Background: Although cone beam computed tomography (CBCT) has been available for over a decade, the pattern of tumor volume change in rectal dancer during preoperative concurrent chemoradiotherapy (CCRT) has been under the veil due to poor soft tissue contrast of registration CBCT. This study was conducted to investigate daily rectal tumor volume change using registration magnetic resonance imaging (MRI). Methods: Patients diagnosed with cT3-4 and/or cN+ rectal adenocarcinoma undergoing preoperative CCRT with capecitabine on the pelvis up to 50 Gy in 25 daily fractions from November 2018 to June 2019 were consecutively included. Rectal tumor volume was uniformly measured by single physician (YKK) in daily 0.35T MRI obtained with ViewRay MRIdian Linac (ViewRay Inc., Oakwood, USA). The mean +/- standard deviation (SD) of daily tumor volume (cc), difference of tumor volume between first fraction and daily fraction (daily tumor volume – baseline tumor volume at first fraction), and tumor volume reduction rate (%, (daily tumor volume – baseline tumor volume at first fraction)/baseline tumor volume at first fraction x 100) were calculated. Statistical significance of differences were tested using the Wilcoxon’s signed rank test and paired t-test. Results: Thirteen patients were included. Median age was 65 and majority of the patients were male (92.3%). Most tumors were T3 (76.9%) and N1-2 (92.3%). Tumors were located median 6 cm (range: 2.4 – 9) above anal verge. Median follow-up was 27.3 months (range: 11.7 – 30.6) and 2 patients had recurred at the time of analysis (1 local and 1 distant). Tumor volume steadily regressed with daily administration of CCRT (mean 2.06 +/-1.73 cc; Figure 1). Significant tumor volume reduction compared to baseline was observed from fourth fraction (mean -12.37 +/- 16.72 cc; P< 0.0005). The difference of tumor volume between baseline and the last fraction was mean -49.33 +/- 68.13 cc (P< 0.0002). The tumor volume reduction rate was significantly increased since fourth fraction (mean - 17.2 +/- 11.78 %; P< 0.0002). The tumor volume reduction rate at the last fraction was - 70.62 +/- 14.01 % (P< 0.0001). Conclusions: For the first time, this study demonstrated daily tumor volume regression in preoperative rectal CCRT with capecitabine using daily MRI. Steady pattern of tumor regression may be explained in part by daily administration of capecitabine. Based on the hypothesis-generating observation, this study may warrant initiation of further investigations such as analysis and comparison with daily tumor volume regression with CCRT using leucovorin and 5-FU.