Magnetic resonance image-guided adaptive radiotherapy enables safe CTV-to-PTV margin reduction in prostate cancer: a cine MRI motion study

Rosalyne L Westley; Sophie E Alexander; Edmund Goodwin; Alex Dunlop; Simeon Nill; Uwe Oelfke; Helen A McNair; Alison C Tree

doi:10.3389/fonc.2024.1379596

Magnetic resonance image-guided adaptive radiotherapy enables safe CTV-to-PTV margin reduction in prostate cancer: a cine MRI motion study

Front Oncol. 2024 Jun 4:14:1379596. doi: 10.3389/fonc.2024.1379596. eCollection 2024.

Authors

Rosalyne L Westley^#^{1

2}, Sophie E Alexander^#^{1

2}, Edmund Goodwin³, Alex Dunlop³, Simeon Nill³, Uwe Oelfke³, Helen A McNair^{1

2}, Alison C Tree^{1

2}

Affiliations

¹ Department of Radiotherapy, The Royal Marsden NHS Foundation Trust, London, United Kingdom.
² Division of Radiotherapy and Imaging, The Institute of Cancer Research, Sutton, United Kingdom.
³ Joint Department of Physics, The Institute of Cancer Research and The Royal Marsden NHS Foundation Trust, Sutton, United Kingdom.

^# Contributed equally.

Abstract

Introduction: We aimed to establish if stereotactic body radiotherapy to the prostate can be delivered safely using reduced clinical target volume (CTV) to planning target volume (PTV) margins on the 1.5T MR-Linac (MRL) (Elekta, Stockholm, Sweden), in the absence of gating.

Methods: Cine images taken in 3 orthogonal planes during the delivery of prostate SBRT with 36.25 Gray (Gy) in 5 fractions on the MRL were analysed. Using the data from 20 patients, the percentage of radiotherapy (RT) delivery time where the prostate position moved beyond 1, 2, 3, 4 and 5 mm in the left-right (LR), superior-inferior (SI), anterior-posterior (AP) and any direction was calculated.

Results: The prostate moved less than 3 mm in any direction for 90% of the monitoring period in 95% of patients. On a per-fraction basis, 93% of fractions displayed motion in all directions within 3 mm for 90% of the fraction delivery time. Recurring motion patterns were observed showing that the prostate moved with shallow drift (most common), transient excursions and persistent excursions during treatment.

Conclusion: A 3 mm CTV-PTV margin is safe to use for the treatment of 5 fraction prostate SBRT on the MRL, without gating. In the context of gating this work suggests that treatment time will not be extensively lengthened when an appropriate gating window is applied.

Keywords: CTV-PTV margins; MRIgART; SBRT; intrafraction motion; prostate; ultra-hypofractionation.

Grants and funding

The author(s) declare financial support was received for the research, authorship, and/or publication of this article. RW is funded by The Royal Marsden NHS Foundation Trust with the funding of the research fellow programme receiving funds from Elekta. The Institute of Cancer Research is supported by Cancer Research UK Programme Grant C33589/A28284. SA is funded by Cancer Research UK Programme Grant C33589/A28284 - Adaptive Data-driven Radiation Oncology. AT is supported by Cancer Research UK at a Cancer Research UK Radiation Research Centre of Excellence at The Institute of Cancer Research and The Royal Marsden NHS Foundation Trust (C33589/A28284 and C7224/A28724). AT receives research funding and has received honoraria and travel grants from Elekta. AT also declares research funding from Accuray and Varian and honoraria from Accuray and Janssen. HM is funded by a National Institute for Health Research and Health Education England (HEE/NIHR), Senior Clinical Lectureship ICA-SCL-2018–04-ST2–002.