In recent years, with the increase in the adoption of anus-preserving surgery, understanding of residual lower rectal cancer distribution to the anal side after chemoradiotherapy (CRT) has become an increasingly important issue. We aimed to clarify the pathological safe distal resection margin for lower rectal cancer after preoperative CRT. This study included 36 patients with lower third rectal cancer, who underwent preoperative CRT. We classified the gross tumor appearance as type 0-II like, Borrmann type 2, and Borrmann type 5. Whole-mount sections were used for pathological examination. We examined all slides and measured the extent of residual cancer spread. In many cases, residual cancer was observed in the deeper layers of the lesion, and in none of the cases was the cancer limited to the superficial layer. With regard to lateral distribution, tumors with a type 0-II like appearance showed a wider extent of lateral cancer spread from the optimal margin. In conclusion, although CRT contributes to tumor reduction, attention should be paid to both circumferential and lateral residual cancer spread. Our results suggest that the lateral distribution of residual cancer spread could be predicted by gross tumor appearance. This is an ongoing study.