam in real irradiation sessions is formed with a help of an individual collimator made of cerrobend alloy - to produce conformity of the dose distribution to the target shape in the cross section. In order to make the proton beam conformal to the distal margin of the target volume, planning system calculates 3D range shifters - boluses that allow for the heterogenic structure of tissues and organs located in the beam path.

Last step of preparation is manufacturing of individual collimators and boluses in the workshop of MTC.

Daily measurement of the proton beam parameters 

Every day before the irradiation session starts, a therapeutic proton beam is delivered to the treatment room, and its thorough dosimetry is conducted. The beam profiles, its depth-dose distribution, the dose rate are measured. Then these parameters are controlled directly during the irradiation session with a help of a special control system.

Implementation and verification of the exposure plan 

 As a rule, proton irradiation is conducted fractionally – every day, except the week-end days. An X-ray image of the patient, taken with an X-ray tube installed behind the patient at the beam axes and the digital X-ray screen, is produced for each irradiation direction proximately before the exposure. At the same time, the screen is exposed by the low intensity proton beam. As a result, the position of the proton beam with regard to the scull anatomical structures is clearly seen in the image.  If this position does not coincide with accuracy to 1 mm with the position that was calculated by the planning programme, the position of the armchair with regard to the beam is corrected. Immediately after that a proton beam therapeutic session is conducted.