A retrospective bilateral breast proton pencil beam scanning and photon volumetric arc therapy planning comparison

Advanced bilateral breast cancers with local and regional nodal involvement are rare and challenging to plan due to conflicting demands of conformality, uniformity and sparing of organs at risk such as heart and lungs. Pencil beam scanning (PBS) protons have been shown to provide improved organ at risk (OAR) sparing, conformality and homogeneity compared to photon techniques including three-dimensional conformal radiotherapy, intensity modulated radiation therapy (IMRT), volumetric arc therapy (VMAT), and tomotherapy. We performed a blinded comparison between VMAT and PBS plans and extend this comparison with a case study of a patient win expander in situ during radiotherapy.

In this case-controlled study, five bilateral breast patients' clinical proton plan datasets were anonymized and replanned using VMAT. Additionally, for one patient with bilateral expanders, we compared two PBS planning techniques that are used clinically.

For the similar target volume coverage, significant differences were found for heart and lung sparing, particularly for lower isodoses. For instance, mean lung V5 was 96.7% vs. 39% for VMAT vs. PBS and mean heart dose was 11.4 vs. 0.69 Gy (RBE) for VMAT vs. protons. The expander patient's plan did not differ significantly from whole breast or chest wall patients in terms of coverage or in gains from PBS compared to VMAT. For example, for comparable homogeneity indexes (HIs) of 1.2 (VMAT) and 1.1 (PBS), the mean heart dose is 11 vs. 1 Gy. The gains from PBS are approximately a factor of 10 compared to a factor of <2 for estimated gains to VMAT plans using deep inspiration breath hold (DIBH) described in the literature.

PBS provides better sparing of heart and lungs compared to VMAT in whole breast, chest wall and expander patients. Further study with more patients is required to validate these conclusions.