Physical and technological principles of radiation therapy with accelerators.

The therapeutic effect is based on the capacity of ionizing radiation to break DNA, thus preventing the proliferation of cancer cells. Since EBRT is a transdermal treatment, the therapeutic effect is inevitably associated with an effect of biological damage to healthy tissues that are passed thru' during irradiation, this damage can be made tolerable by limiting the radiation dose and diluting it over time.

The IORT technique represents a significant advancement in surgical settings and is an innovative, powerful radiation therapeutic strategy.

Linear accelerators, known as LINAC (acronym for linear accelerator), produce high energy electrons for medical applications.
To produce such electrons, accelerating structures are built that contain a high frequency electromagnetic field (GHz) which gives the electrons the accelerating energy.
The source of electrons is a Magnetic gun.

If the accelerator should be used as a source of X-rays, electrons are directed against a high atomic number element target (eg. Tungsten) that produces the phenomenon of X-ray bremsstrahlung, with an energy spectrum characteristic.
The high energy electron beams are then produced by making them collide with a “scattering foil' that increases the beam focus to allow for patient treatment on a large area.

IORT (Intra-Operative Radiation Therapy) which is considered a particular radiation therapy technique requires targeted dosimetric determinations, sometimes different from those required for carrying out treatment with fractionated external beam radiation therapy.

The main objective of IORT is to enhance the therapeutic index and balance between local tumor control and the tolerability of surrounding healthy tissues. All this is obtained through a more precise and visual definition of the target volume and the shielding or dislocation of anatomical organs at risk (OAR) during treatment.
With IORT, a single dose of irradiation with electrons is delivered to a visual target volume, at an extension and depth determined by the Radiation Oncologist during surgery and directly performed in the operating room during surgery. The shape and dimensions of the applicator, the energy and the isodose are also determined during this process.

Another advantage with the IORT technique when compared to EBRT is the use of a range of specific applicators that help determine the physical and geometric characteristics of the electron beam (quality, output, homogeneity, etc.). These applicators are made of perspex (eg Polymethylmethacrylate, PMMA) and are generally of circular section with diameters between 3 cm and 10 cm, for particular treatments rectangular or elliptical sections are used.
Circular applicators may present an oblique terminal end, inclined in respect to the geometric axis of the beam, with angles of 15° up to 45° (angulated applicators).
The applicator's length which can depend on its dimension, in some cases determines the source-skin distance.