Purpose: This study aims to investigate the impact of breathing motion on proton breast treatment plans.
Material and methods: The study cohort was composed of twelve thoracic patients who had CT datasets acquired during breath-hold at inhalation, breath-hold at exhalation and in free-breathing mode. Proton treatment plans were designed for the left breast for the breathhold at inhalation phase and subsequently recalculated for the breath-hold at exhalation phase or designed for the CT acquired in free-breathing mode and recalculated for the extreme breath-hold phases. Four plans were evaluated for each patient: two with three fields and two with one field. The dosimetric features of the plans were compared from the point of view of their coverage of the target and the doses to the organs at risk. The statistical significance of the difference in parameters was tested with a paired, two tailed Student t-test.
Results: Breathing motion led to a degradation of the dose coverage of the target (HI increased from 4-7% to 8-11%). Exhalation tended to decrease the lung burden [average dose 3.1-4.5 Gy(RBE)], while inhalation increased it [average dose 5.8-6.8 Gy(RBE)]. The individual values depended on the field arrangement. Smaller differences were seen for the heart [average dose 0.1-0.2 Gy(RBE)] and the LAD [1.9-4.6 Gy(RBE)]. The differences in dosimetric parameters for various breathing phases were small and their expected clinical impact is consequently quite small.
Conclusion: The results indicated that the dosimetric parameters of the plans corresponding to the two extreme breathing phases are similar, thus suggesting that breathing might have little impact for the chosen beam arrangements with proton scanned beams.