A 60-year-old smoker with a chronic cough presents with haemoptysis for the first time. A chest X-ray reveals a suspicious 3cm mass in the right upper lobe. Further investigation with CT scan confirms the mass and shows mediastinal lymphadenopathy. Biopsy confirms non-small cell lung cancer (NSCLC). According to NICE guidelines, what investigation is essential for staging and determining suitability for potentially curative treatment (surgery or radical radiotherapy)?