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review Breast Protons

Modern proton breast radiotherapy offers cardiopulmonary sparing but randomized clinical benefit remains pending

Modern proton techniques reduce cardiopulmonary dose and can control skin exposure, while randomized cardiac and cancer-control endpoints remain pending.

RadOncToday · ·2 min read

KEY POINTS

  • The review traces proton regional nodal irradiation for locally advanced breast cancer from the first clinical experience in 2010 through modern pencil-beam scanning and IMPT. The principal rationale remains reducing heart and lung exposure when comprehensive nodal treatment—particularly internal mammary coverage—is required.
  • Early dosimetric work in 11 left-sided postmastectomy cases demonstrated the magnitude of potential sparing: mean heart V20 was 1.6% with protons versus 12.0–12.4% with photon techniques, while ipsilateral lung V20 was 16.2% versus 21.7–25.3%.
  • Prospective early clinical experience was encouraging. Among 69 evaluable patients from a phase I/II proton postmastectomy study, five-year locoregional failure was 1.5% and overall survival 91%; one patient developed grade 2 pneumonitis and none developed grade 3 pneumonitis.
  • The first large randomized comparison of proton versus photon regional nodal irradiation completed accrual in 2024 with 1,239 patients. Early quality-of-life results were similarly favorable in both arms, and an analysis of 1,160 participants confirmed substantially lower cardiac and cardiac-substructure doses with protons, but the trial's major cardiac and locoregional-control endpoints remain immature.
  • Skin toxicity is highly technique-dependent and should no longer be considered an unavoidable consequence of proton treatment. In a contemporary cohort of 176 women treated with explicit skin constraints, grade ≥2 dermatitis was 47% with IMPT versus 48% with photons (p=0.91); another modern comparison reported grade 3 dermatitis of 3% versus 7%. The authors suggest making skin dose an explicit planning objective—for example, skin V42 Gy(RBE) <15% in selected conventionally fractionated cases without skin involvement.
  • Proton-specific late effects also require attention. In 225 proton-treated breast patients, three-year in-field rib-fracture incidence was 3.7%, although only 0.4% were symptomatic; emerging data implicate increased LET near distal beam edges as a potential contributor beyond physical dose alone.
  • The authors emphasize that the benefit of protons depends as much on contouring and robustness as on beam physics. Internal mammary nodes, chest wall, ribs and skin require explicit decisions, while implants, tissue-expander ports, respiratory motion, range uncertainty and distal-edge LET all complicate planning; Monte Carlo dose calculation is particularly important for superficial-dose accuracy.

CLINICAL TAKEAWAY

For selected patients requiring comprehensive regional nodal irradiation, especially those with difficult cardiac anatomy or internal mammary disease, modern proton therapy can produce substantial cardiopulmonary sparing while maintaining target coverage. The dosimetric rationale is strong, but whether that translates into fewer major cardiac events or better cancer outcomes remains the key unanswered randomized question.

SOURCE

International Journal of Radiation Oncology, Biology, Physics