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

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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 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 .35 T MR Linac system.
I am frequently asked questions about the importance of magnetic field strength. With regard to pure diagnostics, there is no doubt the bigger the  magnet, that is the higher the T vlaue, the better your imaging for detecting cancer lesions. However, this is not the case with MrgRT. That is because the larger magnet interferes with the delivery of the radiation, causing more uncertainty , and more dose spread due to a physical effect  of what is called the returning effect. Therefore radiation delivery has less pin point accuracy with more chance of the radiation affecting areas other than the target. In addition 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 the isocentre of the machine, that is as close to the middle the bore of the machine as possible. The furhter 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. Its a minor but hugely important point.
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