Radiation therapy has historically used margins for target volume to ensure dosimetric planning criteria. The size of margin for a given treatment site is still uncertain particularly for moving targets along with set-up variations leading to a fuzziness of target volume. In this study, we have estimated the dosimetric benefit of normal structures using biological-based optimal margins. The treatment margins are derived by knowledge-based fuzzy logic technique which is considering the radiotherapy uncertainties in treatment planning.

All treatment plans were performed using stepped increments of asymmetric margins to estimate prostate radiobiological indices such as tumour control probability (TCP) and normal tissue complication probability (NTCP). An absolute NTCP of 5% was considered to be the maximum acceptable value while TCP of 85% was considered to be the minimal acceptable limit for each volumetric modulated arc therapy (VMAT) plan of localised prostate cancer radiotherapy. Results were used to formulate rules and membership functions for Mamdani-type fuzzy inference system (FIS). In implementing the rules for the fuzzy system for ΔNTCP values above 10%, the PTV margin was not permitted to exceed 5 mm to avoid rectal complications due to margin selection. The new margins were applied in VMAT planning of prostate cancer for standard displacement errors. The dosimetric results of normal tissue predictors were estimated such as organ mean doses, rectum V60 (volume receiving 60 Gy), bladder V65 (volume receiving 65 Gy) and other clinically significant dose–volume indicators and compared with VMAT plans using current margin formulations.

Dosimetric results compared well to the results obtained by current techniques. Good agreement was obtained between proposed fuzzy model margins and currently used margins in lower error magnitude, but significant results were observed at higher error magnitude when organ toxicity concerned without compromising the target volumes.

The new margins may be helpful to estimate possible outcomes of normal tissue complications and thus may improve complication free survival particularly when organ motion errors are inevitable, case by case.