Features of Kilovoltage X-rays
Kilovoltage X-ray equipment has been used therapeutically for the treatment of skin cancers for more than 100 years. More recently, in some centres, its use has diminished as megavoltage (the use of high energy X-rays delivered using linear accelerators) and electrons (often preferred because their depth of penetration can be accurately prescribed according to the energy used) have replaced it for many clinical applications. Kilovoltage X-rays units have the advantage of being low in cost compared with megavoltage units because of the relative simplicity of design and operation, and the use of simple collimation and beam shaping. The housing of such equipment requires less in the way of radiation protection compared to linear accelerators. (Linear accelerators are often housed in the basements, whereas superficial machines can be placed on the ground floor.) The use of a kilovoltage machine means that departments can optimise the use of their megavoltage machines for treating other non-cutaneous malignancies. X-ray therapy in the kilovoltage range is conventionally divided into four areas:
- Grenz rays of 10–20kv (kilovolts). These are still used in some centres, particularly in Germany.
- Contact therapy using 40–50kv and short sourcesurface distance (SSD). These are used for treatment depths of 1–2mm as they provide very rapid falling depth-doses.
- Superficial therapy using 50–150kv. With typical filtration, beams with half-value layers (HVLs) in the range of 1–8mm Al. The SSD is normally 10–30cm. Beam characteristics are suitable for treatment of lesions up to 5mm deep, delivering 90% of the dose to the surface.
- Orthovoltage therapy (‘deep therapy’) with 150–300kv. Most clinical units operate at 200–300kv. The treatment distance is about 50cm SSD. Ninety per cent of the dose delivered within approximately 2cm of the surface.