Ch .4 Clinical Radiation Generators

The X-ray therapy in the kilo voltage range has been divided into four categories according to National Council on Radiation Protection.

Clinical Radiation GeneratorsTreatment Unit Kilovoltage Range Half-Value Layer Clinical Use

Grenz Ray » 20 kVp ≈ 0.04-0.09 mm Al Dermatological Use

Contact Therapy 40-50 kVp ≈ 1-2 mm Al Few mmSSD= 1.5 to 5cm Depth. 1-2 mm

Superficial Therapy

50-150 kVp ≈ 1-8 mm Al Skin LesionsSSD= SSD= 20 cm, FSD=15Depth= 5 mm

Orthovoltage 150-500 kVp ≈ 1-4 mm Cu Deep TherapySSD= 50cm, depth 1-2 cm

Supervoltage 500-1000 kVp ≈ 10 mm Pb Replaced by Co-60SSD= 80 cm

1.Grenz rays (border x-rays)1.Grenz rays (10 kV to 20 kV, 0.02 mm to 0.15 mm Al HVL) lie between the hardest ultraviolet rays and X-rays and are named from the German word Grenze (meaning boundary). They are sometimes also called border x-rays. Because of the very low depth of penetration (Fig. 4.1, line a) , such radiations are no longer used in radiation therapy.

2-contact therapy

2. Short distance or contact therapy. (10 kV to 60 kV, 0.02 mm to 3.3 mm Al HVL) was originally developed to reproduce the conditions of surface Radium treatment. The very short source skin distance (SSD) of typically 1.5 cm to 5.0 cm ensures a rapid fall-off in depth dose, even over a wide range of qualities, providing a useful treatment depth up to several millimeters (not deeper than 1-2 mm).

Examples of contact therapy

Cervical Cancer as example for - contact therapy

Methods for Cancer treatment

3-Superficial therapy

3. Superficial therapy (50 kV to 150 kV, 1 mm to 8 mm Al HVL)..The beam characteristics enable lesions to a depth of around 5 mm to be encompassed by 90% of the surface dose. The 90% dose in this range lies 5 mm beneath the incident skin surface at the usual treatment distance of 20 cm SSD while as FSD @ 15 cm.PDD = 100% at skin surfacePDD = 90 % at 5 mm water depth

Kilovoltage unit.

Face treatment using Superficial X-Ray

4. Orthovoltage therapy

4. Orthovoltage therapy or deep therapy (150 kV to 400 kV). Most clinical units in this range are operated with generating potentials from 160 kVp to 300 kVp corresponding to (0.5 mm to 4 mm Cu HVL) for typical filtrations. The 90% dose in this range lies 1 cm to 2 cm beneath the incident skin surface at the usual treatment distance of 50 cm SSD.

The percentage depth dose gradually varies with applied tube potential in this range. Although cones can be used to collimate the beam into a desired size, a movable diaphragm, consisting of lead plates, permits a continuously adjustable field size. Multi-energy units covering both the superficial and orthovoltage ranges are now used in many departments.There are severe limitations to the use of orthovoltage beam in treating lesions deeper than 2 to 3 cm. The greatest limitation is the skin dose, which becomes prohibitively large when adequate doses are to be delivered to deep-seated tumors.

Orthovoltage therapy

5-Supervoltage Therapy

X-ray therapy in the range of 500 to 1,000 kV has been designated as high-voltage therapy or supervoltage therapy.

6- Megavoltage Therapy

X-ray beams of energy 1 MV or greater can be classified as megavoltage beamsExamples of clinical megavoltage machines are accelerators such as Van de Graaff generator, linear accelerator, betatron and microtron, and teletherapy gamma ray units such as cobalt-60.

Clinical Radiation Generators