The present project comprises 3 clinical studies that will 1) test feasibility of PET imaging with tracers such as FLT (proliferation), EF3 and/or FMISO (hypoxia) in head and neck cancer and in colorectal cancer 2) test the improvement in local control in head and neck cancer with FDG-PET based BG-IMRT using a randomized controlled prospective phase II study 3) increaese the rate of negative circumferential resection margins (CRM) in patients diagnosed with locally advanced rectal cancers with positive circumferential resection margin (CRM) in a phase II study.