Understanding the Benefit of Magnetic Resonance-guided Adaptive Radiotherapy in Rectal Cancer Patients: a Single-centre Study. Issue 2 (February 2023) - Ingle, M.
Abstract: Aims: Neoadjuvant chemoradiotherapy followed by surgery is the mainstay of treatment for patients with rectal cancer. Standard clinical target volume (CTV) to planning target volume (PTV) margins of 10 mm are used to accommodate inter- and intrafraction motion of target. Treating on magnetic resonance-integrated linear accelerators (MR-linacs) allows for online manual recontouring and adaptation (MRgART) enabling the reduction of PTV margins. The aim of this study was to investigate motion of the primary CTV (CTVA; gross tumour volume and macroscopic nodes with 10 mm expansion to cover microscopic disease) in order to develop a simultaneous integrated boost protocol for use on MR-linacs. Materials and methods: Patients suitable for neoadjuvant chemoradiotherapy were recruited for treatment on MR-linac using a two-phase technique; only the five phase 1 fractions on MR-linac were used for analysis. Intrafraction motion of CTVA was measured between pre-treatment and post-treatment MRI scans. In MRgART, isotropically expanded pre-treatment PTV margins from 1 to 10 mm were rigidly propagated to post-treatment MRI to determine overlap with 95% of CTVA. The PTV margin was considered acceptable if overlap was >95% in 90% of fractions. To understand the benefit of MRgART, the same methodology was repeated using a reference computed tomography planning scan for pre-treatment imaging. Results: In total, nine patients were recruited between January 2018 and December 2020 with T3a-T4, N0–N2, M0 disease. Forty-five fractions were analysed in total. The median motion across all planes was 0 mm, demonstrating minimal intrafraction motion. A PTV margin of 3 and 5mm was found to be acceptable in 96 and 98% of fractions, respectively. When comparing to the computed tomography reference scan, the analysis found that PTV margins to 5 and 10 mm only acceptably covered 51 and 76% of fractions, respectively. Conclusion: PTV margins can be reduced to 3–5 mm in MRgART for rectal cancer treatment on MR-linac within an simultaneous integrated boost protocol. Highlights: Adaptive radiotherapy on MRgART for rectal cancer enables adaptation of treatment to anatomy of the day. Intrafraction motion is minimal between start and end of treatment; however interfraction motion between reference CT planning scan and treatment is large and may not be fully covered with a reduced PTV margin. Using new RCR guidelines nomenclature, the ICTV absorbs motion effectively, adding a 15 mm PTV margin to the GTV. The ICTV component can be removed from MR-linac treatment and a direct PTV margin can be applied to the GTV. Reducing target volumes on MR-linac can lead to trials in dose escalation in order to increase cPR rates.