# Pelvic lymph node motion during cone-beam computed tomography guided stereotactic radiotherapy

## TL;DR

This study shows that pelvic lymph node movement during a specific type of radiation therapy is minimal, suggesting that smaller treatment margins may be safe.

## Contribution

The study provides new empirical data on pelvic lymph node motion during CBCT-guided SBRT, supporting potential margin reduction.

## Key findings

- Pelvic lymph node intrafraction motion was limited to 3 mm in 97–100% of fractions.
- Para-rectal lymph nodes showed increased inter- and intrafraction motion compared to other pelvic locations.
- A 3-mm PTV margin was associated with a high mean lesion inclusiveness index of 96%.

## Abstract

•Pelvic lymph node intrafraction motion was limited to 3 mm in 97–100 % of fractions.•A 3-mm PTV margin was associated with a high mean lesion inclusiveness index of 96 %•Para-rectal lymph nodes showed increased lesion inter- and intrafraction motion.

Pelvic lymph node intrafraction motion was limited to 3 mm in 97–100 % of fractions.

A 3-mm PTV margin was associated with a high mean lesion inclusiveness index of 96 %

Para-rectal lymph nodes showed increased lesion inter- and intrafraction motion.

Stereotactic body radiotherapy (SBRT) is increasingly applied for pelvic lymph node recurrence. Thus far, knowledge on pelvic lymph node motion during CBCT-guided SBRT is lacking and the applied margins vary between institutions. This study evaluated pelvic lymph node motion during CBCT-guided SBRT and assessed the currently applied PTV margins of 3 and 5 mm.

In total, 45 pelvic lymph node metastases were included. One observer delineated 45 GTVs on planning CT, 224 GTVs on pre-fraction and 216 on post-fraction CBCT. The GTV centroid coordinates were derived from all images for inter- and intrafraction motion analysis. Additionally, we assessed the influence of treatment time and lesion location on lesion motion. The expected coverage of a 3-mm and 5-mm PTV margin was assessed using the inclusiveness index for GTVs on pre- and post-fraction CBCT.

Lymph node interfraction motion was limited to 5 mm in 96–97 % of fractions for all translational directions and intrafraction lesion motion was limited to 3 mm in 97–100 % of fractions. Para-rectal lesions (11 %) were associated with significantly larger inter- and intrafraction motion compared to other pelvic locations and treatment duration showed no correlation with lesion motion. The mean (sd) lesion inclusiveness index was 99 % (5 %) for the 5-mm PTV margin and 96 % (9 %) for the 3-mm margin.

Pelvic lymph node motion during CBCT-guided stereotactic radiotherapy was within the widely applied PTV margin of 5 mm, providing an opportunity to reduce this margin for pelvic lymph node SBRT.