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technical-relevance Breast Protons RBE

Protons produced greater subclinical lung changes after breast nodal irradiation

Proton therapy produced greater CT-detected lung changes than photons despite lower lung dose and no increase in clinical pulmonary toxicity.

RadOncToday · ·1 min read

KEY POINTS

  • This prospective correlative study included patients enrolled in the randomized RADCOMP trial and an institutional imaging substudy. Of 91 enrolled patients, 71 had complete serial imaging, including 36 treated with pencil-beam scanning protons and 35 with photons.
  • Noncontrast CT scans were obtained at approximately 8.6 and 15.3 months after regional nodal irradiation. Planning and follow-up scans were deformably registered, and voxel-level lung-density changes were modeled in 5 Gy dose increments.
  • Proton therapy substantially reduced conventional lung-dose metrics in right-sided free-breathing patients: mean lung dose was 8.0 versus 14.2 Gy, and V20 was 15.4% versus 28.0%, both p < 0.001. However, protons delivered higher localized doses to the upper-lung apices.
  • At the first follow-up, right-sided free-breathing patients had a lung-density TD50 of 33.4 Gy with protons versus 45.1 Gy with photons, corresponding to an estimated RBE of 1.35 (95% CI 1.18–1.44).
  • At the second follow-up, the estimated RBE decreased but remained above the conventional value, at 1.27 (95% CI 1.14–1.36). Across the complete cohort, the corresponding estimates were 1.27 at the first scan and 1.24 at the second.
  • Lung-density changes appeared earlier and were more pronounced after proton therapy, with partial resolution between scans. Photon-associated changes developed more gradually, while remaining relatively stable between follow-ups.
  • No patient developed clinically relevant radiation pneumonitis or pulmonary fibrosis. Cough and dyspnoea rates were similar between modalities (p > 0.40), and CT-density changes were not associated with clinical pulmonary symptoms.

CLINICAL TAKEAWAY

The favorable physical lung dose from proton breast radiotherapy may not translate directly into the biological effect predicted by a fixed RBE of 1.1. These findings support LET- and biological-dose evaluation near the distal beam edge, but they do not demonstrate clinically meaningful harm or justify changing treatment selection based on CT-density changes alone.

SOURCE

International Journal of Radiation Oncology, Biology, Physics