 Patients will be recruited to the trial from the pool of routine referrals for this therapy. Potential participants will receive an information leaflet via the post several weeks in advance of attendance at clinic. Patients will have the opportunity to telephone the study team during this time to answer any questions. Patients who are interested in participating will have the procedure further explained verbally by a physicist on the study team upon attending for therapy. The patient will then have time to consider whether or not to participate in the study. Written consent will be sought before commencing the therapy. 