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technical-relevance kidney Protons SABR

Daily skin variation caused substantial target underdose in kidney proton SABR

Daily surface changes shifted proton beam depth by up to 17.4 mm and reduced accumulated target D99 by up to 24.3%.

RadOncToday · ·1 min read

KEY POINTS

  • This retrospective dosimetric analysis included 15 patients and 75 fractions from kidney stereotactic ablative proton treatments delivered in 2024–2025. Assessment plans prescribed 50 Gy(RBE) in five fractions to 99% of the iCTV using posterior-anterior and ipsilateral proton fields.
  • Daily large-field CBCTs were converted into virtual CTs by mapping planning-CT Hounsfield units onto the treatment-day anatomy. Fraction doses were recalculated and deformably accumulated to quantify how changes in the external body surface affected proton radiological depth and target dose.
  • Skin/anatomy variation was substantial despite conventional setup. Tumor-centroid depth ranged from 17.4 mm shallower to 12.7 mm deeper than at simulation, with posterior-beam depth changes >5 mm in 22.7% of fractions and variations >1 cm in 9.3%.
  • Dose accumulation demonstrated significant target undercoverage: mean D95 changed by −2.6%, D98 by −5.1%, D99 by −6.7%, and minimum dose by −14.5%. The worst accumulated D99 was 37.87 Gy(RBE)—a 24.3% reduction from the intended 50-Gy prescription level.
  • Adding a repeat pre-treatment evaluation CT to robust optimization improved—but did not eliminate—the problem. Mean accumulated D99 loss improved from −6.7% to −4.4%, and worst-case D99 from 37.87 to 42.98 Gy(RBE).
  • Improved robustness came with a small normal-kidney cost: including the evaluation scan increased healthy-kidney V10–V50 by ≤1 cc on average and mean kidney dose by approximately 0.30 Gy(RBE). Actual accumulated kidney V10–V30 changed little, suggesting target coverage was more sensitive than normal-kidney exposure.
  • Importantly, these were nonclinical assessment plans designed to isolate skin-surface effects; clinical plans normally include additional range and 4D robustness. Nevertheless, the unpredictability and magnitude of surface variation led the authors to recommend daily surface assessment and to identify online adaptation as a potential mitigation strategy.

CLINICAL TAKEAWAY

For kidney proton SABR, matching the tumor internally may not be enough: a few millimeters of abdominal skin or soft-tissue change can alter the proton path before the beam ever reaches the kidney. Daily assessment of the full beam-path surface—and potentially adaptive replanning—may therefore be important, particularly for highly abbreviated proton schedules.

SOURCE

Technical Innovations & Patient Support in Radiation Oncology