Prostate Cancer treatment -more on MR Linac

Overview by Dr Philip Camilleri
Consultant Clinical Oncologist
Oxford University Hospitals NHS Foundation Trust
and
Clinical Director – Urological cancers and MR Linac
Genesiscare UK

The MR Linac, also known as MR guided Radiotherapy (MRgRT) is ideal for patients wanting to avoid surgery, the risk of infection and the risk of incontinence.  It is extremely convenient and allows treatment to be over in just 5 visits, which appeals hugely to my patients.

In addition for low-intermediate risk patients, no additional androgen deprivation therapy ( hormone treatment) is required. There may be emerging evidence of benefit with regard to nerve sparing but this is not yet complete.

It is as effective as surgery and there is currently far more data to back it up than for focal therapies such as  Cryotherapy or HIFU.

As stated in earlier information, there are currently two different MR Linac systems. The Elekta Unity system which uses a 1.5 T magnet, and the Viewray MRIdian 0.35 T MR Linac system.

I am frequently asked questions about the importance of magnetic field strength. With regard to pure diagnostics, in other words, when you are at the stage of having your initial investigations, there is no doubt the bigger the  magnet, that is the higher the T value, the better your imaging for detecting cancer lesions. However, this is not the case with MRgRT. The magnet strength does not really add value in terms of the ability to see the target, ie the prostate and the surrounding organs like the bowel and the bladder, all of which are seen in more than enough detail with a 0.35T magnet. The stronger magnetic fields can cause nerve stimulation, tissue heating and even burns. With a 1.5T magnet you cannot have the patient touch the side of the bore due to these effects. this means you are limited to treating centrally placed tumours within the body. With the 0.35T magnet we get excellent image quality with no compromise on the radiation quality and sharpness. in addition there are no tissue stimulatory effects to speak of and we can have patients pressed up against the side of the machine. Why is this important? Because if you have a lateral tumour, say in the lung, kidney or liver the tumour needs to be at or close to the isocentre of the machine, that is as close to the middle the bore of the machine as possible. The further out from the isocentre, the more uncertainties there are around the radiation accuracy. With the MRIdian we can place the tumours in the centre of the bore even if it means squashing the patient up against one side.