Medical Physics Chris Fox Department 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 1895Roentgen discovers x-rays 1895X-rays used to treat breast cancer 1896 Becquerel discovers radiation 1898Radium separated by Curies 1901Radium first used for therapy – skin cancer 1904First text on use of radium for therapy 1951Co-60 used for therapy 1952Linear 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 StageProcess Duration Physical Energy absorption, ionization s Physico-chemicalInteraction of ions with molecules, s formation of free radicals ChemicalInteraction of free radicals with seconds molecules, cells and DNA RepairEnzymes in cellshours BiologicalCell death, change in genetic data tens of minutes in cell, mutationsto 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

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 staff Chilled water for cooling Compressed air Lots 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 dose Ionisation chamber

Measuring Dose Thimble chambers 600cc chamber

Determination of Absorbed dose Absorbed dose to water Corrections for “influence quantities”

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!

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!

Others states do better … NSW has been much more effective at setting conditions Cross-border ‘gravitational field’!

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

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 g z y 

Analysis Angle to vertical can be calculated Use with sensor on abdomen to find hip flexion 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.