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

Abstract

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. 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. 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. 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.