# Impact of intrafraction fiducial tracking on planning target margins during prostate stereotactic body radiotherapy using Brainlab ExacTrac Dynamic with robotic couch correction

## TL;DR

This study shows that using robotic couch corrections during prostate cancer radiotherapy can reduce treatment margins, potentially lowering side effects.

## Contribution

The study evaluates how intrafraction tracking with robotic corrections affects margin reduction in prostate SBRT.

## Key findings

- Most intrafraction fiducial displacements were within 1.5 mm in all directions.
- Pitch deviations most frequently triggered robotic couch corrections.
- Reduced PTV margins of 1.23–1.73 mm were achievable with this technique.

## Abstract

Planning target volume (PTV) margin reduction has been demonstrated to result in decreased acute genitourinary and gastrointestinal toxicity in patients treated with definitive stereotactic body radiotherapy (SBRT) for localized prostate cancer.

The purpose of this study was to assess the impact of stereoscopic x‐ray‐based intrafraction prostate fiducial tracking with six degrees of freedom (6DoF) robotic couch correction on PTV margin reduction during gantry‐based prostate SBRT.

Intrafraction prostate position data was analyzed for patients with localized prostate cancer treated using gantry‐based SBRT with stereoscopic x‐ray intrafraction fiducial monitoring using Brainlab ExacTrac Dynamic (ETD) and 6DoF couch correction at our institution. Fiducial displacements and rotations were calculated between each intrafraction measurement, and mean intrafraction deviations were determined for each fraction, patient, and the overall cohort. Minimum PTV margins required in each dimension were calculated using the van Herk method.

There were 2525 intrafraction stereoscopic fiducial verifications within the cohort. Intrafraction fiducial displacements were within 1.5 mm between 98.7%, 96.1%, and 95.1% consecutive fiducial verifications in the left/right (LR), superior/inferior (SI), and anterior/posterior (AP) dimensions, respectively. Rotational deviations were within 2° between 83.3%, 98.3%, and 99.4% consecutive fiducial verifications in the pitch, roll, and yaw directions, respectively. Twenty‐three percent of stereoscopic verifications triggered a robotic couch correction based upon specified tolerances. Overall, 56% of treatment fractions required a 6DoF couch correction. Fiducial position verifications requiring couch correction were most often triggered by pitch, which occurred in 72% of instances. This corresponded to required minimum PTV margins of 1.23, 1.73, and 1.44 mm in the LR, SI, and AP dimensions, respectively.

Intrafraction fiducial tracking using ETD stereoscopic x‐ray verification with 6DoF robotic couch correction allows for reduced PTV margins when treating localized prostate cancer with gantry‐based SBRT.