Proton Therapy for Breast Cancer: A Consensus Statement From the Particle Therapy Cooperative Group Breast Cancer Subcommittee

Abstract

Radiation therapy plays an important role in the multidisciplinary management of breast cancer. Recent years have seen improvements in breast cancer survival and a greater appreciation of potential long-term morbidity associated with the dose and volume of irradiated organs. Proton therapy reduces the dose to nontarget structures while optimizing target coverage. However, there remain additional financial costs associated with proton therapy, despite reductions over time, and studies have yet to demonstrate that protons improve upon the treatment outcomes achieved with photon radiation therapy. There remains considerable heterogeneity in proton patient selection and techniques, and the rapid technological advances in the field have the potential to affect evidence evaluation, given the long latency period for breast cancer radiation therapy recurrence and late effects. In this consensus statement, we assess the data available to the radiation oncology community of proton therapy for breast cancer, provide expert consensus recommendations on indications and technique, and highlight ongoing trials' cost-effectiveness analyses and key areas for future research.

Figure 1:

Axial CT dose color wash from an IMRT plan (A) and multi-field optimized PBS PT plan (B) for whole breast and RNI demonstrating improved heart, lung, contralateral breast, and other soft tissue sparing with PT. The lumpectomy cavity CTV is highlighted in orange where a simultaneous integrated boost is administered to 56.26 Gy in 25 fractions, with the breast and nodal volumes (not shown) receiving 50 Gy.

Figure 2:

Axial (A), coronal (B), and sagittal (C) 25% color wash images demonstrating comprehensive CTV coverage (pink) with a homogeneous dose (maximum dose 106.4% of prescription) and excellent normal tissue sparing for recurrent breast right breast cancer with ipsilateral and contralateral axillary metastases. (D) Medical comorbidities necessitated an arms down immobilization. The arm is in an akimbo position to attempt to maximize separation between the arm and CTV.

Figure 3:

Axial CT dose color wash images from a PBS PT APBI plan. The 90% color wash (A) demonstrates the high level of conformality and skin-sparing properties of the PBS technique. (B) The 25% color wash at the same level demonstrates exquisite normal tissue sparing, including of non-target breast tissue.

Figure 4:

Axial (A, B, E), sagittal (C, F), and coronal (D, G) 50% color wash images demonstrating a two proton field, multi field optimization (MFO) plan avoiding delivery through the magnetic expander port of the reconstructed breast while achieving comprehensive target coverage. Individual dose deposition profiles from the en face field (B-D) and the more tangential beam angle (D-F) are displayed.

Figure 5:

Sagittal (A) and axial (B) 20 Gy color wash images of a patient with locally advanced left breast cancer and pectus carinatum, a deformity of the chest characterized by protrusion of the sternum and ribs, demonstrating excellent coverage of the chest wall (pink) and IMN (red) CTV and normal tissue sparing despite the unfavorable anatomy.