Radiotherapy Physics

1. Radiotherapy Physics Chris Fox Department of Physical Sciences Peter MacCallum Cancer Centre

2. Cancer: the numbers • In 2004, Victoria lost 9,613 people to cancer • Nearly 30% of all deaths in 2004

3. By site

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

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

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

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

8. 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

9. Basis of Radiotherapy • Radiation disables 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.

10. 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

11. 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!

12. 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.

13. Linear accelerators • High energy x-ray generators • Photon energies between 6MV and 25MV • Microwave devices • Generate x-rays using bremsstrahlung • Accelerate electrons, collide with high-Z material • Convert kinetic energy to radiation

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 • Can still generate therapy beams this way, but lack penetration • Need MV, not kV! + 120kV 0V

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

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

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

20. Waveguide for 4MV

21. Waveguide cont

22. Target and flattening filter • Electrons bent through 270 degrees • Collide with tungsten target • Beam shaped for flatness

23. Linac

24. Linac

25. 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 • Support: Maintenance contract >$100k per year.

26. Measuring the beam • Water used as analogue for tissue

27. X-ray dose Vs Depth

28. The radiation beam 18MV 6MV

29. Combining beams:- a pair

30. Combining beams – three beams

31. A patient plan

32. Measuring dose Ionisation chamber

33. Ionization Chambers 600cc chamber Thimble chambers

34. Determination of Absorbed dose • Absorbed dose to water • Corrections for Influence quantities

35. 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!

36. Medical Physics as a career

37. Training • Minimum honours degree in physics • Training process follows • Employed as “registrar” in a radiotherapy department • Undertake Masters or Doctorate • Accumulate hospital experience • After five years, accreditation exams • Three hour written exam • Half day practical exam • Oral exam • Most recover, with counselling!

38. Some of the staff

39. Physicist numbers • There are 268 ROMPs in Australia employed at ~50 sites • There is a mild shortage of ROMPs • 14% positions unfilled in Australia • Many 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 18 staff, including 3 registrars.

40. Other numbers!

41. Physicists at work

42. Roles within Peter Mac • Radiation protection • Regulating occupational doses • 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

43. Physicists at work

44. Working conditions:

45. 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

46. A project of mine

47. HDR motion study for prostate patients • Background • 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!

48. HDR Motion study • Collection of catheters into prostate • The template is being stitched to the skin

49. HDR Motion study • Problem • The catheters tend to move out of the patient • Question • Is this due to movement of the patient while in bed in hospital, or is it due to swelling?

50. The Project • To get a measure of 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 • If acceleration detected HDD suspends operation