# First experimental demonstration of magnetic resonance-guided multileaf collimator tracking for (ultra-)hypofractionated prostate radiotherapy

## TL;DR

This paper demonstrates how MRI-guided MLC tracking can improve prostate radiotherapy by reducing dose errors caused by patient movement during treatment.

## Contribution

The study experimentally validates MRI-guided MLC tracking for (ultra-)hypofractionated prostate radiotherapy using both 2.5D and 3D imaging workflows.

## Key findings

- MLC tracking improved dose accuracy with 89% and 91% gamma pass-rates for 2.5D and 3D workflows, respectively.
- Dose deviations at the target volume periphery were significantly reduced using tracking.
- Both 2.5D and 3D workflows achieved comparable dosimetric improvements despite differences in latency.

## Abstract

(Ultra-)hypofractionated radiotherapy is an effective treatment for localized prostate cancer, but intrafraction motion can increase toxicity and/or reduce treatment efficacy. Therefore, motion management is essential. This study explores magnetic resonance imaging (MRI)-guided multileaf collimator (MLC) tracking for 2-fraction prostate radiotherapy on an MR-linac.

We compared two MRI-guided MLC centroid tracking workflows, each using a different motion manager to derive and stream target positions to our in-house MLC tracking software. The first workflow relies on interleaved 2D (2.5D) cine-MRI, introducing minimal latency. In contrast, the second workflow utilized 3D cine-MRI, which operates at a relatively lower imaging frequency that introduces more latency.

For experimental validation, we used a motion phantom equipped with an integrated insert that combines film with plastic scintillation dosimetry. A 2x12 Gy 11-beam prostate intensity modulated radiotherapy plan was created for tracking deliveries.

The signal latency introduced by the motion managers was 0.6 s for 2.5D cine-MRI and 6.3 s for 3D cine-MRI. Despite this latency, MLC tracking effectively restored the planned dose, improving the 2%/2mm local gamma pass-rates from 21% (due to linear drift) to 89% (2.5D) and 91% (3D). Plastic scintillator measurements showed reduced dose deviations at the periphery of the clinical target volume from 13–64% (no tracking) to 0–11% (2.5D) and 2–26% (3D).

Our experiments demonstrated the technical feasibility of 2.5D and 3D cine-MRI-based MLC tracking on an MR-linac for 2-fraction prostate radiotherapy, with both motion management strategies achieving comparable dosimetric improvements despite the difference in latency.

•3D multileaf collimator tracking for magnetic resonance guided prostate radiotherapy.•Tracking has clear dosimetric advantages for prostate stereotactic body radiotherapy.•A novel, time-resolved, motion-included, and magnetic resonance compatible dosimeter.

3D multileaf collimator tracking for magnetic resonance guided prostate radiotherapy.

Tracking has clear dosimetric advantages for prostate stereotactic body radiotherapy.

A novel, time-resolved, motion-included, and magnetic resonance compatible dosimeter.