Double-scattered proton-based stereotactic body radiotherapy for stage I lung cancer: a dosimetric comparison with photon-based stereotactic body radiotherapy.

Objective: Stereotactic body radiotherapy (SBRT) has gained popularity in the treatment of early-stage non-small-cell lung cancer (NSCLC) because of its ability to deliver conformal radiation doses to small targets. However, photon-based SBRT (xSBRT) is associated with significant grade 3+ toxicities. In this study, we compare xSBRT treatment plans with proton-based SBRT (pSBRT) to determine whether dose to normal structures could be reduced if SBRT was delivered with protons.

Methods: Eight patients with medically inoperable, peripherally located stage I NSCLC were treated with xSBRT to 48 Gy in 4 12-Gy fractions. These patients were retrospectively re-planned using the same treatment volumes with 3-dimensional conformal double-scatter proton therapy. A Wilcoxon paired test compared dosimetric parameters between the plans for each patient.

Results: Compared with xSBRT there was a dosimetric improvement with pSBRT for these volumes: lung V5 (median difference [MD]=10.4%, p=0.01); V10 (MD=6.4%, p=0.01); V20 (MD=2.1%, p=0.01); V40 (MD=1.5%, p=0.05); and mean lung dose (MD=2.17 Gy, p=0.01). There were also benefits (p=<0.05) in D0.1cm3 and D5cm3 with pSBRT to the heart, esophagus, and bronchus.

Conclusions: In a dosimetric comparison between photon and proton-based SBRT, protons resulted in lower doses to critical organs at risk and a smaller volume of non-targeted normal lung exposed to radiation (V5, V10, V20, and V40). The clinical significance and relevance of these dosimetric improvements remain unknown.