This study expands on protocol (NCT01908504"PET adaptive RT") designed to evaluate the utility of adaptive PET-CT planning for radiation therapy (RT). Radiation therapy is used in many malignant diseases as a curative treatment modality. However, critical normal tissue is often in close approximation to disease, and portions of such tissue must receive high doses of radiation for appropriate treatment. Positron Emission Tomography (PET) adapted radiation therapy, as defined in the current protocol, may allow for a means of determining the eventual response to therapy, at a time point when adaptation of treatment plan may be possible to improve outcomes. This protocol will build upon the findings the previous protocol (NCT01908504 "PET adaptive RT") that evaluated the utility of intra-treatment PET imaging in multiple types of cancers. The current focus will be more specific to certain types of gastrointestinal and gynecologic cancers treated with RT, identified from the prior study to warrant further research.