Part I | Chapter 5 96 Materials and methods Steps of the sub-fractionation workflow The presented sub-fractionation workflow is an adaptation of the commercially available clinical software for a 1.5 T MR-Linac system (Unity, Elekta AB, Stockholm, Sweden).2 The workflow consists of several steps and the exemplary workflow applied in prostate cancer patients is displayed in Figure 1. The first part of the sub-fractionation workflow (steps 1-5) is similar to the current commercially available interfraction-adaption (‘Adapt-to-Shape’ [ATS]) method.2 Before treatment, a pre-treatment (offline) MRI is acquired, which is used for offline delineation of the target(s) and OARs and pre-treatment planning. During actual treatment, a daily MRI scan (PRE MRI) is acquired on the MR-Linac (step 1). Next, the ATS procedure is initiated (step 2). During this procedure, the PRE and offline MRI are registered using deformable image registration (DIR) software that is part of the commercially available Monaco® (Elekta Inc., Sunnyvale, California, USA) treatment planning software, after which the contours from the offline scan are non-rigidly propagated to the PRE scan. The contours are visually checked and – if necessary – manually edited by the operator. After approval of the contours, treatment plan re-optimisation is initiated. When the DVH constraints are met, the treatment plan is approved. Just prior to finishing treatment planning (after the first fluence optimisation step), a position verification (PV) MRI scan (PV1) is acquired (step 3). In case shifts of > 1mmoccurred, an ‘Adapt-to-Position’ (ATP) procedure or ‘virtual couch shift’ (VCS) is applied, in which the dose is shifted to correct for translations (step 4).2 Next, treatment delivery is started (step 5). These steps combined constitute one cycle of the sub-fractionation workflow, in which a predefined dose of X Gy is delivered without interruption. This first initiation cycle comprises interfraction adaptation. The next cycle is initiated during beam-on time of the previous cycle and thus steps 6 and 7 are executed in parallel with treatment delivery (step 5). First, right after start of beam-on, a new PV MRI scan (PV2) is obtained (step 6), which is rigidly registered to the initial daily MRI using the ATP procedure (step 7). Contours are now rigidly propagated, followed by treatment plan optimisation. In case the treatment plan complies with the DVH constraints, treatment delivery is initiated (step 8). Simultaneously, new imaging can be acquired (step 9). Steps 6-8 can be repeated a predefined number of times, based on e.g. the expected organmotion and/or deformations within a predefined time frame. Figure 1 – Schematic overview of the sub-fractionation workflow. Legend: ATS = Adapt-to-Shape. ATP = Adaptto-Position. PV = Position verification.
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