Medical Physics Physicists in hospitals Chris Fox

Medical Physics Physicists in hospitals? ? Chris Fox Department of Physical Sciences Peter Mac. Callum Cancer Centre

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 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 • • 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

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, 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.

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 0 V • Electrons accelerated by E field • Energies < 120 k. V +V

Therapy Needs Megavolts • Diagnostic energies of k. V • 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 = 10 cm 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 4 MV

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: 6 Gy/min at 1 m. Lethal dose in ~ 10 min. • Weight: ~ 8 tonnes • Cost: $2. 5 m to $4 m • Lifespan: ~10 y • Facility: 1. 2 m to 2. 4 m concrete as shielding for staff Chilled water for cooling Compressed air Lots of electricity! • Support: Maintenance contract >$200 k per year.

The radiation beam 6 MV 18 MV

X-ray dose Vs Depth 18 MV

Combining beams -- a pair

Combining beams – three beams

A patient plan

Measuring dose Ionisation chamber

Measuring Dose 600 cc chamber Thimble chambers

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 Mac. Callum 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

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 i. Phone/i. Pad has one inside

Inclinometer • • Device measures ‘static acceleration’ due to gravity Can easily calculate angle to vertical z g 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.