# Evaluating intra‐fraction motion and its dosimetric impact in ethos online adaptive radiotherapy for prostate cancer

## TL;DR

This study assesses how prostate and surrounding organs move during radiotherapy and how this affects treatment accuracy.

## Contribution

The study introduces optimized PTV margins for prostate and seminal vesicles in online adaptive radiotherapy.

## Key findings

- Prostate motion was smaller than seminal vesicles motion during treatment setup.
- Reduced PTV margins led to a 37% dose reduction to critical organs without compromising target coverage.
- Maximum prostate motion occurred in inferior and anterior directions, while seminal vesicles moved posteriorly and to the left.

## Abstract

This study evaluated the geometric and dosimetric uncertainties during online adaptive radiotherapy (ART) for prostate cancer.

Sequential cone beam computed tomography (CBCT) scans from 52 sessions involving 13 patients were analyzed. An ART plan was generated using CBCT1, followed by a verification scan (CBCT2) acquired 13.1 ± 3.0 minutes before treatment delivery. New contours (prostate, seminal vesicles, bladder, rectum, and bowel) were delineated on CBCT2 and transferred to CBCT1 for dose distribution analysis. Three dimensional contour variations were quantified, new planning target volume (PTV) margins were calculated, and their dosimetric benefits were assessed.

No significant differences were observed in the high‐dose volumes (V60 Gy and V57 Gy) of the bladder and rectum (P > 0.05). PTV margins were 2.6 mm, 2.4 mm, and 2.6 mm in the lateral, vertical, and longitudinal directions for the prostate, and 3.9 mm, 3.8 mm, and 4.3 mm for the seminal vesicles, respectively. These margin reductions led to a 37% reduction in the dose to the surrounding critical organs, while maintaining consistent target coverage.

This study supports symmetric PTV margins of 3 mm for the prostate and 4.5 mm for the seminal vesicles in online ART, thereby contributing to the development of optimized treatment strategies for prostate cancer.

The figure illustrates their cumulative frequency distributions of motion. The data indicate that movements of the prostate were generally smaller than those of the seminal vesicles. Specifically, the prostate exhibited maximum motion predominantly in the inferior and anterior directions, whereas the seminal vesicles showed maximum motion in the posterior and left directions. This distinction highlights the differing patterns of intrafractional motion between these anatomical structures.

Figure G1. Cumulative frequency distribution of intrafractional motion for the prostate.

Figure G2. Cumulative frequency distribution of intrafractional motion for the seminal vesicles.