Part II | Chapter 9 184 Target definition Pre-treatment delineations for the salvage FS-HDR-BT were performed using the in-house developed software Volumetool®10 by experienced prostate cancer radiation oncologists on diagnostic MRI and prostate-specific membrane antigen (PSMA) positron emission tomography (PET) images. The Gross Tumour Volume (GTV) consisted of the visible tumour and was expanded uniformly by 5 mm to create the Clinical Target Volume (CTV), staying inside the prostate body and/or seminal vesicle anatomical boundaries. Prostatic urethra, rectum, and bladder were delineated. For FS-HDR-BT treatment planning, the Planning Target Volume (PTV) was equal to the CTV (i.e. a CTV to PTV margin of 0 mm). For MR-Linac treatment planning, a CTV to PTV margin of 1 mmwas adopted. This 1 mmmargin has to account for additional uncertainties that are introduced with the use of an MR-Linac system compared to FS-HDR-BT. These uncertainties consist of intrafraction prostatic motion and geometrical (in)accuracy of the system. Prior investigations by our department showed that the geometrical accuracy of the Elekta MR-Linac system is on average 0.3 mm (range 0.2-0.4 mm).6 Concerning prostate motion, we have developed an accurate soft tissue prostate tracking algorithm with a mean error of 0.07 mm (SD 0.22 mm).11,12 Combined with new treatment planning systems developed at our department that facilitate online (intrafraction) replanning to account for anatomy shifts and changes, we assumed that a 1 mm PTV margin is achievable.9,13,14 Figure 1 – Different tumour locations treated with FS-HDR-BT. A: peripheral tumour, not near any of the organsat-risk (OARs). B: central tumour adjacent to the urethra. C: peripheral/lateral tumour between rectum and urethra. D: central tumour between rectum and urethra. E: tumour adjacent to bladder and urethra. F: Peripheral tumour adjacent to rectum and not near urethra. G: tumour in seminal vesicles, adjacent to rectum and/or bladder. H: Tumour in base of the prostate, adjacent to rectum and bladder, without seminal vesicle involvement. Legend: GTV = Gross Tumour Volume. CTV = Clinical Target Volume. PTV = Planning Target Volume.
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