Tumor Regression Grade Assessment

Pathological Assessment Methods

The National Comprehensive Cancer Network recommends a modified 4-tier or 5-tier grading system to measure tumor regression grade, with Grade 0 indicating a complete response and no viable tumor cells, Grade 1 indicating 1-10% residual cancer cells, Grade 2 indicating 11-50% residual cancer cells, and Grade 3 indicating more than 50% residual cancer cells or gross residual carcinoma 1, 2
For a complete pathological response designation, both the primary site and lymph nodes must be free of viable tumor cells, according to the American College of Surgeons 1
The pathology report should include an assessment of the entire tumor bed, requiring thorough sampling of the primary tumor site, particularly when no grossly obvious residual tumor is present, as recommended by the National Comprehensive Cancer Network 2

MRI tumor regression grading provides complementary prognostic information and can identify treatment responders, using T2-weighted imaging to assess fibrosis versus residual tumor, with diffusion-weighted imaging improving interobserver agreement, according to the European Society for Medical Oncology 7, 8
Sequential MRI comparison provides better discrimination between good and poor responders than single post-treatment imaging, as reported by the European Society for Medical Oncology 7

The American College of Radiology recommends that PET/CT combined with MRI increases accuracy for predicting complete response to 94.7% compared to MRI alone at 76% 8
The European Society for Medical Oncology notes that grading cannot be reliably assigned after preoperative treatment in certain contexts, such as sarcomas, due to major therapy-related changes that prevent standard malignancy grading 4, 5, 6
The National Comprehensive Cancer Network emphasizes the importance of standardized protocols to reduce interobserver variability among expert pathologists 2
The American College of Surgeons recommends that the pathological assessment of margins be performed in collaboration with the surgeon to ensure accurate evaluation of treatment response 4, 5, 6