# Radiotherapy volume delineation based on MRI and 18F-FDG-PET/MRI in locally recurrent rectal cancer

## TL;DR

This study compares MRI and PET/MRI for defining radiotherapy targets in recurrent rectal cancer, finding that PET/MRI improves accuracy.

## Contribution

The study demonstrates that PET/MRI improves target delineation accuracy in recurrent rectal cancer radiotherapy.

## Key findings

- PET/MRI and MRI delineations showed strong correlation for GTV, CTV, and GTVn.
- PET/MRI and MRI had similar efficiency in detecting metastatic lymph nodes.
- Spatial heterogeneity was observed in GTV and GTVn between the two imaging methods.

## Abstract

To evaluate the value of 18F-FDG-positron emission tomography (PET)/magnetic resonance imaging (MRI) functional imaging in the radiotherapy of locally recurrent rectal cancer by comparing the target volume delineation based on PET/MRI and MRI.

Twenty-six patients who were diagnosed with locally recurrent rectal cancer were included in this study. Patients underwent PET/MRI, and the target volume was delineated independently by three radiation oncologists. The degree of overlap, spatial consistency, and difference in the target volume delineated based on the two methods were compared. The efficacy of PET/MRI and MRI in detecting metastatic lymph nodes was analyzed.

In radiotherapy for patients with recurrent rectal cancer, the gross tumor volume (GTV), clinical target area (CTV), and nodal gross tumor volume (GTVn) delineated based on MRI and PET/MRI were correlated (P < 0.001, P < 0.001, and P < 0.001, respectively). Differences in CTV were statistically significant (P < 0.001), and the CTV greatly overlapped spatially. There is spatial heterogeneity in GTV and GTVn based on the two imaging modalities. Metastatic lymph node analysis revealed that the detection efficiency of the two modalities was the same at the population level. There was no significant difference in the number of metastatic lymph nodes detected (P = 0.521).

PET/MRI can improve the accuracy of target volume delineation and has similar advantages to MRI in assessing the number of metastatic lymph nodes in patients with recurrent rectal cancer.