What is the difference between photon xray treatment and electron treatment in Radiation Therapy

1
Question

• What is the difference between photon (x-ray)
  treatment and electron treatment in Radiation
  Therapy?

2
Photons (X-Rays)

• Photons are light (electro magnetic) waves
• They behave like packets of energy
• no electric charge, not particles
• Photons travel through the patients body,
  setting electrons in motion as they go
• Its the freed electrons that do damage to cells

Atoms in a cell
3
Photons (X-Rays)

• Used for tumors deep in the body
• Prostate
• Lung
• Brain

• Or, when there are no sensitive structures along
  the path
• breast
• using tangential beams

4
Electrons

• Electrons are negatively charged particles
• Electrons stop in tissue - do not exit the body
• The energy of the electrons, 6 MeV, 12MeV, etc.,
  determines the depth the electrons can travel in
  the body

Atoms in a cell
5
Electrons

• Types of cancer treated with electrons
• Skin Cancer
• Chest wall (post mastectomy)
• Scar Boosts

6
10 MV Photon Beam
7
10 MeV Electron Beam
110 100 90 80 70 60 50 40 30 20 10
Dmax
Field size 15 x 15 (cm)
Percentage Relative Dose
Definition Depth in water where 80 of Max
ionization occurs Field size 15 x 15 cm on
surface Characteristics High Surface dose Low
penetration Steep fall off
X Ray contamination generated by electrons
stopped in filters , Air, Water
Penetration depth
Depth in water
8
(No Transcript)
9
Siemens Primus Electrons Measured Depth
Ionization Ratios
10
So what really happens when radiation hits a cell?
11
Cell Survival Curve
Suppose you irradiate a petri dish with a single
layer of cells...
How many survive?
If things were simple, youd expect this

• D0 mean lethal dose (reduce survivors to 37)

12
Cell Survival Curve
The low levels of radiation produce no cell
kill!
Higher levels of radiation produce cell kill in
equal increments.

• D0 mean lethal dose (reduce survivors to 37)
• Dq quasi-threshold dose

13
Time Scale of Radiation Effects
seconds
Free Radical Reactions
human lifespan
1 day
Enzyme Reactions
Early Effects
Excitation
Repair Processes
Ionization
Late Effects
Carcinogenesis
Cell Proliferation
14
What the radiation hits makes all the difference

• Many areas of the cell will repair
• Some types of damage cause a cell to commit
  suicide
• Apoptosis programmed cell death
• But, what if the damage affects the blueprint
  for making a new cell?

15
Cancer is a Disease of the Genes

• Genetic Code - contained in nuclear
  chromosomes- double stranded helix of
  DNA-composed of 4 basic units (nucleotides)
  named after their bases Adenine (A - T)
  Cytosine (C - G) Guanine Thymidine

16
Types ofRadiation Damage
The double strand break is the most difficult
type of damage for a cell to repair. It also
correlates well with experimental data on cell
death and multiple-target theory
17
2 Ways to Do Damage
The radiation breaks chemical bonds in water
60 of all damage
The free radicals damage the DNA chemically
In healthy cells
40 of all damage
The electron crashes into the DNA
18
Radiation Damage Sites
The density of damage sites determines the
likelihood the cell will be able to repair
19
What do cells do? PHASES of the CELL CYCLE
Mature, highly specialized cells
Period of greatest resistance late S
Radiosensitivity changes as cell moves through
the cell cycle most sensitive in M and G1 -
boundary
20
What happens to the cell after radiation?
Mutated Cell
Not damaged
Damaged, but correctly repaired
Damaged, but incorrectly repaired
Damaged, not repaired
Healthy Cell
Cell slated for (reproductive) death
21
How does this affect the tumor?
Radiation kills the active, oxygenated cells
The previously hypoxic cells become oxygenated
and active again
Hypoxic means low oxygen, due to inadequate
blood supply These cells are not radiosensitive!
Necrotic means dead.
22
Fractionation
A typical RT prescription might be to treat the
tumor to a total of 60 Gy. This would then be
divided into 1.8-2.0 Gy fractions per day.
The patient needs to return to the clinic 5 days
a week for approximately 6 weeks to complete a
course of therapy
23
Why Fractionate Treatment?

• The biological response of tissue to radiation
  depends on the combined action of 4 different
  factors (the 4 Rs)Repair of sub-lethal DNA
  damage
• Repopulation or the division of surviving cells
• Reoxygenation of hybernating cells located far
  from a blood supply
• Reassortment of cells in all phases of the
  cycle

Remember Normal tissues repair well if the
damage is not too intense. And We cant kill
all the tumor cells at once due to the latter 3
Rs.
24
Tolerance Dose for Normal Tissue

• Attempt to express minimal and maximal injurious
  dose acceptable to the clinician.
• Affected by Volume Fraction

25
Minimal Tolerance Dose

• TD 5 Dose that gives 5of a population of
  patients a severe complication in 5 years
  
• - e.g., paralysis, blindness, brain damage,
  incontinence, sexual dysfunction, bone necrosis

26
Maximal Tolerance Dose

• TD 50 Dose that gives 50of a population of
  patients a severe complication in 5
  years
• - e.g., paralysis, blindness, brain damage,
  incontinence, sexual dysfunction, bone necrosis

27
Tolerance Doses (For Fractionated Doses)
Target Organs Complication TD 5 - TD 50,
(Gy) Lens Cataract 6-12 Bone
Marrow Hypoplasia 25-35 Liver Hepatopat
hy 35-40 Gastrointestinal Infarction
necrosis 50-55 Spinal Cord Myelopathy
45-50 Brain Encephalopathy
60-65 Bladder Ulcer
65-75 Lung Pneumonitis Fibrosis 20-30
28
The Therapeutic Window
Dose Range where we can treat
Tumor control
IMRT improves the therapeutic window
normal tissue damage
Complication - free control
29
Tumor Growth Rate and Prognosis

• Fast Growing tumors
• Generally highly responsive to Radiotherapy and
  Chemotherapy.
• Slow growing tumors
• Less responsive to these modalities of treatment.
• Benefit more from surgical resection and
  associated treatment with RT.

30
Dose Response for Different Cancers
31
General Treatment Goals

• Cancer treatment may be either Curative or
  Palliative
• Curative goal is to erradicate the tumor and
  keep the patient disease free (5 years)
• Palliative goal is to prolong life and minimize
  discomfort when a cure is impossible

32
Dose Prescription

• The Radiation Oncologist has to decide how to
  prescribe the best treatment possible
• predict the optimal dose to the entire tumor
• cause minimal damage to the surrounding normal
  tissues.
• The physicist or dosimetrist will decide how to
  deliver it

33
Dose Prescription

• Take into account
• Tumor type and location
• Normal tissue to be affected
• Stage of disease
• Metastases
• Previous treatment the body remembers
• Desired outcome - palliation or cure
• Quality of life

34
Conclusions
Radiation Therapy is one of the most important
treatment modalities for cancer It works by
creating charged particles in the body, which
interact with cells If the cellular DNA is
damaged, the cell may mutate or die Tumor cells
are slightly more vulnerable to DNA damage than
healthy tissue A good treatment plan will spare
healthy tissue and kill enough cells in the tumor
to stop the disease