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Physics

IORT using low-energy X-rays differs in some specific aspects from IORT using electrons from linear accelerators or dedicated IORT machines. The 50-kV X-rays of the INTRABEAM system have an increased RBE and a steeper dose falloff. Furthermore, the dose is delivered with a low dose rate over a prolonged time.

Soft X-rays (kV) have an increased RBE compared to standard linear accelerator X-rays (MV). Irradiation types which produce more locally concentrated ionisations, although at the same total amount of ionisations, are more effectively killing tumour cells and thus have an increased Relative Biological Effectiveness (RBE).

This is especially true for high Linear Energy Transfer (LET) irradiations like neutrons or heavy ions. Although soft X-rays have a somewhat higher LET than highenergy linac X-rays, they are not considered to be high LET irradiation. The increased RBE of soft X-rays is most likely a consequence of the fact that the total energy of the beam is deposited within a relatively short distance of the track ends, which means that the ionisation events are locally more concentrated within this area. The absolute value of the RBE of 50 kV X-rays depends on the dose, endpoint, tissue and many other factors.

With appropriate shielding, the risk to staff is very low, with doses to staff involved in the procedure at potentially negligible levels compared to natural background . Direct reading personal dosimeters can be used to monitor staff remaining to operate the equipment, or the anaesthetist. These have the advantage of instantaneous feedback of any higher doses.

Applying the principles of time and distance lead to the following practical measures:

  • All non-essential staff to leave the treatment area during irradiation;
  • Doors are locked to restrict access;
  • No pregnant staff to be involved in the irradiation, unless a specific individual risk assessment has been carried out.
  • INTRABEAM can be used safely in standard operating theatres.
  • Medical physicists should be closely involved.
  • Radiation protection is straight-forward but important .