Tumor volume and-orTumor thickness should be considered in TNM classification of rectal tumors Tumor volume and/orTumor thickness should be considered in TNM classification of rectal tumors P. A. Coucke, A. Zouhair, H. Bouzourene, J.-Y. Meuwly Radiation Oncology, Pathology, Radiology, CHUV-Lausanne Purpose: Very often tumor size is considered for the TNM classification of human tumors. This is not the case for rectal tumors in contrast to anal tumors. We aimed at investigating in Trial 93-01 (HART in a preoperative setting) whether, initial tumor size, assessed by measurements on pathological specimen is predictive for outcome. We also plan to demonstrate that size assessment by ultrasound is the most important prognostic factor, already obtained at initial work-up. Patients and methods: Carefull assessment of tumor size has been performed on all pathological specimens of 105 patients introduced in Trial 93-01 in Lausanne. These volume estimates are currently compared to those obtained by ultrasound (work in progress). Results: Tumor thickness or tumor volume are independent predictors of survival and disease free survival in rectal tumors Interestingly, cT/pT classification is eliminated in the multivariate analysis when parameters linked to size are considered, whereas pN remains in the model. The analysis of ultrasound tumor size estimation is currently ongoing. Conclusions: We suggest that tumor volume is related to clonogen number and metastatic potential, and therefore it should be considered in the recording and reporting of results in rectal trials.