A system for assessing the response to treatment of advanced breast cancer was proposed by the Union Internationale Contre le Cancer (UICC). The system was developed as part of a project initiated by the UICC's Programme on Clinical Oncology in 1975, aiming to create a standardized method for evaluating treatment responses internationally. Advanced breast cancer was chosen as the first tumor type for this assessment due to the existing variety of evaluation methods and the potential for agreement on a common system. The guidelines outline the criteria for eligibility, exclusions, baseline studies, and methods for measuring and recording lesions. They emphasize the importance of objective evaluation, with specific criteria for measuring and documenting the response to treatment. The system includes definitions for different categories of response, such as complete response, partial response, no change, and progressive disease. It also specifies the duration of response and survival, and recommends extra-mural review of patient records. The system replaces the previously published assessment method of the British Breast Group and is intended to be widely adopted as a model for assessing therapy in other tumors. The guidelines provide detailed instructions for physical examination, imaging, and laboratory studies, as well as for the recording and interpretation of lesion measurements. They also address the stratification of patients based on factors such as menopausal status, disease-free interval, and other clinical characteristics to ensure fair comparison of treatment outcomes. The document was developed through collaboration with international experts and is supported by various references and studies.A system for assessing the response to treatment of advanced breast cancer was proposed by the Union Internationale Contre le Cancer (UICC). The system was developed as part of a project initiated by the UICC's Programme on Clinical Oncology in 1975, aiming to create a standardized method for evaluating treatment responses internationally. Advanced breast cancer was chosen as the first tumor type for this assessment due to the existing variety of evaluation methods and the potential for agreement on a common system. The guidelines outline the criteria for eligibility, exclusions, baseline studies, and methods for measuring and recording lesions. They emphasize the importance of objective evaluation, with specific criteria for measuring and documenting the response to treatment. The system includes definitions for different categories of response, such as complete response, partial response, no change, and progressive disease. It also specifies the duration of response and survival, and recommends extra-mural review of patient records. The system replaces the previously published assessment method of the British Breast Group and is intended to be widely adopted as a model for assessing therapy in other tumors. The guidelines provide detailed instructions for physical examination, imaging, and laboratory studies, as well as for the recording and interpretation of lesion measurements. They also address the stratification of patients based on factors such as menopausal status, disease-free interval, and other clinical characteristics to ensure fair comparison of treatment outcomes. The document was developed through collaboration with international experts and is supported by various references and studies.