OTHER MACHINES

1
OTHER MACHINES

• Chapter 10 SS
• Chapter 7 W/L Part 2

2
Low Energy Machines

• Limitations
• Can not reach deep-seated tumors with an adequate
  dosage of radiation.
• Do not spare skin and normal tissues.
• Kilovoltage filtration
• Inherent glass envelop (similar to x-ray tube),
  insulating oil between tube and housing.
• Added clinical use, thin sheets between target
  and collimator to decrease weak x-rays that add
  skin dose (Cu, Al).

3
Central Axis Depth Dose

• Central axis depth dose and physical penumbra are
  related to beam quality.
• The central axis depth dose distribution for a
  specific beam depends on the energy.

4
Isodose Curve

• Isodose curve a line representing various points
  of similar value in a beam along the central axis
  and elsewhere
• The depth of an isodose curve increases with beam
  quality (tells depth dose for different energy
  beams)
• The absorbed dose in the medium outside the
  primary beam (physical penumbra) is greater for
  low-energy beams than for those of a higher
  energy.
• Limited scatter outside the field for megavoltage
  beams occurs because of predominantly forward
  scattering of the beam.

5
(No Transcript)
6
Radiation Therapy Equipment

• Low-energy machines Uses x-rays generated at
  voltages up to 300 kVp
• Primary application is in the treatment of
  superficial lesions.
• Kilovoltage units include
• Grenz 10-15 kVp
• Contact 50 kVp
• Superficial 50-150 kVp
• Orthovoltage 150-500 kVp
• Super Voltage 500-999 kVp use???

7
Grenz rays

• 10-15kvp
• Grenz rays are almost entirely absorbed in the
  first 2 µm of skin and have a useful depth dose
  range of about 0.5 µm.
• Treatment of inflammatory disorders (langerhans
  cells), bowens disease, patchystage mycosis
  fungoides, herpes simplex

8
Contact therapy

• Superficial skin lesions, treatment unit comes in
  contact with patient.
• Endocavitary treatments for curative intent
  (rectal). Advantage- preserves sphincter
• Low to middle third of the rectum
• Confined to bowel wall
• Maximum tumor size of 3 x 5 cm
• Hemangiomas
• 4 treatments of 3000 cGy each, separated by a 2
  week interval
• SSD 4 cm
• 1 µm aluminum filtration

9
Superficial equipment

• 50-150kVp
• 1-6 µm aluminum filtration
• Cones 2-5 cm diameter
• Pb cutouts
• SSD 15-20 cm
• Skin cancer and tumors no deeper than 0.5 cm
  treated as a result of the rapid falloff of the
  radiation.

10
Orthovoltage machines

• 150-500kVp
• HVL 1-4 mm Cu
• SSD 50 cm
• Skin, mouth, and cervical carcinoma treatment
  last several minutes
• Experience limitation in the treatment of lesions
  deeper than 2 to 3 cm.

11
Cobalt-60 Machines

• Introduced in the 1950s, being replaced by
  linacs.
• The first practical radiation therapy treatment
  unit to provide a significant dose below the skin
  surface and simultaneously spare the skin the
  harsh effects of earlier methods.
• Still used in developing countries simpler
  design, cost, little tech support.

12
Cobalt 60 Machines

• Constantly emit radiation.
• The 60Co source must be shielded in a protective
  housing (source head).
• The source head is a steel shell filled with lead
  (may be up to 2 ft in diameter).
• Uses a counterweight to balance the lead
  shielding in the head of the machine housing,
  (similar to the beam stopper on linac) absorbs a
  significant amount of radiation transmitted
  through patient.

13
Linear Accelerators vs. Cobalt 60

• Linear accelerators
• Provide better isodose distribution (greater dose
  to the tumor and less dose to normal tissue)
• Higher and faster dose rate
• More manageable radiation protection concerns.

14
Application

• The 60Co unit is commonly used today to treat
  cancers of the head and neck area, breast, spine,
  and extremities.
• Areas just below the skin surface.
• Ideal in treating lymph nodes.
• A 60Co beam can provide an adequate distribution
  of dose by using parallel opposed fields.

15
Cobalt-60 Production

• Cobalt produced in nuclear reactors by the
  irradiation of neutrons of the common stable form
  of 59Co.
• The 59Co nucleus absorbs a neutron in the reactor
  and becomes 60Co.

16
Cobalt 60

• Radioactive 60Co produces a useful therapy beam
  when it undergoes beta decay.
• The nucleus emits a beta particle and then two
  photons, 1.17 MeV and 1.33 MeV for an effective
  energy of 1.25 MeV
• 60Co ? 60Ni B- neutrino (v) gamma rays
• Radioactive 60Co emits radiation in the form of
  high energy gamma rays in an effort to return to
  its more stable state.

17
60Co Source

• The overall diameter of a 60Co source is 1 to 3
  centimeters.
• Consists of pellets of radioactive 60Co encased
  in multiple layers of welded metal to prevent
  contamination of the environment and to absorb ß-
  particles produced by the decay process.
• Or 60Co sources are made with the 60Co fused into
  a solid cylinder. Advantages
• Smaller source with less penumbra for the same
  beam intensity
• Less hazard of contamination should a source ever
  become exposed to the environment.

18
60Co Activity

• 60Co activity may be expressed in curies (Ci),
  the historical unit of radioactivity
• 3.7 x 1010 Becquerel (Bq)
• 1 Bq 1 disintegration per second
• May also be defined in rhm units (roentgens per
  hour at 1 meter)
• Most sources have an activity of 750-9000 Ci,
  typically 3000-9000 Ci used in radiation therapy

19
Half-life

• Half-life the time necessary for a radioactive
  material to decay to half or 50 of its original
  intensity.
• Requires a correction factor for this decay of
  about 1 per month in all treatment calculations.
• Source must be replaced at about five year
  intervals.
• The half-life of 60Co is 5.26 years.

20
60Co Shielding

• Cerrobend (Lipowitz metal) lower melting point
  than Pb, cheaper
• 50 Bismuth
• 26.7 Lead
• 13.3 tin
• 10 Cadmium (a toxic metal can get into
  bloodstream)
• Density ratio of Cerrobend to Lead 1.2 cm
  Cerrobend to 1 cm lead.
• 5 HVL is needed to reduce intensity
• A thickness of 7.2 cm of Cerrobend needed, 6 cm
  lead.

21
Penumbra

• Penumbra the area at the edge of the radiation
  beam at which the dose rate changes rapidly as a
  function of distance from the beam axis.
• Describes the edge of the field having full
  radiation intensity for the beam compared with
  the area at which the intensity falls to 0.
• The larger the source size, the larger the
  penumbra
• P S(SSD-SDD)/SDD
• When depth is given
• P S(SSD D -SDD)/SDD
• Larger field sizes are necessary to cover the
  same amount of tissue adequately compared to the
  linac.

22
Penumbra

• Geometric penumbra the place where a lack of
  sharpness or fuzzy area occurs at the edge of the
  beam
• Occurs at the skin surface and greater depths in
  tissue.
• Transmission penumbra occurs as the radiation
  passes through the edge of the primary
  collimators
• Occurs at the edge of the patients shielding
  blocks mounted or placed below the collimator
• Correlates with the size of the collimator
  opening- larger field sizes have more
  transmission penumbra

23
Reducing Penumbra

• The transmission penumbra can be reduced by using
  satellite collimators, penumbra trimmers or
  trimmer bars.
• Trimmers are metal bars that attenuate the edge
  of the beam providing a sharper field edge.
• Should be placed no closer than 15 cm from the
  patients skin to reduced electron contamination
  (increased skin dose) by metal devices.
• Provides enough distance for the secondary
  electrons produced by the trimmer bars to lose
  sufficient energy

24
Dose Maximum

• Dose maximum (Dmax) when a greater percentage of
  dose occurs below the skin surface
• Dmax is the depth of maximum buildup, in which
  100 of the dose is deposited.
• For 60Co, Dmax occurs at 0.5 cm below the skin
  surface.
• Electron equilibrium is another term used to
  describe Dmax. As energy increases, so does the
  depth of electron equilibrium.

25
Beam On/Beam Off

• Turning the beam on requires physically exposing
  the source either by moving it into position or
  by removing shields around the source.
• Air pressure the compressor generates air
  pressure by pushing the source horizontally into
  position over the collimator opening.
• Rotating wheel the motor rotates a wheel 180
  degrees by placing the source over the collimator
  opening.

26
Warning Lights

• Red light Radiation present- do not enter room
• Green Light time elapsed
• Malfunction both red and green lights still on-
  means that machine is still in on position after
  prescribed dose has been delivered. Remove
  patient.

27
Timer Error

• Timer error (TE, or travel time correction,
  shutter error) is a consequence of the physical
  motion of the source. Even if its less than 1
  second.
• The difference between the beam on time setting
  and the time that the source is in the treatment
  position, the time it takes to advance and
  retract the source
• Timer error is determined at the time of machine
  beam calibration and is checked monthly.
• TE (R1t1-R2t2)/R1-R2
• The timer setting necessary for a treatment is
  the time calculated plus the timer error.

28
Quality Assurance

• Required be the Nuclear Regulatory Commission
  (NRC)
• A qualified radiation physicist must perform full
  calibration testing annually. More frequent if
• The source is replaced
• A 5 deviation is noticed during a spot check
• A major repair requiring the removal or
  restoration of major components is done.

29
Quality Assurance

• Written directive the prescription, must be
  clear, unambiguous, and signed by a licensed
  authorized user.
• Recordable event must be reported to the
  RSC-Radiation Safety Committee
• A delivered dose more than 15 in excess of the
  weekly prescription
• No WD
• Lack of daily recording of dose
• Misadministration must be reported immediately to
  the NRC
• Treatment of the wrong patient, wrong site
• Weekly dose more than 30 in excess of the
  prescription
• The delivery of more than 20 (25???) total dose
  

30
Advisory Agencies

• Analyze and assess data, and then develop
  recommendations for dose limits
• National Council on Radiation Protection and
  Measurement (NCRP) and ICRP
• Recommendations may be made into law by state or
  federal government.

31
Regulatory Agencies

• License users of radioactive materials and
  radiation producing equipment inspection
  enforcement of laws.
• Nuclear Regulatory Commission (NRC) oversees
  isotopes produces in nuclear reactors,
  teletherapy, and brachytherapy.

32
Tests

• The integrity of the source must be tested twice
  a year to ensure the sealed source continues to
  be totally sealed.
• Wipe tests wipe collimator edges with a filter
  paper, background radiation reading done,
  activity is determined (acceptable under 0.005
  mCi)
• Radiation and light field coincidence
• Timer accuracy
• Dose rates
• more.

33
Leakage Limits

• Leakage in the off position
• Cannot exceed 2 mrem/hr at 1 meter
• Maximum of 10 mrem /hr at 1 meter at any
  measurable location at any time
• Leakage in the on position
• Cannot exceed 0.1 of the useful beam at 1 meter
  from the source.

34
(No Transcript)
35
Betatrons

• Developed by Kerst in 1941
• Can provide x-ray and electron therapy beams from
  less than 6 to more than 40 MeV.
• Applied to industrial radiography- used in World
  War II
• Operation is based on the principle that an
  electron in a changing magnetic field experiences
  acceleration in a circular orbit.

36
Betatrons

• Accelerating tube shaped like a hollow doughnut
  and placed between the poles of an AC magnet.
• A pulse of electrons is introduced at the moment
  the AC cycle begins
• As the magnetic field rises, the electrons
  accelerate continuously around the tube.
• Energy depends on the number of revolutions and
  the speed.
• By the end of the first quarter cycle, electrons
  are made to spiral out of orbit, then strike a
  target to produce x-rays or a scattering foil to
  produce a broad stream of electrons.
• By choosing the timing of electron injection and
  extraction, continuous control of electron energy
  is possible

37
Betatrons

• Most often used for electron therapy
• Can produce x-ray beams with energies over 40MeV,
  though cannot compare with dose rates of linacs.
• Used Lucite cones to treat gynecologic, bladder,
  and prostate carcinomas.
• Treatment times 3-5 min.
• Disadvantage Machine is noisy, bulky- isocentric
  design impossible, non-isocentric, slower dose
  rates
• Makes small field sizes and precise patient setup
  cumbersome.

38
(No Transcript)
39
Van de Graff Generator

• Developed by R.J. Van de Graff while working at
  the MIT
• First electrostatic Linear accelerator
• Capable of accelerating either positive or
  negative ions.
• Disadvantages
• Size
• Warm-up took as long as 1 hour.
• Used a front pointer device to measure the
  distance to the patient because no ODI was
  available.

40
Van de Graff Generator

• An insulating belt transports electric charge to
  a collector screen within a metal dome
• The accumulation of charge produces the high
  voltage used to accelerate charged particles
• The maximum possible x-ray energy in MeV is
  directly proportional to the voltage on the dome
  and on the total length of the machine- limits
  the voltage of clinical machines to a few MV
• Smaller source size ? smaller penumbra

41
Van de Graff Generator

• Operate at 200cGy/minute.
• Provide a standard SSD of 100 cm and can
  approximate much greater treatment distances-
  useful in the treatment of extended fields.
• Used to treat seminoma, whole brain, and mantle
  field (used to treat lymph nodes in the neck and
  thorax for Hodgkins disease).

42
(No Transcript)
43
Cyclotrons

• Developed by E.O. Lawrence in 1928.
• Accelerate heavy charged particles (protons,
  deuterons, positive nuclei, etc)
• Use large magnets to confine the charged
  particles to a circular or spiral path.
• Particles are accelerated by an oscillating
  electric field.
• Medical uses of the cyclotron
• Production of radionuclides applied in nuclear
  medicine.
• The use of neutrons and protons in radiation
  therapy.

44
Dees

• Metal half-disks with a hollow evacuated center
  through which the particles can travel
• Two dees of opposite polarity are separated by an
  alternating voltage produced by a high frequency
  generator, pull the particles back and forth in a
  spiral pattern.
• Charged particles enter the center of one of the
  dees with a moderate velocity and are bent in a
  circular path by the magnet.
• When the particles enter the gap between the
  dees, they experience the high frequency, high
  voltage electric field and are accelerated,
  reenter the opposite dee, only magnetic force
  within the dee- no velocity increase.
• Final energy is equal to the sum of the energy
  gained at each gap crossing.

45
(No Transcript)
46
(No Transcript)
47
Theory of Relativity

• Acceleration causes particles to gain mass.
• The increase in weight causes to particles to
  slow down and be out of sync with the frequency
  of the alternate potential applied to the dee.
• Electrons cannot accelerate in a dee.

48
PET

• Cyclotrons are used for the production of
  radionuclides used in PET.
• PET scanning technique that involves the
  systemic administration of a radiopharmaceutical
  agent labeled with a positron emitting
  radionuclide.
• PET scanners are used in nuclear medicine to
  measure important physiologic and biomedical
  processes such as blood flow, oxygen, glucose and
  metabolism of free fatty acids, etc.

49
PET

• PET scanners use radiation emitted from within
  the patient to produce images.
• Anatomy can be evaluated in microscopic detail-
  patients receive a small amount of an agent that
  closely resembles a substance naturally found in
  the body.
• PET imaging involves positrons emitted during the
  breakdown of the nuclei of certain radioisotopes.
• Pure energy, released as gamma rays, is a result
  of the collision and subsequent annihilation of
  matter and antimatter.
• Radiation from the positron emitting isotope is
  detected be the PET scanner and displays in
  microscopic detail the chemical processes
  occurring.

50
Neutrons

• To produce neutron beams, deuterons are
  accelerated to high energies and then forced to
  strike a beryllium target, producing neutrons via
  nuclear reactions.
• Use Teflon flattening filters, polyethylene
  filters.
• Neutrons effective in penetrating nuclei and
  producing reactions by a process called
  stripping.
• Neutron studies abandoned due to the high
  concentration of hydrogen in the fatty tissue
  (lack of information about its RBE).

51
Neutrons

• Advantages
• Good for radioresistant tumors
• GBM
• Salivary gland
• Soft tissue sarcoma
• No oxygen effect
• Disadvantages
• Poor depth dose
• Poor skin sparing
• Extra shielding due to neutron contamination
• Fixed horizontal beam

52
Protons

• To produce a usable beam of protons, strip off
  hydrogen electrons by subjecting it to an
  electric current
• Protons are then subjected to an oscillating
  electric field, which accelerates them to half
  the speed of light and a strong magnetic field
  which keeps them contained in a spiral
  configuration.

53
Protons

• Precision controlled, accuracy critical within 1
  mm.
• Scattering is minimal compared with that from
  x-rays, neutrons and cobalt radiation.
• Filtration brass cutouts
• They have a characteristic distribution of dose
  with depth.
• Most of the energy is deposited near the end of
  the range, where the dose peaks to a high value
  and then drops rapidly to zero (Bragg peak).
• Lucite compensators used to spread out Bragg
  peak.
• Treatment of choice for lesions close to
  sensitive areas of the body, usually given in a
  single large dose.

54
Brachytherapy

• Brachytherapy a form of radiotherapy where a
  radioactive source is placed inside or next to
  the area requiring treatment, usually after the
  tumor has been surgically removed.
• Remote After loading this technique relies on
  the use of hollow tubes which are connected to a
  safe containing a small radioactive source welded
  to a wire that is driven out by a stepping motor
  to predetermined positions to deliver radiation
  dose.
• Personnel do not touch the source

55
High Dose Rate

• Greater than 1200 cGy per hour or more than 20
  cGy per minute.
• Can be fractionated 4-5 treatments over 2 weeks.
• Most use Iridium-192
• Advantages
• Treatment can be given on an outpatient basis.
• Treatment time is extremely short compared to
  LDR- implant reproducibility more precise
• Radiation protection for staff
• No general anesthesia or bed rest with decreased
  complications.
• Increased comfort for patient.
• Higher dose rate- tumor gets more of the dose and
  cannot repopulate.

56
High Dose Rate

• Disadvantages
• Labor intensive
• Longer setup time
• Treatment plan changes are difficult to make
  before the treatment is completed

57
Low Dose Rate

• 40- 200 cGy per hour.
• Use Cesium-137, requires bed rest for 2 days
• Used for prostate
• Platinum
• Iodine