Hypoxic (low oxygen) cells have long been known to exist in animal tumors. It is also known that hypoxic cells are more difficult to eliminate with radiotherapy than tumor cells at normal levels of oxygen (normoxic cells). However, the extent to which hypoxic cells limit the curability of human tumors is uncertain. To determine if hypoxic cells exist in human tumors and how hypoxic cells might influence the efficacy of radiotherapy, this study involves direct measurements of oxygen levels in human tumors compared to the tumor uptake of the experimental drug, 18F-fluoromisonidazole (18F-FMISO), visualized with PET scanning. 18F-fluoromisonidazole has been used with PET imaging to tell the difference between growing tumors which have high and low oxygen content.