Hazards associated with higher doses of radiation have led to strict legal requirements, which will be specific to each country. Most of Europe derives its laws from the European Union directives, given in Table 5.2. There will also usually be a national license or authorisation to use radiation, which must be obtained before beginning to use any material or equipment.

In the UK, safety of staff and members of the public is covered by the Ionising Radiation Regulations (1999) of the Health and Safety at Work Act (1974). These define the role of a Radiation Protection Adviser, who is the qualified radiation expert for the organisation, and a Radiation Protection Supervisor, who supervises compliance with the specific procedures relating to the safety of the radiation technique (the Local Rules). Safety of the patient is covered by the Ionising Radiation (Medical Exposure) Regulations (2000), which require any radiation exposure to be justified by a defined practitioner (usually a radiation oncologist) and delivered by defined operators. Both the staff operating the INTRABEAM system and the surgeon who positions the applicator are considered to be operators and should be named in the local procedures.

In the USA, the system is still considered to be an emerging technology and although it has received Food and Drugs Administration clearance, it is not currently covered by Nuclear Regulatory Commission regulations. The specific requirements of each state will also vary. Guidelines have been produced by the American Association of Physicists in Medicine (AAPM, Thomadsen et al. 2009), which are summarised in Table 5.3.

All of these documents describe the close involvement of experts in radiation physics, who should be consulted with regard to both commissioning a new service and ongoing clinical support.

The philosophical basis underlying radiation safety is to keep doses to staff (and others not undergoing treatment) as low as reasonably practicable (ALARP).²

Four general principles can be used to achieve this:

Radiation exposure is directly proportional to time, but follows an inverse square dependence on distance. For example, if the distance is doubled, the exposure is reduced by a factor of 4. Therefore distance can have a large impact on doses received. As mentioned earlier, less shielding is required for lower energy X-rays.

Because the INTRABEAM system is an electronic X-ray source, there is no issue of contamination, since the radiation exposure only occurs when the beam is on. If any intervention for the patient is required, the radiation can be switched off (paused) and poses no further hazard during this time.

Radiation dose may be measured using a number of different units. The most fundamental is the Gray (Gy), which is commonly used to describe the amount of radiation given therapeutically, since it corresponds to the energy deposited per unit mass and can be readily measured. However, effective doses to individuals for protection considerations are quoted in Sieverts (Sv), which take account of the relative biological effect of different types of radiation and different sensitivities of different tissues in the body. Typical values are thousandths (mSv) or millionths (μSv) of this unit, and the latter will be used for all subsequent quoted values, since many detectors read in this scale.

Although current models for risk from radiation equate any dose with some risk, it is important to consider small exposures in context. Every individual receives some exposure to ionising radiation from natural background sources, listed in Table 5.4, as well as any medical exposures. Some comparative doses for common activities, legislative limits and adverse effects are given in Table 5.5.