Optimizing the prescription isodose level in stereotactic volumetric-modulated arc radiotherapy of lung lesions as a potential for dose de-escalation

BACKGROUND: To derive and exploit the optimal prescription isodose level (PIL) in inverse optimization of volumetric modulated arc radiotherapy (VMAT) as a potential approach to dose de–escalation in stereotactic body radiotherapy for non–small cell lung carcinomas (NSCLC). METHODS: For ten patients, inverse Monte Carlo dose optimization was performed to cover 95% PTV by varying prescription isodose lines (PIL) at 60 to 80% and reference 85%. Subsequently, these were re–normalized to the median gross tumor volume dose (GTV–based prescription) to assess the impacts of PTV and normal tissue dose reduction. RESULTS: With PTV–based prescription, GTV mean dose was much higher with the optimized PIL at 60% with significant reduction of normal lung receiving 30 to 10 Gy (V(30–10Gy)), and observable but insignificant dose reduction to spinal cord, esophagus, ribs, and others compared with 85% PIL. Mean doses to the normal lung between PTV and GTV was higher with 60–70% PIL than 85%. The dose gradient index was 5.0 ± 1.1 and 6.1 ± 1.4 for 60 and 85% PIL (p < 0.05), respectively. Compared with the reference 85% PIL plan using PTV–base prescription, significant decreases of all normal tissue doses were observed with 60% and 70% PIL by GTV–based prescription. Yet, the resulting biological effective (BED) mean doses of PTV remain sufficiently high, ranging 104.2 to 116.9 Gy (α/β = 10). CONCLUSIONS: Optimizing the PIL with VMAT has notable advantage of improving the dosimetric quality of lung SBRT and offers the potential of dose de–escalation for surrounding tissues while increasing the GTV dose simultaneously. The clinical implication of re–normalizing plans from PTV–prescription at 60–70% to the GTV median dose requires further investigations.