Dosimetric comparison of intensity modulated proton therapy and knowledge-based volumetric arc therapy for left-sided breast cancer.

[INTRODUCTION] There is increasing interest in the application of proton beam therapy to left-sided breast cancer, due itspotential to reduce heart dose. However, state-of-the-art volumetric modulated arc therapy (VMAT) in conjunction with deep-inspiration breath hold (DIBH) and knowledge-based (KB) planning has demonstrated that low heart doses can be achieved in left-sided breast cancer with X-ray therapy.

[METHOD] Fourteen left-sided breast cancer patients with comprehensive lymph node involvement previously treated at Royal North Shore Hospital (RNSH) were included in the project. All patients were scanned in DIBH and free-breathing (FB) during simulation. Photon plans were generated using the RNSH KB planning model on the DIBH scan. Intensity modulated proton therapy (IMPT) plans were generated on the FB datasets using a two-field approach with multifield optimisation (MFO).IMPT plans were designed to provide equivalent robust target coverage of the VMAT plans with the intention of comparing dose differences to organs at risk (OARs).

[RESULTS] Mean heart dose was significantly reduced (p < 0.0001) for IMPT plans. However, mean heart doses were all below 2 Gy for VMAT plans. Contralateral breast and ipsilateral lung were also significantly reduced with IMPT. Other OARs, including left anterior descending coronary artery, oesophagus, thyroid, spinal cord, humeral head and trachea did not show significant difference between modalities.

[CONCLUSION] Although statistically significant dose reductions for IMPT plans relative to VMAT were reported, the low mean heart doses observed with DIBH KB VMAT plans may limit clinical benefit when considering adverse cardiac events.