Impact of Air Gap on Intensity-Modulated Proton Therapy Breast Plans

Abstract

ObjectiveThe primary purpose of this study was to investigate the impact of an air gap (AG) on breast cancer plans that are generated using the intensity-modulated proton therapy technique. MethodsIn this retrospective study, we have included a total of ten female patients with left breast cancer. A total clinical target volume (CTV_Total) was generated by combining breast or chest wall, axillary level I–III nodes, internal mammary nodes, and supraclavicular nodes. Monte Carlo algorithm used within RayStation was used for both the robust plan optimization and dose computation. For each case, intensity-modulated proton therapy breast plan with a 4 cm AG was considered as the base plan. To test the impact of the AG, the base plan (AG = 4 cm) of a given case was copied and recalculated for additional AG of 6, 8, 10, 12, and 14 cm. ResultsAn increase in the AG yielded a decrease in the dosimetric and tumor control probability (TCP) values in all ten patients. As the AG increased from 4 cm to 14 cm, the impact of the AG on the dosimetric indices was the most severe for the target volume received by 95% of the prescription dose (CTV_Total V95%) and dose received by 99% of the target volume (CTV_Total D99%), which showed the average reduction of 7% ± 3% and 6.3% ± 2.8%, respectively. Similarly, the average decrease in dose received by 99% of the volume (CTV_Total D95%), CTV_Total mean dose, Homogeneity Index of CTV_Total, and TCP of CTV_Total were 4.7% ± 1.8%, 2.1% ± 0.6%, 0.03 ± 0.02, and 1.3% ± 0.4%, respectively. ConclusionsIn the present study, an increase in 2 cm AG provided a reduction in the dosimetric and TCP results by ≤1%. An increment of the AG by 4 cm (ie, from 4 cm to 8 cm) resulted in a loss of ≤2% and ≤0.6% in the dosimetric and TCP results, respectively. There was no distinct trend between organ at risk results and variation in the AG.