Measurement of intramural tumor spread distance and determination of optimal distal resection margins after neoadjuvant radiotherapy for rectal cancer under naked eyes

Abstract Background It is not clear about the safe distance between the intra-operative resection line and the distal rectal tumor visible margin after preoperative radiotherapy. We aimed to investigate the furthest tumor intramural spread distance in fresh tissue to determine safe distal intraoperative resection margin length. Methods Twenty specimens of rectal cancer after preoperative radiotherapy were collected. Tumor intramural spread distances were defined between tumor visible margin and microscopic margin. Tumor visible margins in fresh specimen were identified during operation and were labeled with 5 − 0 sutures under naked eyes at distal 5, 6, 7 o’clock direction of visible margins right after removal of the tumor. Labels of suture were injected with nanocarbon particles by gauge 27 syringe after fixation with formalin. Longitudinal tissues were collected along three labels and stained with hematoxylin and eosin. The spread distance after formalin fixation was measured between the furthest intramural spread tumor cell and the nanocarbon under the microscope. Positive intramural spread distance meant the furthest tumor cell was distal to the nanocarbon, and negative value meant tumor cell being proximal to the nanocarbon. Tumor intramural spread distance in fresh tissue during operation was 1.75 times of tumor intramural spread distance after formalin fixation according to literature. Results At distal 5, 6 and 7 o’clock direction, seven (35%), five (25%) and six (30%) patients had distal tumor cell intramural spread distance > 0 mm. The median tumor cell intramural spread distance in fresh tissue during operation was − 0.9 (IQR, -4.8, 3.5) mm, 0 (IQR, -4.8, 3.1) mm and − 3.5 (IQR, -5.3, 5.3) mm, respectively. The maximal intramural spread distances in fresh tissue during operation were 8.75, 7 and 7 mm, respectively. Conclusions The intraoperative distance between the distal resection line and rectal tumor visible margin after radiotherapy should be around 1cm to ensure oncological safety.