Simultaneous tumour dose escalation and liver sparing in Stereotactic Body Radiation Therapy (SBRT) for liver tumours due to CTV-to-PTV margin reduction

PurposeTo quantify potential benefits of CTV-to-PTV margin reduction for SBRT of liver tumours, as allowed by enhanced treatment precision.Materials and methodsFor 14 patients plans were generated for the clinical margin and for 3 tighter margins. An in-house developed algorithm was used to optimise beam directions, shapes, and weights for generation of the plan with the highest isocenter dose (Diso), while keeping the minimum PTV dose at least 65% × Diso and strictly adhering to all imposed hard OAR constraints. Each plan contains 10 optimal beam directions, automatically selected from up to 252 coplanar and non-coplanar input directions.ResultsApart from the expected tumour dose escalation (Diso, EUDPTV, gEUDPTV) with decreasing margin, a simultaneous improved sparing of the normal liver (D33%, D50%, Dmean) was also observed. The smaller the margin was, the bigger both effects were. For renormalized plans with Diso equal to the clinical value (3 × 19.2 Gy), and a margin reduction of 50% (2.5 mm laterally, 5 mm longitudinally), normal liver D33% and D50% reduced on average by 22% (maximum 38%), and 26% (maximum 47%), respectively.ConclusionsUsing an algorithm for beam direction, shape and weight optimisation, large increases in the therapeutic ratio of liver plans could be obtained for reduced margins.