Dosimetric impact of spatial separation in combined SBRT and mediastinal radiotherapy for peripheral small cell lung cancer.

PURPOSE: To evaluate the dosimetric feasibility of combining stereotactic body radiotherapy (SBRT) for peripheral tumors with conventionally fractionated mediastinal lymph node (MLN) radiotherapy in small cell lung cancer (SCLC), with a focus on tumor-nodal spatial separation and geometry-dependent dose redistribution. MATERIALS AND METHODS: A retrospective virtual planning study was conducted in 21 patients with peripheral SCLC. Three treatment strategies were generated: (1) SBRT 40 Gy in 5 fractions plus MLN radiotherapy 60 Gy in 30 fractions, (2) SBRT 50 Gy in 5 fractions plus MLN radiotherapy, and (3) conventional thoracic radiotherapy (60 Gy in 30 fractions). Target coverage and dose-volume parameters for organs at risk (OARs) were compared. Biological dose normalization using equivalent dose in 2 Gy fractions (EQD2), based on the linear-quadratic model, and prescription-matched sensitivity analyses were performed. RESULTS: All strategies achieved adequate target coverage. SBRT-based approaches significantly reduced low-dose lung exposure compared with conventional radiotherapy, particularly in the 40 Gy regimen (lung V5 - 7.34%, V10 - 3.34%, V20 - 2.20%; all p ≤ 0.003), along with a reduction in mean lung dose (-125.74 cGy, p < .001). In exploratory EQD2-normalized analysis, the reduction in mean lung dose remained statistically significant (-0.95 Gy EQD2, p < .001). In prescription-matched analyses, lung V5 remained significantly lower, indicating that differences were not solely attributable to prescription dose. Patients with tumor-nodal spatial separation ≥2 cm exhibited lower doses to multiple OARs, supporting a geometry-dependent reduction in normal tissue exposure. CONCLUSIONS: Combining SBRT with MLN radiotherapy is dosimetrically feasible in selected patients with SCLC, with feasibility strongly influenced by tumor-nodal spatial separation. Reduced low-dose lung exposure appears to be associated with geometry-dependent dose redistribution. These findings provide a rationale for further investigation of geometry-informed treatment strategies in prospective clinical studies.