Peter MacCallum PD 8/12/07

These are notes taken on a Physics teachers’ PD session at Peter MacCallum Cancer Centre on 8/12/07. Thanks to Dan O’Keeffe for organising and for the Kate Wilkinson and Chris Fox at Peter Mac who kindly donated their time to speak to us and demonstrate.

Dave Campbell

The focus of the day was about radio therapy – using high doses of radiation to target cancer cells while attempting to minimise the effects on the rest of the body

  • Linear accelerator (linac)
  • Accelerates electrons in a straight line (as opposed to cyclotrons which use magnets to bend particles in a circle)
  • RF power in a waveguide produces a standing wave which provides the energy to accelerate the electrons
  • Electrons accelerated to almost the speed of light
  • They are rotated 270 degrees. Why 270 not 90?
  • When the particles are rotated 90o, they spread out slightly.
  • If they are rotated through 270o, they can be spread out and then focussed again
  • They are then slammed into 120 tungsten leaves
  • This produces x-rays
  • Energy levels
  • 6 MeV / 18 MeV
  • 1 mm lead normally shields x-rays
  • this level of energy needs 70mm of lead to shield
  • Beam can be shaped to target tissue
  • Softer radiation – can be used for skin cancers as they are on the surface
  • Dose :
  • 25 Gray once / week for four weeks
  • Or every week day with one fifth the weekly dose given (fractionation)
  • Fractionation – as you increase fractionation you reduce the risk of long-term side affects
  • Dose-effect curve – sigmoid shape
  • You need a threshold dose for the radiation to have any affect at all
  • But you don’t want to go over to avoid harming healthy tissue
  • Number of cells over time
  • Both healthy and cancer cells recover in number between treatments
  • There is a slight difference in the gradient – normal body cells can recover more quickly than cancer cells
  • Effects of radiation :
  • Aim is to target DNA. Disruption of the DNA will mean a reduced capacity of the cells to reproduce over time
  • The x-rays act on the body by ionising atoms. These atoms can become free radicals and disrupt the cell.
  • Direct action – x-rays cause ionisation of an atom in the DNA itself
  • Indirect action – ionise water, create free radicals which disrupt DNA
  • Tumour cells are more poorly organised, have less backup support from the body systems so don’t recover as well from damage
  • Different types of cancer need to be tackled differently.
  • Eg prostate cancer may be growing only very slowly, less susceptible to DNA damage
  • Some cancers may be located close to vital or susceptible organs
  • Targetting
  • It is possible to cure any cancer in vitro with enough radiation. However indiscriminate doses in the body could cause too much damage to healthy body tissue
  • Huge advances in technology / computing power have mean that radiation therapy can be targeted more precisely than ever before (see below)
  • Positioning :
  • The medical staff need to be confident that, over a number of treatmet sessions, the radiation dose is being administered to exactly the same place in the body.
  • A mould is made of the relevant part of the persons body
  • A plastic mask is created to hold them in the correct position so that each dose over a number of days is targeted at exactly the same spot (this can be confronting!)
  • Imaging :
  • Advances in imaging / processing power have meant that medical staff can have a much better idea of where the cancer is, how to access it
  • CT scan – gives an image where the scale is accurate
  • MRI – gives a much better picture of what is happening with the tissues, but can be distorted – not spatially correct
  • Can use computers to combine the two images to produce one that is spatially correct and contains all of the information about the tissues.
  • The latest linacs have CT scanners built in to the sides.
  • Targetting dose :
  • Once all information about the location of the cancer and other organs is available, can develop a plan
  • If all of the radiation came from one angle, the area just below the skin would end up getting a very high dose.
  • But if the radiation is given from a number of different angles, the cumulative dose that the cancer cells receive is the same, but other individual parts of the body receive a much lower does
  • Complicated arrangements with up to 15 different beams (not at the same time!) have been devised in special cases.
  • Is this expensive ? Yes, but radiation therapy compares favourably with invasive surgery
  • Maintenance :
  • Equipment can be quite fussy – will refuse to work unless everything is exactly right
  • Need teams of engineers / physicists to monitor
  • Economies of scale if you have a large number of machines (eg at Peter MacCallum).
  • Treatments :
  • 10-15 minutes per patient
  • 150-200 patients per day
  • CT scans :
  • 2mSv is normal annual dose received by a person
  • 8-10mSv from CT scan
  • 2% of cancers in US caused by CT !
  • Operation of linac
  • Controlled by a bank of computers in the next room
  • One displays a camera picture of what is happening inside the room
  • The patient lies on a hard bench
  • The beam can be rotated and applied from different angles
  • Can carry out a CT and automatically adjust the bench on which the patient lies if they are slightly out of position
  • Pastoral aspect :
  • Medical staff often build up a strong relationship with the patient as they are coming in every day for a number of weeks.
  • Radiation therapy can bring up a number of non-medical issues, and the staff work through these with the patients