This is used to determine the optimum voltage from the deflection coils in the X and Y directions and isotropy settings of the XRS prior to treatment. It contains five diodes placed on the four side faces and the top face of a cube, such that all are equidistant from the centre of radiation of the XRS probe. When the probe is inserted into the PDA, the axis markings on the side of the PDA housing must be aligned with the matching axis marking on the XRS housing. Signals from the PDA will be displayed on the user terminal measuring the distribution (isotropy) of X-rays emitted from the tip of the probe.

The PAICH (Fig. 3.1) has two primary functions: (1) to maintain straightness of the XRS probe and (2) to provide a means of accurate positioning of an ionisation chamber in relation to the probe tip. It is essential for the XRS probe to be straight in order to maintain an isotropic radiation output. The “runout” (deviation from perpendicular) of the probe is determined using the probe adjuster. Probe straightness is measured optically to 0.1 mm by a method in which the probe intercepts a portion of a light beam from an LED across one edge of the tip of the tube. The light beam is detected as it strikes an opposing photodiode. As the PAICH is rotated around the probe, any mechanical deviation of the tube from the axis of rotation will be represented by a change in the light detected.

A shielded IC (type PTW 23342 for very low radiation therapy energies) parallel plane chamber is used to verify the output of the XRS and to calibrate the IRM readings. The electrometer (PTW Unidos E) measures and displays the current generated by the IC. The customised shield serves not only to provide radiation safety but also to reproducibly position the chamber inside the PAICH with respect to the tip of the probe whenever taking output measurements.

Older INTRABEAM systems (i.e. PRS 400) utilised a radiation detector external to the XRS, measuring the radiation exiting the patient. The ERM used detectors with the purpose of independently monitoring the constancy of the radiation emitted by the XRS and during the course of treatment, thus providing a further backup to the timer for controlling dose delivery and end of treatment. New systems do not include this external monitor.

Spherical applicators, ranging from 1.5 to 5 cm in 0.5-cm increments, fill the entire cavity left after tumour excision. The XRS probe is located in the centre of the applicator, and thus the tumour bed. The material of the applicator is medical-grade acrylic and it can be steam sterilised up to 100 times. For smaller applicators, aluminium is inserted around the cavity where the tip of the probe will be present (Fig. 3.2). This is to remove lower energy characteristic line from the radiation spectrum due to the presence of components at the probe tip other than the gold target material. For larger applicators, the thickness of the material will be sufficient to harden the beam. The aluminium will also reduce the higher signal that would otherwise be obtained from radiation passing back through the probe as these photons go through less material with decreasing applicator diameter.

The treatment time for each applicator is worked out using the “transfer function” for the relevant applicator and PDD data of the bare probe as given in the calibration files.

The stand allows positioning of the XRS with 6 degrees of freedom. Electromagnetic clutches lock the radiation source in the treatment position. The stand requires careful balance setup for weight compensation.

The whole system features various interlock mechanisms to avoid unintended radiation emission and loss or incorrect data transmission. Radiation emission is audio and visually indicated on the X-ray source and user interface. Further safety systems such as a theatre door switch can be added to an external interlock switch on the INTRABEAM control console. The count rate is constantly monitored by the IRM and its ratio with the planned treatment time is checked via the control console. Any deviation from tolerance will stop any emission of radiation. The system also features an optical interlock able to detect whether the appropriate verification device has been correctly attached to the X-ray source, whether the source has been mounted correctly onto the floor stand and whether an applicator has been correctly attached to the floor stand. The INTRABEAM PRS500, and older versions, is not capable of detecting the applicator size. For this reason, it is recommendable to independently verify such parameters once the surgeon has attached it to the stand. Finally, the software follows a series of verification steps such that only when they have been correctly completed by trained and authorised staff will emission of radiation be allowed.

Ancillary components include connectors that will provide signal feedback and low-voltage power between different components; V&X block guides for the safety coupling between the XRS and pre-treatment components; and a protector sheath to cover the XRS probe while not in use or out of its carrying tray.

When logging in the user terminal, the software will display the “System Quality Assurance” tab by default. Once the XRS in use has been selected, the following optional and mandatory tests are performed prior to treatment: