We know that X-rays are injurious to our bodies and thus we tend to limit the number of X-ray images that we have over our lifetime. However, X-rays, particularly high energy X-rays, are lethal to cancer cells.
The objective of Radiotherapy, therefore, is to kill off cancerous cells in the body. However, good cells are also going to be damaged as the X-ray beam passes through healthy tissue on its way to the tumour. This situation is ameliorated by applying multiple X-ray beams to the body from different directions, but ALWAYS passing through the tumour, thereby spreading out the unwanted tissue damage.
The machine which delivers the X-ray beams is called a Linear Accelerator and as the beams are generated externally to the body the process is called “External Beam Radiation Therapy” or EBRT.
In the past typically 3 beams at 120 degrees with each other or 4 beams at right angles were employed. But cutting edge Linear Accelerators now perform better by delivering a full arc of 360 degrees to the body, always trained on the tumour. These modern machines have an accuracy of a couple of millimetres in targeting the tumour whilst sparing good tissues. I was treated on a modern Linear Accelerator.
As the Prostate Gland is very close to both the Bladder and the Rectum, tissue sparing can be quite important, but it is impossible to avoid some limited damage. Side effects develop that get somewhat more pronounced as the treatments proceed – typically 20 treatments, called “fractions” over 4 weeks, delivered on weekdays only. In my case I developed a very small amount of bleeding from the back passage and had pain passing urine. The former seemed to clear up as the treatment progressed but the latter required a medication which I was given at the end of my treatments – one tablet per day for several weeks. However, the pain practically disappeared with the first tablet.
Slightly older Linear Accelerators give off a high pitched whine when the beam is on, but newer ones are practically silent. This silence, on the first couple of fractions, gave me a feeling that the whole experience was very surreal.
Remember, Radiotherapy is performed with high energy X-rays. There is no radioactivity employed and the body does not contain any radiation at the end of the treatment fractions.
A real example of the process
If you want to know more about how the treatment proceeds you may like to watch this video from the NHS Oxford University Hospitals. You will hear a reference to “Gold Seeds”. Prior to treatment, 3 Gold Seeds are inserted into the Prostate Gland in the shape of a triangle. They can be seen with conventional X-ray machines and are used to ensure that the patient’s Prostate is in the correct place for each treatment fraction to millimetre accuracy. If the patient moves during the treatment, the beam automatically shuts down, and the Radiographers enter the room and re-position the patient – following which, the treatment continues from where it stopped. Further, the beam is continually shaped to the outline of the tumour as the arc progresses, and this process is called “Conformal” – strongly sparing nearby tissues.