A single centre evaluation of the 2019 UK SABR consortium guidelines for primary lung cancer: correlation between Prescription Dose Spillage and inverse Paddick Conformity Index

BACKGROUND: The aim of the study was to determine level of agreement between RTOG Conformity Index (RTOG-CI), Paddick Conformity Index (PCI) and Prescription Dose Spillage (PDS) in describing lung stereotactic ablative radiotherapy (SABR) plan conformity; to elucidate any limitations, in practice, of PCI and PDS. International Commission on Radiation Units and Measurements report 91 (ICRU 91) aimed to reduce inconsistencies in dose prescription and normalisation between centres by specifying SABR reporting rules, and suggested using PCI. UK SABR Consortium 2019 guidelines adopted PDS to measure plan quality, but not the PCI. MATERIALS AND METHODS: 51 consecutive lung SABR plans received 54 Gy in 3 fractions (54 Gy/3 Fr), 55 Gy/5 Fr or 60 Gy/8 Fr. Plans were developed according to 2016 UK SABR consortium guidelines, which did not specify PCI or PDS; these values were retrospectively calculated. As PCI varies from 0 to an optimum of 1, inverse PCI (invPCI) was used for calculations. RESULTS: PTV-adjusted PDS tolerances were met in 80.4% of studied plans. A near-perfect positive correlation between invPCI and PDS (R2 = 0.978) was found — stronger than between invPCI and the previously-used RTOG-CI (R2 = 0.915). CONCLUSIONS: The strong invPCI-PDS correlation is likely dependent on adequate PTV coverage, present in our cohort. This supports the UK SABR Consortium’s adoption of PDS provided PTV coverage is ensured. Plan conformity should be confirmed by visual slice-by-slice review.