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Proton hypofractionation achieved durable control in heavily reirradiated spine metastases

Proton hypofractionation produced 10% one-year local failure despite extensive prior irradiation, with grade 3 toxicity confined to multiply reirradiated patients.

RadOncToday · ·1 min read

KEY POINTS

  • This retrospective two-institution cohort included 52 patients treated to 58 spinal lesions with proton hypofractionated radiotherapy in ≤10 fractions between 2015 and 2023. Median imaging follow-up was 14 months, and the population was heavily pretreated: only 10 lesions represented first-time RT, while 48/58 (83%) had at least one previous overlapping radiation course.
  • Treatment was highly consistent: 47/58 lesions (81%) received 40 Gy in 5 fractions. Median cumulative target EQD2 from all radiation courses was 115 Gy, while median cumulative spinal-cord Dmax was 60 Gy EQD2. Proton plans used robust optimization with approximately 3.5% range uncertainty and posterior-oblique beam arrangements.
  • Local control remained strong despite the reirradiation burden. There were 15 local failures; cumulative local-failure incidence was 3.4% at 6 months, 10% at 12 months and 22% at 24 months with death treated as a competing risk.
  • Target coverage mattered. Higher CTV D95 BED10 was associated with lower local-failure risk (HR 0.96 per Gy; p=0.035), as was higher CTV V95 (HR 0.97 per percentage point; p=0.006). GTV and PTV coverage metrics were not statistically significant in the univariable analysis.
  • The trade-off between OAR protection and target coverage was substantial: median CTV V95 was only 86% and median PTV V95 85%, reflecting deliberate compromise in heavily reirradiated cases where cord, cauda or esophageal tolerance frequently constrained planning.
  • Seven treatment-related adverse events were recorded, including five grade 3 events: bowel perforation, tracheoesophageal fistula, esophageal stricture, wound dehiscence and radiation myelitis. All grade 3 events occurred after three or four overlapping RT courses; there were no grade 4–5 events.
  • The radiation-myelitis case followed four overlapping courses and a cumulative cord Dmax of approximately 97.9 Gy EQD2. No radiation plexopathy or vertebral compression fractures were observed after proton hypofractionation.

CLINICAL TAKEAWAY

Hypofractionated proton therapy—most commonly 40 Gy in 5 fractions—can provide strong local control even after multiple previous spine radiation courses, where exit-dose reduction may be particularly valuable. The safety signal becomes much less reassuring after a third or fourth overlapping course, and cumulative-dose reconstruction plus careful target–OAR trade-offs remain essential.

SOURCE

International Journal of Radiation Oncology, Biology, Physics