Colorectal cancer is the second leading cause of cancer-related deaths worldwide, and one third of these carcinomas are rectal cancer [[1], [2], [3]]. Radical surgery is the only curative treatment for most cases of extraperitoneal rectal cancer, with a 5-year overall survival rate of up to 95 % [4]. However, survival is poorer in patients with locally advanced rectal cancer, including a TNM classification T3/T4 (invasion of tumors in muscular layers) or N1/N2 (invasion of regional lymph nodes). In these patients, survival rates are around 89 % without metastasis and 32 % with metastasis after neoadjuvant chemoradiotherapy and radical resection [5,6].

Several neoadjuvant and adjuvant treatments have been introduced to improve the survival of patients with rectal cancer after radical surgery. In patients with locally advanced rectal cancer, the European Society for Medical Oncology has recommended preoperative short-course radiotherapy or preoperative chemoradiotherapy [7]. The National Comprehensive Cancer Network has also suggested short-course radiotherapy or chemoradiotherapy as a neoadjuvant treatment for T3/T4 tumors without distant metastasis [7,8]. These recommendations are based on the advantages of neoadjuvant therapies in patients with rectal cancer; these advantages include anal sphincter preservation, preoperative tumor downstaging, lower local recurrence rate, and fewer treatment-related toxicities than adjuvant chemoradiotherapy [9]. Although complete and partial tumor regression after neoadjuvant short-course radiotherapy or chemoradiotherapy improves long-term outcomes in patients with rectal cancer, complete regression is only observed in 10–25 % of patients [9,10].

Correlation between preoperative factors such as the tumor regression grade (TRG) and improvement of long-term outcomes after radical surgery for rectal cancer is still under debate [11], and it is not clear which TRG leads to the best long-term outcomes. It is of great importance to identify those patients who would benefit most from neoadjuvant chemoradiotherapy followed by radical surgery. To address this issue, we evaluated the impact of the TRG on long-term oncological outcomes of patients with rectal cancer who underwent neoadjuvant chemoradiotherapy prior to radical surgery.