Four-dimensional proton treatment planning for lung tumors.

PURPOSE In proton radiotherapy, respiration-induced variations in density lead to changes in radiologic path lengths and will possibly result in geometric misses. We compared different treatment planning strategies for lung tumors that compensate for respiratory motion. METHODS AND MATERIALS Particle-specific treatment planning margins were applied to standard helical computed tomography (CT) scans as well as to "representative" CT scans. Margins were incorporated beam specific laterally by aperture widening and longitudinally by compensator smearing. Furthermore, treatment plans using full time-resolved 4D-computed tomography data were generated. RESULTS Four-dimensional treatment planning guaranteed target coverage throughout a respiratory cycle. Use of a standard helical CT data set resulted in underdosing the target volume to 36% of the prescribed dose. For CT data representing average target positions, coverage can be expected but not guaranteed. In comparison to this strategy, 4D planning decreased the mean lung dose by up to 16% and the lung volume receiving 20 Gy (prescribed target dose 72 Gy) by up to 15%. CONCLUSION When the three planning strategies are compared, only 4D proton treatment planning guarantees delivery of the prescribed dose throughout a respiratory cycle. Furthermore, the 4D planning approach results in equal or reduced dose to critical structures; even the ipsilateral lung is spared.

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