# Multidisciplinary approach to target volume delineation in locally recurrent rectal cancer: An explorative study

## TL;DR

This study explores how multidisciplinary collaboration can reduce variability in defining cancer treatment areas for recurrent rectal cancer.

## Contribution

The study introduces a multidisciplinary approach to reduce interobserver variation in target volume delineation for locally recurrent rectal cancer.

## Key findings

- Radiological input improved delineation consistency in 29% of cases.
- Geographical miss occurred in 7% of cases after radiological input.
- Fibrotic and intraluminal recurrences showed the largest interobserver variation.

## Abstract

•Interobserver variation in locally recurrent rectal cancer remains a clinical challenge.•A small overall improvement is observed when delineating multidisciplinary.•An improvement of IOV is seen in 29 % of cases when exposed to radiological contours.•Geographical miss occurred after radiological input in 7 %.•Sub-analyses indicate differences in interobserver variation between recurrence types.

Interobserver variation in locally recurrent rectal cancer remains a clinical challenge.

A small overall improvement is observed when delineating multidisciplinary.

An improvement of IOV is seen in 29 % of cases when exposed to radiological contours.

Geographical miss occurred after radiological input in 7 %.

Sub-analyses indicate differences in interobserver variation between recurrence types.

Interobserver variation (IOV) in locally recurrent rectal cancer (LRRC) delineations is large, possibly because of different interpretations of imaging. An explorative study was performed to investigate the benefit of additional delineations by expert radiologists.

14 cases of LRRC were delineated on planning CT by 8 radiologists (RADs) to construct a median and total radiology contour, followed by 12 radiation oncologists (ROs), without (GTV−) or with (GTV+) the additional contours. IOV was calculated separately for RADs, GTV− and GTV+. The following metrics were used: the Surface Dice Similarity Coefficient (SDSC), Dice similarity coefficient (DSC), and Hausdorff Distance at the 98th percentile (HD98%). The median SDSC, DSC, and HD98% of GTV− and GTV+ were compared. Sub-analyses of IOV in different recurrence types were performed.

Median SDSC significantly improved from GTV− to GTV+ overall, but a significant benefit could not be proven in individual cases. Additional radiological input consistently improved all parameters in 4/14 cases (29 %). Geographical miss occurred after radiological input in 7 %. Subgroup analyses show large IOV in mainly fibrotic and intraluminal recurrences. Little IOV is seen in solitary nodal recurrences.

This study highlights target volume delineation challenges in LRRC. Overall, radiological input reduced IOV amongst ROs in target volume delineation for LRRC. Large differences do however exist amongst recurrence types. A standard terminology for LRRC and close collaboration between radiologists and radiation oncologists seems necessary to reduce IOV and improve quality of care.