Hypertrophic cardiomyopathy (HCM) is a common disease of the heart which causes thickening of the heart muscle.

HCM primarily affects the muscle of the main pumping chamber of the heart (the left ventricle) and particularly the septum (this is the muscular wall which separates the right and left side of the heart). In a subgroup of patients, the thickened heart muscle at the septum prevents blood from leaving the heart during contraction (this is called obstruction). This form of the disease is called hypertrophic obstructive cardiomyopathy (HOCM).

HOCM is a common cause of shortness of breath, chest pain and dizzy spells. These symptoms are treated with tablets and if symptoms are uncontrolled, patients are often offered invasive treatment to get rid of some of the thick heart muscle and reduce obstruction. This is achieved either by:

1. open heart surgery (myectomy) where a surgeon cuts out the thick muscle
2. injection of alcohol to the thick heart muscle via a tube in the wrist or groin (alcohol septal ablation). The alcohol thins the heart muscle at the point of obstruction, mimicking the effects of myectomy.

Unfortunately, some patients are not suitable for both these procedures.

This study will test whether radiotherapy, usually used for the treatment of tumours, can be used to destroy the thick heart muscle at the point of obstruction safely and effectively. Study patients will be monitored following the procedure and the investigators plan to measure the levels of heart muscle thinning, reduction of obstruction and improvement in symptoms and importantly document any side effects.

Radiotherapy works by precisely targeting high energy X-rays (ionising radiation) at a specific area of the body with the aim of destroying abnormal tissue. CyberKnife is one of the latest radiotherapy delivery systems, which will deliver highly focussed and accurate radiotherapy.

Inclusion Criteria:

1. Limiting, drug refractory symptoms secondary to left ventricular outflow tract obstruction
2. Previously failed invasive septal reduction therapy (ethanol septal ablation/surgical myectomy) or inability to perform invasive septal reduction due to increased risk/co-morbidities/anatomical considerations following specialist MDT review
3. A device (permanent pacemaker or implantable cardioverter defibrillator) in situ.
4. Ventricular septal thickness at site ablation ≥ 16mm.
5. Patient able to tolerate lying flat for one hour
6. High target-surrogacy of ICD/pacing lead for cardiac motion (from cardiac-gated MRI or cardiac-gated CT with the patient in pacing mode used for treatment)
7. Successful completion of ICD lead tip tracking test ("patient dummy run") with the patient in pacing mode used for treatment
8. Adult Patients aged 18 and over willing and able to give written informed consent

Exclusion Criteria:

1. New York Heart Association I-II
2. Canadian Cardiovascular Society class 1-2
3. Follow-up impossible (e.g. no fixed abode)
4. Weight of patient that exceeds the maximum limit of CMR table (170kg)
5. Subjects of childbearing potential unless βHCG negative and on contraception
6. Lack of cardiac device with anti-bradycardia pacing capabilities
7. Previous chest radiotherapy
8. Inability to provide informed consent.