Proton versus Photon Breath-Hold Radiation for Left-Sided Breast Cancer after Breast-Conserving Surgery: A Dosimetric Comparison

Purpose: Radiation to breast, chest wall, and/or regional nodes is an integral component of breast cancer management in many situations. Irradiating left-sided breast and/or regional nodes may be technically challenging because of cardiac tolerance and subsequent risk of long-term cardiac complications. Deep inspiratory breath-hold (DIBH) technique physically separates cardiac structures away from radiation target volume, thus reducing cardiac dose with either photon (Ph) or proton beam therapy (PBT). The utility of combining PBT with DIBH is less well understood. Methods and Materials: We compared photon-DIBH (Ph-DIBH) versus proton DIBH (Pr-DIBH) for different planning parameters, including target coverage and organ at risk (OAR) sparing. Necessary ethical permission was obtained from the institutional review board. Ten previous patients with irradiated, intact, left-sided breast and Ph-DIBH were replanned with PBT for dosimetric comparison. Clinically relevant normal OARs were contoured, and Ph plans were generated with parallel, opposed tangent beams and direct fields for supraclavicular and/or axillae whenever required. For proton planning, all targets were delineated individually and best possible coverage of planning target volume was achieved. Dose-volume histogram was analyzed to determine the difference in doses received by different OARs. Minimum and maximum dose (D-min and D-max) as well as dose received by a specific volume of OAR were compared. Each patient's initial plan (Ph-DIBH) was used as a control for comparing newly devised PBT plan (Pr-DIBH). Matched, paired t tests were applied to determine any significant differences between the 2 plans. Results: Both the plans were adequate in target coverage. Dose to cardiac structure subunits and ipsilateral lung were significantly reduced with the proton breath-hold technique. Significant dose reduction with Pr-DIBH was observed in comparison to Ph-DIBH for mean dose (D-mean) to the heart (0.23 Gy versus 1.19 Gy; P < .001); D-mean to the left ventricle (0.25 Gy versus 1.7 Gy; P < .001); D-mean, D-max, and the half-maximal dose to the left anterior descending artery (1.15 Gy versus 5.54 Gy; P < .003; 7.7 Gy versus 22.15 Gy; P < .007; 1.61 Gy versus 4.42 Gy, P < .049); D-max of the left circumflex coronary artery (0.13 Gy versus 1.35 Gy; P < .001) and D-mean, the volume to the ipsilateral lung receiving 20 Gy and 5 Gy (2.28 Gy versus 8.04 Gy; P < .001; 2.36 Gy versus 15.54 Gy, P < .001; 13.9 Gy versus 30.28 Gy; P = .002). However, skin dose and contralateral breast dose were not significantly improved with proton. Conclusion: This comparative dosimetric study showed significant benefit of Pr-DIBH technique compared with Ph-DIBH in terms of cardiopulmonary sparing and may be the area of future clinical research.