Determination of institute specific CTV to PTV prostate margins using the 2020 BIR IGRT geometric uncertainty guidelines for a conventional linear accelerator

OBJECTIVES: A multitude of differing clinical practice exists for prostate radiotherapy, this includes dose prescription, planning methods, asymmetric margins and the presence, or not, of fiducial markers. Institute specific CTV to PTV margins expansion is desirable for optimum treatment of patients. Margins can be determined by assessing geometric errors inherent in radiotherapy. METHODS: The institutional CTV to PTV margins for fiducial free prostate treatments are calculated based on the BIR 2020 methodology derived from the van Herk formulism. Systematic (Σ_tot) and random (σ_tot) errors including delineation, matching and technical accuracy were measured locally while rotational, deformation and intra-fraction errors were quantified from the literature.Σtot and σtot and the penumbral width (σp) are combined with statistical parameters (α) and (β) to calculate the institutional CTV to PTV margin expansion (M) for prostate treatments. RESULTS: An anisotropic CTV to PTV margin expansion for prostate EBRT was calculated. The resulting CTV to PTV margin was 4 mm left/right, 7.5 mm superior, 7 mm inferior, 6 mm anterior and 5.5 mm posterior, rounded to the nearest 0.5 mm. CONCLUSIONS: Systematic errors contributed substantially to the overall margin particularly due to the magnitude of delineation errors. The margin calculated was smaller than the previously set institutional margin and ensures coverage of the PTV by the 95% isodose for 90% of prostate treatments at our centre. ADVANCES IN KNOWLEDGE: Institute specific margins for prostate radiotherapy on a linear accelerator are provided. These are calculated using the BIR 2020 methodology of CTV-PTV margin calculation.