Intensity modulated proton therapy and its sensitivity to treatment uncertainties 2: the potential effects of inter-fraction and inter-field motions

Simple tools for studying the effects of inter- fraction and inter- field motions on intensity modulated proton therapy ( IMPT) plans have been developed, and have been applied to both 3D and distal edge tracking ( DET) IMPT plans. For the inter- fraction motion, we have investigated the effects of misaligned density heterogeneities, whereas for the inter- field motion analysis, the effects of field misalignment on the plans have been assessed. Inter- fraction motion problems have been analysed using density differentiated error ( DDE) distributions, which specifically show the additional problems resulting from misaligned density heterogeneities for proton plans. Likewise, for inter- field motion, we present methods for calculating motion differentiated error ( MDE) distributions. DDE and MDE analysis of all plans demonstrate that the 3D approach is generally more robust to both inter- fraction and inter- field motions than the DET approach, but that strong in- field dose gradients can also adversely affect a plan's robustness. An important additional conclusion is that, for certain IMPT plans, even inter- fraction errors cannot necessarily be compensated for by the use of a simple PTV margins, implying that more sophisticated tools need to be developed for uncertainty management and assessment for IMPT treatments at the treatment planning level.