Prostate cancer treatment – MR Linac

Overview by Dr Alison Tree
Consultant Clinical Oncologist
The Royal Marsden NHS Foundation Trust
and honorary faculty at the Institute of Cancer Research

MR Linac treatment for prostate cancer is the most state of the art radiotherapy treatment available

Up until now, radiotherapy for prostate cancer has been directed using scans (usually X-Rays), to try and make sure the radiotherapy treatment is focused on the cancer lesion and avoid the healthy tissues around the prostate, including the rectum (back passage) and bladder. Until the advent of MR Linac, the image guidance for radiotherapy either used small gold seeds placed inside the prostate identified by X – Rays, or performing a CT scan of the prostate daily. Both these methods are accurate and effective but have drawbacks and inaccuracies due to the changing shape of the prostate, the rectum and the bladder and the clarity of the image. Despite this, the vast majority of men with localised prostate cancer have treatment which cures their cancer, and this is associated with very few side effects in the majority regardless of which method we use to guide the radiotherapy.

The gold standard for viewing the prostate is an MR scan which shows the edge of the prostate much more clearly as well as showing the aggressive cancer lesions within the prostate.

There are two commercially available MR Linac systems, The Viewray MRIdian which incorporates a 0.35T MR scanner and the Elekta Unity MR Linac which integrates a Philips 1.5T MR scanner and an Elekta Linac.

This allows not only exceptional accuracy but also incredible confidence in where the beams are going and what is happening to the target and any organs at risk near enough to be affected.

As well as giving Clinical Oncologists a much clearer picture, the MR Linac machine allows change the radiotherapy plan when they observe changes in the shape and position of the prostate.These changes are mainly due to changes in the rectum which is very dynamic due to rectal filling. Because of this added complexity, a treatment session on the MR-Linac can take longer than usual – around 45 minutes compared to 10 minutes for a standard session, but for some patients (depending on their type of prostate cancer) this can be done in as few as five treatments.

All other radiotherapy technologies give the same radiotherapy daily. This means the Clinical Oncologist has to treat a larger ‘safety margin’ around the prostate to allow for these day to day changes in anatomy.

The first prostate cancer patient was treated with the Elekta Unity MR Linac in September 2018 over 20 sessions at the Royal Marsden Hospital. Since then, we have treated many men with 20, 5 and even just 2 radiotherapy treatments, as part of our research studies.

MR Linac treatment for prostate cancer is not yet widely available across the UK but offers an excellent option, especially when delivered as part of a clinical trial.