Delivered dose quantification in prostate radiotherapy using online 3D cine imaging and treatment log files on a combined 1.5T magnetic resonance imaging and linear accelerator system.

Charis Kontaxis,Eline N De Groot-Van Breugel,Jochem R.N Van Der Voort Van Zyp,Thomas Willigenburg,Daan M De Muinck Keizer,Bas W Raaymakers,Hans C.J De Boer,Mariska D Den Hartogh,Linda G.W Kerkmeijer,Jan J.W Lagendijk,Jochem Hes

doi:10.1016/j.phro.2020.06.005

Charis Kontaxis, Eline N De Groot-Van Breugel + Show 9 more

Open Access

https://doi.org/10.1016/j.phro.2020.06.005

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Abstract

Monitoring the intrafraction motion and its impact on the planned dose distribution is of crucial importance in radiotherapy. In this work we quantify the delivered dose for the first prostate patients treated on a combined 1.5T Magnetic Resonance Imaging (MRI) and linear accelerator system in our clinic based on online 3D cine-MR and treatment log files. A prostate intrafraction motion trace was obtained with a soft-tissue based rigid registration method with six degrees of freedom from 3D cine-MR dynamics with a temporal resolution of 8.5-16.9s. For each fraction, all dynamics were also registered to the daily MR image used during the online treatment planning, enabling the mapping to this reference point. Moreover, each fraction's treatment log file was used to extract the timestamped machine parameters during delivery and assign it to the appropriate dynamic volume. These partial plans to dynamic volume combinations were calculated and summed to yield the delivered fraction dose. The planned and delivered dose distributions were compared among all patients for a total of 100 fractions. The clinical target volume underwent on average a decrease of 2.2%±2.9% in terms of D99% coverage while bladder V62Gy was increased by 1.6%±2.3% and rectum V62Gy decreased by 0.2%±2.2%. The first MR-linac dose reconstruction results based on prostate tracking from intrafraction 3D cine-MR and treatment log files are presented. Such a pipeline is essential for online adaptation especially as we progress to MRI-guided extremely hypofractionated treatments.

Highlights

With the introduction of methods to quantify intrafraction prostate motion during radiotherapy [1,2,3,4], its impact on dose blurring has become more conspicuous [5]
For five out of 100 fractions it was necessary to adjust the plan based on the intrafraction motion observed on the position verification (PV) scan, for which the registration and dose accumulation reference was appropriately modified
Four of these cases were due to intrafraction motion and one case was due to technical difficulties

Summary

Introduction

With the introduction of methods to quantify intrafraction prostate motion during radiotherapy [1,2,3,4], its impact on dose blurring has become more conspicuous [5]. While the impact of prostate intrafraction motion over a fully fractionated scheme was found to be relatively small [6], current routine clinical practice and subject of recent studies is geared towards hypofractionated treatments [7,8] as well as the use of reduced margins [9] This has led to increasing interest of the radiotherapy community for accurate reconstruction of the delivered dose to the prostate and surrounding Organs At Risk (OAR). While several methods for dose reconstruction are described in literature, popular approaches are based on combining the simulated or delivered beams with the prostate intrafraction motion, obtained by tracking implanted fiducial markers with kilovoltage (kV) imaging [10], megavoltage (MV) imaging [5] or using magnetic implanted markers with the Calypso system [6]. Such a pipeline is essential for online adaptation especially as we progress to MRI-guided extremely hypofractionated treatments

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