Successful completion of this study will provide data on changes in oxygenation in irradiated tissues. If changes are documented, this study would provide evidence supporting current models of tissue injury following radiotherapy. In addition, these data may provide a mechanistic rationale for clinical approaches to ameliorate the toxicities of radiotherapy. Finally, if a correlation exists between tissue oxygenation and toxicity, the rapid and simple tissue oxygenation test may provide a quantitative measure of toxicity and allow earlier and more precise assessment of both toxicity and efficacy of ameliorative therapies.