Medical Physics
description
Transcript of Medical Physics
Medical Physics
Chris FoxDepartment of Physical Sciences Peter MacCallum Cancer Centre
Physicists in hospitals??
How we die
Source: NEJM. Quoted New Scientist, 25 June 2012
Cancer: the numbers• In 2008, Victoria lost 10,538
people to cancer
• More than 30% of all deaths in 2008
Source: Cancer Council Victoria Canstat 2008Google “Canstat”
Mortality By Site
By time
• Generally steady decline in mortality
Incidence -- men
Incidence -- women
Mortality: men
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• 1895 Roentgen discovers x-rays• 1895 X-rays used to treat breast cancer• 1896 Becquerel discovers radiation• 1898 Radium separated by Curies• 1901 Radium first used for therapy – skin cancer• 1904 First text on use of radium for therapy• 1951 Co-60 used for therapy• 1952 Linear accelerator used for therapy
Biological Basis of Radiotherapy
• Radiation disables cancer 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, 10-6 sformation of free radicals
Chemical Interaction of free radicals with secondsmolecules, cells and DNA
Repair Enzymes in cells hours
Biological Cell death, change in genetic data tens of minutesin cell, mutations 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.
Linac
Bremsstrahlung• Example of conservation of energy• Radiative energy loss by fast electron when slowed near nucleus• Results in spectrum of energies from many interactions
Diagnostic x-ray production
• Electrons accelerated by E field
• Energies < 120kV
0V + V
Therapy Needs Megavolts• Diagnostic energies of kV
• Lack penetration for deep seated lesions
• Need MV
• Can’t accelerate using millions of volts!!
MV x-ray production• Carefully tuned microwave source • ~ 3 GHz = 10cm wavelength
• Intense electric field• Phase problem!
Microwave resonance cont.• Sideline every second cavity• Solves phase problem
Operation• Inject bunches of electrons into cavity• Time to coincide with pulses of microwaves
• Makes compact system
Waveguide for 4MV
Waveguide cont
High Energy Waveguide
MV X-ray Production
• Electrons bent through 270 degrees• Collide with tungsten target• Beam shaped for flatness
Linac
Linac
Vital statistics
• Output: 6Gy/min at 1m. Lethal dose in ~ 10 min.
• Weight: ~ 8 tonnes
• Cost: $2.5m to $4m
• Lifespan: ~10y
• Facility: 1.2m to 2.4m concrete as shielding for staffChilled water for coolingCompressed airLots of electricity!
• Support: Maintenance contract >$200k per year.
The radiation beam6MV 18MV
X-ray dose Vs Depth 18MV
Combining beams -- a pair
Combining beams – three beams
A patient plan
Measuring doseIonisation chamber
Measuring Dose
Thimble chambers
600cc chamber
Determination of Absorbed dose • Absorbed dose to water
oo QQQwDQrefQw kNMzD ,,,,
• Corrections for “influence quantities”
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Corrections• Accurate dosimetry requires many small corrections• E.G. Temperature/Pressure
– Ionisation charge collected depends on amount of air in chamber– Correct by
• Other corrections for chamber characteristics– Recombination, polarity effects
• Complex business, keeps us in work!
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Medical Physics as a career
Training• Minimum honours degree in physics
• Training process follows– Employed as “registrar” in a radiotherapy department
• Masters or Doctorate will be completed during this time• Five years hospital experience
– After five years, accreditation exams• Three hour written exam• Half day practical exam• Oral exam
• Most recover, with counselling!
• “ROMP”
Physicist numbers• There are 314 ROMPs in Australia employed at ~50 sites
– 254 in Rad Onc– 37 in Nuclear Medicine– 33 in Diagnostic Imaging
• There is a shortage of ROMPs – 10% positions unfilled in Australia– vacancy rate projected to be 25% - 35% in 10 years– Most vacancies are filled from overseas
• Very international flavour to most departments
• Peter MacCallum Cancer Centre is one of Australia’s largest employers of ROMPs with 32 staff, including 6 registrars.
Some of the staff
Other numbers!Annual Salary
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$250,000.00
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Seniority --> promotion Level
Others states do better …• NSW has been much more effective at setting conditions• Cross-border ‘gravitational field’!
$40,000
$60,000
$80,000
$100,000
$120,000
$140,000
$160,000
$180,000
$200,000
MeS3 (
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MeS5 (
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Relative to SA Medical Scientist MeS Levels (brackets indicate increments)
Rat
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Pay
(per
ann
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NSW AccrediatedNSW Non-AccrediatedSA with A&R AllowanceSA without A&R Allowance
Roles within Peter Mac • Radiation protection
– Targeting lowest possible occupational doses – Patient dose always justifiable
• Dosimetry– Checking output against national laboratory standards
• Brachytherapy– Clinical work treating patients using radioactive sources
• Teaching/lecturing– Medical registrars
• Quality assurance– After hours work checking machine outputs and alignments
• Research– Many clinical projects trialling new approaches to treatment
• Development towards improved treatment– Application of new technology
Physicists at work
Physicists at work
Physicists at work
Working conditions:
So, what else do we do?• About 50% (+/-30%!) of our time is unscheduled
• Most work is project based and open ended
• Most physicists have a specialty and pursue a project in that area
• My interest is in setup correction– Study of position accuracy for patients on treatment– New imaging tools have become available– New treatment techniques
A project of mine.
HDR motion study • Background
– Therapy for prostate cancer– Hollow plastic catheters implanted through the skin into the prostate– Implant locked together and stitched to the patient’s skin– A tiny radioactive source moved through the catheters in the prostate
and treats it from the inside– Very tightly defined dose distribution– Called brachytherapy and is a very successful treatment– Patients lie in hospital and get 2 treatments over 2 days
Next slide not for the squeamish!
HDR Motion study • Collection of catheters into prostate• The template is being stitched to the skin
HDR Motion study• Problem
– The catheters tend to move out of the prostate• Question
– Is this due to movement of the patient while in bed in hospital, or is it due to swelling?
The Project• Aim: To measure patient movement while in bed
• Uses electronic inclinometers to measure angles of legs and torso
• Based on solid state accelerometer
• Now cheaply available since used in laptops to detect motion
• Your iPhone/iPad has one inside
Inclinometer• Device measures ‘static acceleration’ due to gravity• Can easily calculate angle to vertical
zyaTan
g
z
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Analysis
• Angle to vertical can be calculated
• Use with sensor on abdomen to find hip flexion
Leg Angle (rel horizontal)
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Time (s)
Ang
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•Search for correlation between hip flexion/extension angles and implant movement.
Results• No relationship between patient movement and implant
displacement.
• Published Medical Physics
• An opportunity to review restrictions on patient movement
• May reduce need for patients to lie still
Lie Still Please!
Thank You!
Spares• Following slides just junk for recycling
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Prevalence Vs Incidence
• Autopsy results
• Patients did not die from the cancer.