# Contouring atlas and essential points for radiotherapy in rectal cancer

## TL;DR

This paper discusses the changing role of radiotherapy in rectal cancer and the need for updated guidelines on irradiation field boundaries.

## Contribution

The paper highlights the lack of consensus on irradiation field boundaries and proposes the need for collaboration between specialists.

## Key findings

- The upper margin of the irradiation field varies across atlases and is not consistently set to the root of the IMA.
- For patients undergoing preoperative radiotherapy plus TME, the entire mesorectum to the IMA root may not need irradiation.
- For NOM patients, irradiation may need to include the mesorectum to the IMA root, but risks must be considered.

## Abstract

In the last decade, the role of radiotherapy in rectal cancer has changed significantly with the introduction of total neoadjuvant therapy (TNT) and nonoperative management (NOM). For the setting of irradiation field in rectal cancer, the pararectal, lateral lymph nodes, and those along the inferior mesenteric artery (IMA) are most important. In total mesorectal excision (TME), the root of the IMA is dissected. In the atlas of pelvic irradiation for rectal cancer, the setting of the upper margin of the mesorectum varies from atlas to atlas, and no atlas sets the upper margin of the mesorectum to the root of the IMA. In particular, there is no consensus on the definition of anatomical boundaries regarding the lymph nodes along the superior rectal artery (SRA). The upper margin of the irradiation field in clinical trials of preoperative radiotherapy and TNT is generally set at the level of the internal and external iliac artery branches, L5/S1, or S2/S3. However, it is not necessary to include the entire mesorectum to the root of the IMA in patients undergoing preoperative radiotherapy plus TME. Conversely, for patients receiving NOM, the irradiation field may have to include the mesorectum to the IMA root, though the incidence of lymph node metastasis and gastrointestinal adverse events merits consideration. It is increasingly important to determine the extent of clinical target volume around the SRA region and the setting of the upper margin of the irradiation field after formulating the treatment policy together with the surgeons and medical oncologists.