Download Applying Physics to Patient Care - Part 1

Radiotherapy Physics
Chris Fox
Department of Physical Sciences
Peter MacCallum Cancer Centre
Cancer: the numbers
• In 2004, Victoria lost 9,613
people to cancer
• Nearly 30% of all deaths in
2004
By site
By time
Incidence -- men
Incidence -- women
Mortality: men
• Generally steady decline in mortality
Mortality: women
Treatment
• The gap between
incidence and
mortality is treatment
Survivable?
• M/I = Mortality/Incidence ratio
– Good guide to survivability
• Low M/I
– high likelihood of surviving
– Treatment effective
Treatment
• Three main forms of treatment
– Radiotherapy
– Chemotherapy
– Surgery
• Radiotherapy used in 30% – 50% of cases
Radiotherapy: quick history
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1895
1895
1896
1898
1901
1904
1951
1952
Roentgen discovers x-rays
X-rays used to treat breast cancer
Becquerel discovers radiation
Radium separated by Curies
Radium first used for therapy – skin cancer
First text on use of radium for therapy
Co-60 used for therapy
Linear accelerator used for therapy
Basis of Radiotherapy
• Radiation disables cells
• Disrupts DNA
• Attack via
– direct ionisation/excitation
– Free radicals formed from
water in cell
• Some repair may follow
• Cell may not be killed, but
can’t reproduce. Disabled.
Timeline
Stage
Process
Duration
Physical
Energy absorption, ionization
10-15 s
Physico-chemical
Interaction of ions with molecules,
formation of free radicals
10-6 s
Chemical
Interaction of free radicals with
molecules, cells and DNA
seconds
Repair
Enzymes in cells
hours
Biological
Cell death, change in genetic data
in cell, mutations
tens of minutes
to tens of years
Discrimination
•
Cancer tissue is poorly organised. DNA repair less effective than normal tissue
•
Therefore more sensitive to radiation than normal tissue = therapeutic advantage
•
Advantage often slender. Accuracy needed with dose!
Radiation dose delivery
• Three approaches used:
– Beaming high energy x-rays into patient from outside
• External beam Radiotherapy (EBRT)
• Linear accelerators (Linacs) generate the x-rays
– Radioactive sources inside diseased tissue
• Brachytherapy
– Administering radioactive solutions that concentrate in diseased tissue
• Often part of Nuclear Medicine (NM)
• We’ll focus on EBRT
• Most widely used.
Linear accelerators
• High energy x-ray generators
• Photon energies between 6MV
and 25MV
• Microwave devices
•
Generate x-rays using
bremsstrahlung
– Accelerate electrons, collide with
high-Z material
– Convert kinetic energy to
radiation
Linac