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Electrons are emitted by thermoionic emission when the filament contained in the cathode (electron gun) is heated. These electrons are then accelerated by anodes along different accelerating sections from 50 to 0 V, in 10-kV increments. The effective emitted radiation energy is approximately 20 keV. Following this acceleration, the electrons enter a 10 cm long, 3.2 mm diameter probe. The first part of this probe, the snout or mounting collar, contains deflecting coils that steer the electron beam down the evacuated drift tube towards a thin (approx. 1 μ m) concave gold target at the hemispherical tip of the probe. Part of the energy produced from the interaction of the electrons with the target material is converted into radiation in the form of characteristic and bremsstrahlung radiation. These X-rays are generated in a nearly spherical distribution centred at the tip of the probe. The radiation that passes back along the path of the electron beam is detected by the IRM. The signal from the IRM is calibrated to the dose rate before the probe is placed in the patient for treatment. The integrated IRM output is used during treatment as a direct measure of treatment dose to the patient. The total exposure is then controlled by the accumulated counts monitored by the IRM as well as by a timer which serves as a backup.

The X-ray generator INTRABEAM delivers a highly localised dose of low-energy photons (50 kV) to the tumour bed immediately after wide local excision for early breast cancer. This modality differs from conventional external beam fractionated radiotherapy (EBRT) in several respects. First, the highly localised radiation field is characterised by a non-uniform dose distribution. In fact, the dose delivered to the cavity with INTRABEAM abruptly decreases in a manner proportional to the cube of the distance from the applicator. In contrast, EBRT delivers the same dose to the whole breast. A second important aspect to be taken into account is that intraoperative radiotherapy (IORT) distributes the total dose in a single fraction during surgery, thereby avoiding geographical miss and unwanted delay in RT application. A third important difference is that low-energy photons have an increased relative biological effect (RBE) compared with highenergy photons. Finally, the typical duration of the application (from 30 to 50 min), results in sublethal damage to the irradiated tissue. Since residual proliferating cancer cells are much less able to repair sublethal DNA damage in comparison with normal epithelial cells, the long treatment time of IORT with INTRABEAM is likely to result in a better therapeutic index.