Radiology 101 » Radiotherapy

History of Radiation Therapy

Hariette A.W. — Fri, 27 Feb 2009 14:30:28 +0000

Radiation therapy has been in use as a cancer treatment for more than 100 years, with its earliest roots traced from the discovery of x-rays in 1895 by Wilhelm Röntgen.

The field of radiation therapy began to grow in the early 1900s largely due to the groundbreaking work of Nobel Prize-winning scientist Marie Curie, who discovered the radioactive elements polonium and radium. This began a new era in medical treatment and research. Radium was used in various forms until the mid-1900s when cobalt and cesium units came into use. Medical linear accelerators have been used to as sources of radiation since the late 1940s.
With Godfrey Hounsfield’s invention of computed tomography (CT) in 1971, three-dimensional planning became a possibility and created a shift from 2-D to 3-D radiation delivery; CT-based planning allows physicians to directly measure the dose delivered to the patient’s anatomy based on axial tomographical images. Orthovoltage and cobalt units have largely been replaced by megavoltage linear accelerators, useful for their penetrating energies and lack of physical radiation source.
The advent of new imaging technologies, including magnetic resonance imaging (MRI) in the 1970s and positron emission tomography (PET) in the 1980s, has moved radiation therapy from 3-D conformal to intensity-modulated radiation therapy (IMRT) and image-guided radiation therapy (IGRT). These advances have resulted in better treatment outcomes and fewer side effects.

Brief Overview of Radiotherapy

Hariette A.W. — Fri, 06 Feb 2009 05:33:21 +0000

Radiotherapy also known as Radiation Therapy, is a treatment of disease primarily malignant tumors using electromagnetic and particulate radiation.

Aims of Radiotherapy:

1. Radical Care – treatment is intended to cure the patient of his/her disease.

2. Palliative Care – relieve the patient of the distressing symptoms of the advance disease or cancer
- ease up the pain.

Two Types:

1. Teletherapy/ External Beam Therapy
application o radiation beam at a distance from the patient’s body.

3. Brachytherapy/ Plesiotherapy/ Internal Radiotherapy/ Intaluminal/ Intracavitary
- Introduction of radioactive source near or within the tumor.
- Very expensive

• Application of the Radiotherapy depends on the patient’s condition.

Positron Emission Tomography (PET) Cyclotron

Hariette A.W. — Fri, 06 Feb 2009 05:25:50 +0000

Built by Lawrence in 1929
Accelerates positive particles rather than the electrons
Uses positively charged particles such as proton, neutron, alphas, deuterons, short- lived radioisotopes
Particles are accelerated in a circular chamber called dees
The particles are accelerated by electric fields between the dees until the particle acquires the desired energy
Powerful magnetic filed are employed to confine the particle to a circular path
Protons are not commonly used for radiation therapy
In some centers, deuterons produced by cyclotron are brought out and allowed to strike targets of beryllium leading to the production of neutrons, such neutrons can be used for radiation therapy
Neutron beam is not universally available for therapy but have been tried clinically by various group in the US
Short- lived isotopes are used in radiopharmaceuticals research

LINACS (Linear Accelerators) and Its Major Components

Hariette A.W. — Fri, 06 Feb 2009 05:20:11 +0000

- gradually replaces the cobalt-60
- produces gamma rays, x-rays and electrons; more energetic than cobalt-60.
- operates at 4 MeV to 20 MeV
- the charged particles travel in a straight line as they gain energy from alternating EM field.
- higher energy beams can be generated with greater skin sparing.
- field edges are more abruptly design with less penumbra and personnel receive less exposure to radiation leakage.
- provides better isodose distribution (greater dose to the tumor and less dose to normal tissues)
- fastest dose rate and more manageable radiation protection concerns.

• LINACS producing electrons- for shallow lesions and superficial lesions

Major Components:

1. Drive Stand- contains the apparatus that drives the LINACS like gauges, tanks an buttons.

a. Klystron/ Magnetron- most important part of LINAC; provides the source of microwave power that is used to accelerate electrons.

b. Waveguide- hallow tubular structures.

c. Circulator- directs the RF energy into the waveguide and prevents any reflected microwaves from returning to klystron/magnetron.

d. Cooling System- allows many components or assemblies in the gantry drive stand to operate the constant temperature.
- absorbs the heat generated by the machine.

2. Gantry- most important; contains the source.
- responsible for directing the x-ray photon or the electron beam at the patient’s tumor.

a. Electron Gun- responsible for producing electrons and injecting them into the accelerator structure.

b. Accelerator Structure or Guide- It is where the microwave power is transported on which corrugations are used to slow up the waves.

c. Treatment head- contains the source; most important in the gantry.

d. Beam Stopper or Counterweight- optional; balances the lead shielding.
-made of lead and helps the machine rotate smoothly and provides additional shielding.

3. Control Console- monitors and controls the LINAC.

4. Treatment Couch or Patient Support Assembly (PSA)
- area on which the patients are positioned to receive their radiation treatment.
- looks like a table.

LINACs facility should be securely installed in steel buildings to protect both the workers and the patients.