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 2008

Google “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 beam

6MV 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”

spolelecPTQ kkkkMM ,1

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

$0.00

$50,000.00

$100,000.00

$150,000.00

$200,000.00

$250,000.00

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18

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

MeS

3 (1

-2)

MeS

3 (3

)

MeS

4 (1

-2)

MeS

4 (3

)

MeS

4 (4

)

MeS

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)

MeS

5 (2

)

MeS

5 (3

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MeS

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MeS

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MeS

5 (3

)

Relative to SA Medical Scientist MeS Levels (brackets indicate increments)

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ay (

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NSW Accrediated

NSW Non-Accrediated

SA with A&R Allowance

SA 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

z

yaTan

g

z

y

Analysis

• Angle to vertical can be calculated

• Use with sensor on abdomen to find hip flexion

Leg Angle (rel horizontal)

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0 5 10 15 20 25 30 35 40 45

Time (s)

An

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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.