Pimonidazole administration: This well-established drug has the valuable property of binding exclusively to oxygen- starved tissues in a form that can be detected as a colour change in cell fragments released into the blood stream. A single oral dose of pimonidazole 500 mg/m2 will be prescribed and administered in a Royal Marsden outpatient clinic after confirmation of eligibility and completion of written informed consent. A member of the research team will monitor subjects to ensure that the prescribed drug is successfully taken (swallowed with water) before leaving the hospital (or going back to work/home in the case of healthy volunteers).