Background: The treatment of rectal cancer has shifted from a primarily surgical approach with radiation therapy or combined chemotherapy and radiation therapy (CRT) toward the preoperative use of neoadjuvant CRT. This shift has resulted in a decrease in the local recurrence rate and an increase in the rates of long-term survival and sphincter preservation. The aim of the study is to investigate & describe the role of whole tumor volume analysis generated from diffusion weighted imaging (DW-MRI), in patients with rectal cancer, as a predictor of response to chemoradiotherapy (CRT). Methods: This study included forty patients diagnosed with rectal cancer. They managed according to national treatment protocols.  Magnetic resonance imaging (MRI) including diffusion-weighted imaging (DWI), before and after CRT was done to all patients. Endoscopy and/or surgery was done to obtain biopsy for histopathological analysis which is the reference standard for pathologic complete response (pCR). Results: the tumor volume was shown to be substantially correlate with tumor response, with the responding group (33.5cm³) having a smaller tumor volume than the non-responding group (65 cm³). Conclusion: histogram analysis of the ADC can evaluate the biologic heterogeneity of a tumor, which may have prognostic and predictive consequences.