  Intensity modulated proton therapy (IMPT) provides highly conformal dose
distributions through the application of multiple, angularly spaced fields,
each applying an ad-hoc pattern of spatially varying particle fluences. In
particular, Bragg peaks are simultaneously optimized for all the fields. Once
the number and the direction of the fields are set, the dose distribution of
the IMPT plan is determined by the dose constraints assigned to specific
organs-at-risk (OARs) surrounding the tumour. In this work, we introduce a new
feature for the OARs, called "opacity", aimed at improving the quality and the
robustness of IMPT plans, by modifying, in the pre-optimization stage, the
fluence of the pencil beams, that intersect those structures. The proposed IMPT
planning strategy is addressed to those clinical cases, where metallic
prothesis or fast density-varying organs can compromise the stability of the
dose distribution. We show how the usage of this additional parameter can lead
to more accurate IMPT plans with respect to the situation in which only dose
constraints are adopted.
