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Medical Physics

Medical Physics. Physicists in hospitals??. Chris Fox Department of Physical Sciences Peter MacCallum 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

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Medical Physics

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

  2. How we die Source: NEJM. Quoted New Scientist, 25 June 2012

  3. 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”

  4. Mortality By Site

  5. By time • Generally steady decline in mortality Incidence -- men Incidence -- women Mortality: men Mortality: women

  6. Treatment • The gap between incidence and mortality is treatment

  7. Survivable? • M/I = Mortality/Incidence ratio • Good guide to survivability • Low M/I • high likelihood of surviving • Treatment effective

  8. Treatment • Three main forms of treatment • Radiotherapy • Chemotherapy • Surgery • Radiotherapy used in 30% – 50% of cases

  9. 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

  10. 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.

  11. Timeline Stage Process Duration Physical Energy absorption, ionization 10-15 s Physico-chemical Interaction of ions with molecules, 10-6 s formation of free radicals Chemical Interaction of free radicals with seconds molecules, cells and DNA Repair Enzymes in cells hours Biological Cell death, change in genetic data tens of minutes in cell, mutations to tens of years

  12. 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!

  13. 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.

  14. Linac

  15. Bremsstrahlung • Example of conservation of energy • Radiative energy loss by fast electron when slowed near nucleus • Results in spectrum of energies from many interactions

  16. Diagnostic x-ray production • Electrons accelerated by E field • Energies < 120kV + V 0V

  17. Therapy Needs Megavolts • Diagnostic energies of kV • Lack penetration for deep seated lesions • Need MV • Can’t accelerate using millions of volts!!

  18. MV x-ray production • Carefully tuned microwave source • ~ 3 GHz = 10cm wavelength • Intense electric field • Phase problem!

  19. Microwave resonance cont. • Sideline every second cavity • Solves phase problem

  20. Operation • Inject bunches of electrons into cavity • Time to coincide with pulses of microwaves • Makes compact system

  21. Waveguide for 4MV

  22. Waveguide cont

  23. High Energy Waveguide

  24. MV X-ray Production • Electrons bent through 270 degrees • Collide with tungsten target • Beam shaped for flatness

  25. Linac

  26. Linac

  27. 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.

  28. The radiation beam 18MV 6MV

  29. X-ray dose Vs Depth 18MV

  30. Combining beams -- a pair

  31. Combining beams – three beams

  32. A patient plan

  33. Measuring dose Ionisation chamber

  34. Measuring Dose 600cc chamber Thimble chambers

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

  36. 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!

  37. Medical Physics as a career

  38. 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”

  39. 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.

  40. Some of the staff

  41. Other numbers!

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

  43. 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

  44. Physicists at work

  45. Physicists at work

  46. Physicists at work

  47. Working conditions:

  48. 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

  49. A project of mine.

  50. 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!

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