The Radiation Oncology Feed For radiation oncologists · physicists · RTTs · dosimetrists
technical-relevance nsclc carbon-ion robust-optimization

Robust optimization minimized lung-dose trade-offs in carbon-ion planning for stage I NSCLC

Robust optimization improved target coverage with less lung-dose penalty than margin expansion, while density override was particularly useful for solid tumors.

RadOncToday · ·2 min read

KEY POINTS

  • This retrospective planning study included 30 patients with peripheral stage I NSCLC, equally divided between 15 ground-glass opacity lesions and 15 solid tumors, previously treated with pencil-beam scanning carbon-ion radiotherapy. Prescription was 72 Gy(RBE) in 9 fractions of 8 Gy(RBE) with respiratory gating.
  • The internal GTV incorporated respiratory motion, and the CTV was generated with a 7-mm isotropic expansion. Investigators then created eight plans per patient by combining three approaches: density override, an additional 7-mm margin expansion, and robust optimization.
  • All three strategies improved accumulated target coverage, but with very different lung-dose costs. Density override increased CTV V95% by 3.3–4.1 percentage points while increasing mean ipsilateral lung dose by only 0.15–0.92 Gy(RBE). Robust optimization improved CTV V95% by 4.8–5.1 points with approximately 1.77–1.79 Gy(RBE) additional mean lung dose.
  • Additional margin expansion produced a target-coverage gain comparable to robust optimization but imposed by far the largest lung penalty: mean ipsilateral lung dose increased by approximately 3.78–4.86 Gy(RBE). This makes simply adding geometric margin substantially less dose-efficient than uncertainty-aware optimization.
  • Tumor density changed the preferred strategy. For ground-glass lesions, CTV plus robust optimization provided adequate coverage with the lowest lung exposure among the high-coverage strategies. For solid tumors, adding density override to robust optimization improved the lower end of the CTV V95% distribution from 85.2% to 92.8%, with only about 0.15 Gy(RBE) additional mean lung dose.
  • The findings remained broadly consistent after introducing ±3.5% range uncertainty and 3-mm setup shifts. Under setup uncertainty in solid lesions, robust optimization alone produced median CTV V95% of 97.04%, compared with 95.44% using margin expansion and 88.90% using density override alone.
  • Where robust optimization is unavailable, the authors recommend retaining the larger margin; adding density override appears particularly valuable for solid lesions and can also improve robustness for ground-glass tumors. This is a dosimetric study, however, and does not demonstrate improved tumor control or reduced pulmonary toxicity.

CLINICAL TAKEAWAY

For gated pencil-beam scanning carbon-ion RT of peripheral stage I NSCLC, robust optimization appears considerably more dose-efficient than simply adding another margin. Density override adds little lung dose and may be especially useful for solid tumors, but these recommendations remain planning-based rather than outcome-validated.

SOURCE

International Journal of Radiation Oncology, Biology, Physics