Treatment Volume Definitions

Treatment Volume

• International Commission on Radiation Units and Measurements (ICRU) Reports No. 50 and 62 define target and critical structure volumes.
• These volumes are useful aids for treatment planning and allow for comparison of treatment outcomes.
• The volumes are:
  • Gross Tumor Volume
  • Clinical Target Volume
  • Internal Target Volume
  • Planning Target Volume

Gross Tumor Volume (GTV)

• It is the gross palpable or visible/ demonstrable extent and location of malignant growth
• This is according to the ICRU Report No. 50

Clinical Target Volume (CTV)

• It is the tissue volume containing a demonstrable GTV and/or sub clinical microscopic malignant disease.
• It must be eliminated through adequate treatment to achieve therapy, cure, or palliation.
• This definition is per ICRU report no. 50

Internal Target Volume (ITV)

• Consists of the CTV plus an internal margin

Planning Target Volume (PTV)

• PTV is a geometrical concept to select appropriate beam arrangements.
• It considers the net effect of possible geometrical variations.
• It ensures the prescribed dose is absorbed in the CTV, as noted in ICRU Report No. 50.

Methods in Radiation Therapy

• Teletherapy or External Beam Radiation Therapy (EBRT) is a radiotherapy technique.
• EBRT involves a radiation source at a distance from the patient.
• Brachytherapy or Plesiotherapy means placing radioactive substances or nuclides in or on a neoplasm.
• The purpose of Brachytherapy or Plesiotherapy is to deliver a cancercidal dose.

Classification of EBRT Machines

• Grenz Ray Therapy
• Contact Therapy or Endocavitary
• Superficial Therapy Units
• Orthovoltage Therapy or Deep Therapy Unit
• Supervoltage Therapy Unit
• Megavoltage Therapy Unit

Grenz Ray Therapy

• Uses very soft (low-energy) x-rays produced at potentials below 20kV
• The radiations are no longer used in radiotherapy because of their very low depth of penetration
• Formerly used for skin lesions

Contact Therapy or Endocavitary

• Operates at potentials of 40 to 50 kV
• Facilitates irradiation of accessible lesions at very short source (focal spot) to the surface of the skin (SSD of 2 centimeters)
• It operates with a tube current of 2 mA

Superficial Therapy Units

• Superficial therapy applies to treatment with x-rays produced at potentials ranging from 50 to 150 kV
• This equipment produces a large amount of soft (low energy) x-rays which can produce severe skin reaction.
• Useful for irradiating tumors to about 5mm depth
• Source-to-Skin Distance (SSD) typically ranges from 15 to 20 cm
• Operates at a tube current of 5 to 8 mA

Orthovoltage Therapy or Deep Therapy Unit

• Orthovoltage therapy, or deep therapy, describes treatment with x-rays produced at potentials ranging from 150-500 kV
• Reasonable tissue penetration of the resultant x-ray beam is achieved with filters

Supervoltage Therapy Unit

• X-ray therapy within the range of 500 to 1,000 kV is designated as high-voltage therapy or supervoltage therapy
• The electron attains high energies before striking the target so a transmission type target is an option

Megavoltage Therapy Unit

• X-ray beams of energy 1 MV or greater can be classified as megavoltage beams.
• The gamma ray beams produced by radionuclides are included in the category if their energy is 1 MeV or greater

Van de Graaf Generator

• It is an electrostatic accelerator designed to accelerate charged particles
• Accelerates electrons to produced high energy x-rays, typically at 2MV

Betatron

• It was developed by Donald Kerst
• Operation is based on the principle that an electron in a changing magnetic field experiences acceleration in a circular orbit
• The accelerating tube is shaped like a hollow doughnut and is placed between the poles of an alternating current magnet
• Betatrons were first used for radiotherapy in the early 1950s

Microtron

• It is a type of particle accelerator concept originating from the cyclotron
• Accelerating field is not applied through large D-shaped electrodes, but through a linear accelerator structure
• It was developed by Vladimir Veksler in 1944

Cobalt 60 Unit

• Production involves irradiating ordinary stable Co-59 with neutrons in a reactor to produce Co-60
• The Co-60 source is usually in the form of solid cylinders, discs, or pellets
• Beta decay energy is 2.824 MeV, and Gamma rays are produced at 1, 173,210 and 1,332,470 Ev (1.17MeV & 1.33 MeV) energies with nearly 100% frequency of occurrence.
• Features a half-life of 5.27 years.
• Delivers an energy average of 1.25 MV gamma ray
• Cobalt 60 undergoes beta decay to become activated Nickel 60

Gamma Knife

• Invented shortly after cobalt teletherapy
• Uses multiple cobalt sources (usually 201 sources) with holes (apertures) that direct focused radiation to a spherical isocenter
• The size of the isocenter sphere can be changed by changing the size of the apertures.
• The position of the isocenter can only be changed by moving the head to different positions within the helmet.
• It is still used today to treat intracranial lesions

Linear Accelerator (LINAC)

• Most commonly used machine for external beam radiation treatments
• Used to treat all parts/organs of the body
• Delivers high-energy x-rays or electrons to the region of the patient's tumor
• Developed by Wilderoe in 1928 to accelerate heavy ions
• Electron accelerators were first developed during the late 1940s

Parts of LINAC

Gantry

• In external beam radiotherapy, the gantry moves the radiation source around a patient

Drive Stand

• A large part of a linear accelerator and supports the gantry
• Allows the gantry to rotate on a fixed horizontal axis
• It is a cabinet in the shape of a rectangle and is attached to the floor within the treatment room
• The horizontal axis bearings the gantry rotates on are positioned within the drive stand

Modulator

• Provides high voltage pulses to the microwave transmitter (klystron)

Electron Gun

• It produces electron to be accelerated
• Located in the stand of the linac

Magnetron

• It is an electron tube that provides microwave power to accelerate electrons
• Often chosen for linacs needing smaller amounts of electron energy such as 4 MeV to 6 MeV linacs
• It can be used in place of a klystron

Klystron

• Provides microwave power to accelerate electrons
• This process occurs through intensification of present RF (Radio Frequency) electromagnetic waves
• It functions as an RF amplifier
• Often chosen for LINAC needing larger amounts of electron energy

RF Wave Guide

• It is a vacuum tube where electrons are accelerated
• The microwave powered RF electromagnetic waves accelerate, from the Klystron or Magnetron.

Bending Magnet

• It is a component of a LINAC
• It changes the direction of the beam toward the patient
• It bends the beam towards the target, producing different paths for different energy needs

Target

• Made of tungsten and is transmission type
• It is the source of x-rays

Flattening Filter

• Placed to attain homogenous intensity of the x ray beam in the entire field

Scattering Foil

• Used in electron mode of linac operation
• The narrow pencil electron beam is made to strike an electron scattering foil to spread the beam
• Obtains a uniform electron fluence across the treatment field

Ion Chamber

• It is an integrated ion chamber (IC) that monitors the radiation beam in the linac
• Consists of two layers for dosimetry and one to measure the spectrum of the beam
• Symmetry is measured with two sets of plates
• These plates measure a tilt in symmetry as percentage of dose

Collimators

• Restrict the x-ray beam to a desired field size

Primary Collimator

• May be constructed of depleted uranium, which is approximately 1.6 times denser than lead
• It is a Fixed collimator

Secondary Collimator

• The secondary motorized collimators define the beam shape
• They are constructed of lead or tungsten 84 and are Movable

Multi-Leaf Collimator

• Multi-leaf collimators (MLCs) are in widespread use in medical linacs
• Consist of two collimator banks of thin tungsten 'leaves', with each bank consisting of 40 to 80 leaves
• Each collimator leaf can move independently under computer control

Other Collimation Systems

Cerrobend

• Prior to the development of MLCs, radiation beams were shaped by pouring physical blocks in the shape of your target out of liquid metal
• It is a mix of bismuth, cadmium, and lead, which creates an alloy with a melting point of 70 degrees Celsius
• Intensity of a beam may be shaped by making some parts thicker and some parts thinner in a process called compensation

Apertures

• Small holes allow radiation through with a specific shape (usually a circle) and size
• Some apertures change size using an iris mechanism, and are often used in stereotactic systems

Wedges

• Made of metal, often steel
• Multiple gradients of wedges can be inserted into the head, giving a wedged profile to the beam

Light Fields

• A light within the head of the linear accelerator shines through the collimators to give shape to the field
• Used as a guideline for treatment setup
• Field size is defined at isocenter, meaning unless the patient’s skin is at the isocenter, the actual field size will be different

Electron Applicator

• It is an Electron collimator
• Restricts the electron beam to a desired field size because the restriction of the primary and secondary collimators is not enough