Highlights
- •Radiotherapy is often dealing with extended abdomen and pelvic tumours.
- •These districts are featured by tumour and OARs inter/intra- fraction variation.
- •These tumors can exceed the maximum longitudinally field size of the MRgRT system.
- •Dual isocenters treatment plans can increase significantly the treatment field size.
- •Dosimetric accuracy of dual isocenter has been assessed in MRgRT system.
Abstract
Purpose
Due to limited field size of Magnetic Resonance Linear Accelerators (MR-Linac), some
treatments could require a dual-isocenter planning approach to achieve a complete
target coverage and thus exploit the benefits of the online adaptation. This study
evaluates the dosimetric accuracy of the dual-isocenter intensity modulated radiation
therapy (IMRT) delivery technique for MR-Linac.
Material and Methods
Dual-isocenter multi leaf collimator (MLC) and couch accuracy tests have been performed
to evaluate the delivery accuracy of the system. A mono-isocenter plan delivered in
clinical practice has then been retrospectively re-planned with dual-isocenter technique.
The dual-isocenter plan has been re-calculated and delivered on a 3-dimensional (3D)
ArcCHECK phantom and 2-dimensional (2D) films to assess its dosimetric accuracy in
terms of gamma analysis. Clinical and planning target volume (CTV and PTV respectively)
coverage robustness was then investigated after the introduction of ± 2 mm and ± 5 mm
positioning errors by shifting the couch.
Results
MLC and couch accuracy tests confirmed the system accuracy in delivering a dual-isocenter
irradiation.
2D/3D gamma analysis results occurred always to be above 95% if considered a gamma
criteria 1%/2 mm and 1%/1 mm respectively for the 2D and 3D analysis.
The mean variations for CTV D98% and PTV V95% were 0.2% and 1.1% respectively when
positioning error was introduced separately in each direction, while the maximum observed
variations were 0.9% (CTV) and 3.7% (PTV).
Conclusion
The dosimetric accuracy of dual-isocenter irradiation has been verified for MR-Linac,
achieving accurate and robust treatment strategy and improving dose conformality also
in presence of targets whose extension exceeds the nominal maximum field size.
Keywords
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Article info
Publication history
Published online: April 23, 2021
Accepted:
March 30,
2021
Received in revised form:
March 23,
2021
Received:
November 23,
2020
Identification
Copyright
© 2021 Associazione Italiana di Fisica Medica. Published by Elsevier Ltd. All rights reserved.
