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position of the inhomogeneity. Margins for range uncertainties due the CT number variations were not considered in this study so as to focus only on the range changes caused by the displacement of inhomogeneity. An expanded or contracted contour of the high density or low density object was created by adding or subtracting 3 mm margin, respectively to the actual contours of the inhomogeneities (blurred image set). A single field optimization plan was created both in the original and blurred image sets for SSPBT. The robustness of these three plans for distal target coverage for up to 3 mm changes in the inhomogeneity position were studied. Results: The treatment plans for the SSPBT using the blurred image set showed similar robustness as the PSPBT plan with compensator smearing for dose coverage of the distal edge of the target. On the other hand, the distal target coverage in the treatment plan for the SSPBT, that used the image set without inhomogeneity blurring, was reduced when the location of the beam isocenter was changed, making the plan less robust compared to the PSPBT plan. Conclusions: The use of an expanded or contracted margin around high or low density objects in the images, respectively for SSPBT planning appears to be equivalent to the compensator smearing effect in PSPBT planning. Results of application of the proposed approach for designing treatment plans for SSPBT for prostate and targets in brain will be presented.

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