The Medical Physics Committee is a core resource to the RTOG that provides medical physics and dosimetry quality management services, as well as intellectual input into clinical trial design and analysis from the medical physics leadership. The RTOG seeks to determine the appropriate role for the many new technologies currently available in radiation oncology. The Medical Physics Committee has been especially active in the quality assurance aspects of protocols that use state-of-the-art treatment planning and treatment delivery techniques, including stereotactic radiosurgery (SRS), three-dimensional (3D) conformal radiotherapy (CRT), prostate brachytherapy, and now intensity-modulated radiotherapy (IMRT). The successful testing of these technologies within the cooperative group setting requires a common vocabulary, a set of minimal technical capabilities demonstrated through a credentialing process, appropriate dosimetric specifications that focus on the critical clinical question, and a comprehensive central review. The RTOG has been successful in establishing mechanisms that address each of these requirements. 1.The RTOG Medical Physics Committee, either acting alone or in coordination with the Radiologic Physics Center and/or the 3D Quality Assurance (QA) Center, has adopted and popularized a common vocabulary so that concepts such as clinical target volume and planning target volume (PTV) are now commonly integrated into protocols.2.The Medical Physics Committee has credentialed >90 institutions to participate in RTOG SRS protocols. The 3D QA Center has credentialed 41 institutions to participate in RTOG 94-06, the 3D prostate cancer protocol; 19 institutions to participate in RTOG 93-11, the 3D lung cancer protocol; and 15 institutions to participate in RTOG 98-03, the 3D brain tumor protocol. The Radiologic Physics Center (RPC) has credentialed 41 institutions to participate in RTOG 98-05, the first permanent prostate implant protocol, and subsequent prostate implant protocols. The importance of these substantial credentialing efforts is to ensure that patients who are entered in these protocols are treated in a manner specified in the protocols, so that high-quality data are available for analysis.3.The Medical Physics Committee, together with the 3D QA Center in St. Louis, has successfully defined an RTOG external beam communications protocol, which has been implemented by five different commercial software companies. These standards have resulted in the central review of the dosimetry of patients entered into RTOG 3D-CRT protocols.4.IMRT is an exciting new treatment approach. The Medical Physics Committee is in the initial phase of establishing a credentialing process that will include both a treatment verification component and a data transfer central review component. Substantial efforts have been expended to ensure that the dosimetric specifications are complete, consistent, and obtainable with the current technology. In addition, the Medical Physics Committee is performing initial data exchange, dosimetry evaluation, and educational activities. During the next grant period, the Medical Physics Committee will increase its efforts on IMRT protocols, prostate brachytherapy protocols, and 3D-CRT protocols. It can be expected that all commercial planning systems will incorporate Digital Imaging and Communication in Medicine (DICOM) RT, so that an even larger number of institutions can participate in the 3D-CRT protocols. The dosimetry associated with prostate brachytherapy protocols will be reviewed using a digital approach, as opposed to the nondigital approach used in RTOG 98-05. The IMRT dose specifications and treatment delivery approaches will be refined.