The Radiotherapy Application NMT 232 L 8 1

Radiotherapy Generators Kilovoltage X-rays are generally used in the treatment of skin cancers and superficial tumors, whereas megavoltage X-rays are used in the management of deeply seated tumors. Megavoltage electrons, on the other hand, are used in the treatment of superficial tumors. 2

Kilovoltage Machines (<500 kV) Contact therapy machines Contact therapy machines * 40–50 kV * Filtered with 0.5–1.0 mm aluminum. * SSD = 2 cm *50% depth dose is 5 mm. Superficial therapy machines Superficial therapy machines 50–150 kV Filtered with 1–4 mm aluminum. SSD = 20 cm 50% depth dose is 1–2 cm. 3

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Megavoltage Therapy Machines (>1 MV) Van de Graaf generator Van de Graaf generator Also known as an electrostatic generator Produces energies of up to 25 MV 5

Cobalt-60 teletherapy unit (Co-60) Manufactured in Two gamma rays with energies of 1.17 and 1.33 MV Dose rate >150 cGy/min. SSD = 80–100 cm (for field sizes of 35 × 35 or 40 × 40 cm) 50% depth dose is 10 cm. The half-life of Co-60 is 5.27 years. Source is 2 cm in diameter, and has a large penumbra in comparison to a 4 MV liner accelerator. 6

Cyclotron A circular particle accelerator in which charged subatomic particles generated at a central source are accelerated spirally outward in a plane perpendicular to a fixed magnetic field by an alternating electrical field. A cyclotron is capable of generating particle energies of between a few million and several tens of millions of electron volts 7

Cyclotron 8

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Orthovoltage treatment machines These machines have similarities with machines used in diagnostic radiology. X-rays are produced by accelerating electrons into tungsten. These X-rays are shaped by collimators before they reach the tumor tissue. Essentially all of the dose is directed onto the surface of the skin; the percentage depth dose (PDD) curve drops sharply. Thus, deeply seated tumors cannot be treated using these machines. 11

Megavoltage treatment machines Megavoltage treatment machines are the foundation of modern radiotherapy techniques. Photon beams are produced in megavoltage treatment machines. Their one difference from orthovoltage machines is the skin-sparing effect (i.e., the maximum energy is delivered in the subepidermal region). The skin-sparing effect is directly proportional to the energy of the photons. 12

Cobalt-60 Teletherapy Unit Natural cobalt is a hard, stable, bluish-gray, easily breakable metal with properties similar to iron and nickel. Its atoms contain 27 protons, 32 neutrons, and 27 electrons. Nonradioactive cobalt can be found mixed with various minerals in nature, and has been used to impart a blue color to glass and ceramics for thousands of years. 13

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15 Cobalt-60 teletherapy unit Co-60 teletherapy units have a cylindrical source 2 cm in diameter. The activity of the source is generally between 5,000 and 15,000 Ci. A source with an activity of less than 3,000 Ci is replaced with a new one; this is necessary after 5–7 years of use. Co-60 teletherapy units provide good performance for tumors with depths of <10 cm. Thus, the use of a linac is recommended for more deeply seated tumors.

Treatment Head and Collimator 1- The cobalt source (orange) is situated in a drawer, and surrounded by lead 2- The collimator system; 3- manual system that can pull the source to the resting position in case of emergency 4- pneumatic system 5- link between the head and the rotating part of the machine that is used to change the source when its activity is no longer sufficient for treatment. 16

Gantry The source–isocenter distance (SAD) is 80–100 cm. The rotational movement of the gantry is motorized and controlled in two directions continuously; its rotation speed can be adjusted. The gantry can rotate by 360°. 17

Cobalt-60 head. 1- Cobalt-60 source; 2- tungsten cylinder; 3- enriched uranium; 4- lead; 5- laser source; 6- collimator; 7- γ-rays 18

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