This study is looking at improving radiotherapy treatment for head and neck cancers by:

1. Repeating the radiotherapy planning scan at weeks 2 and 4 of treatment so that investigators can adapt the radiotherapy to changes to the shape of the cancer and the patient's body. These changes can affect the accuracy and the radiotherapy doses delivered.
2. Using a MR (magnetic resonance) scans to view and target the cancer with more precision.
3. Identifying HPV negative oropharyngeal cancer who are non-responders and increasing the radiotherapy dose.

The 3 groups of patients are:

1. Cancers of the oropharynx (middle of the throat) that test positive for HPV (human papilloma virus). If HPV is present, the cancer responds better to treatment and there is a higher chance of cure. In this group, the investigators aim is to reduce radiotherapy associated long-term side effects by sparing healthy tissue from high doses.
2. If the oropharyngeal cancers test negative for HPV, they are less likely to respond well to treatment. The investigator's department has shown that investigators can predict which patients will respond to treatment using a special type of MR scan. Investigators will increase the dose of radiotherapy to HPV negative patients who are predicted to be non-responders with the aim of improving the chance of cure.
3. Cancers that located at the base of the skull are not seen very well on CT scan. By using MR imaging, investigators can visualize the surrounding normal organs and the cancer better, target the cancer with more precision and adapt to changes to the healthy organs and tumour.

Investigators will also test if they can predict response to treatment by checking blood for fragments of the cancer and using a special MRI.

The study will be conducted at the Royal Marsden in Sutton only and will be followed up for 2 years.