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Published on: February 6, 2019
Impact of Air Gap on Intensity-Modulated Proton Therapy Breast Plans
Suresh Rana1, Jaafar Bennouna1
1Department of Radiation Oncology, Miami Cancer Institute, Baptist Health South Florida, Miami, Florida, USA; Department of Radiation Oncology, Herbert Wertheim College of Medicine, Florida International University, Miami, Florida, USA.
Objective:
The 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.
Methods:
In 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.
Results:
An 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.
Conclusions:
In 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.
