Proton Improves BED Compared With Photon Beam-based Partial Stereotactic Ablative Boost Radiotherapy for Large Non-small Cell Lung Cancer

Abstract Purpose: Increased volume ratio of Biologic Effective Dose (BED) was associated with higher local control rate in a setting of large (greater than 5 cm) non-small cell lung cancer (NSCLC) via photon partial stereotactic ablative boost radiotherapy (P-SABR). Taking advantage of unique physics characteristic of the Bragg peak, we exploit the potential dosimetric and clinical benefit of using proton P-SABR.Methods: Thirty patients’ CT structure set with large NSCLC previously treated with photon P-SABR were re-planned with three different treatment modalities: intensity modulated proton therapy (Proton-IMPT), proton volumetric modulated arc therapy (Proton-Arc) and photon volumetric modulated arc therapy (Photon-VMAT). Proton-Arc was mimicked with 8 to 14 beams (10 to 20 degree apart). Dosimetric metrics were compared among all of three treatment modalities in which tumor and organ at risks (OARs) doses were compared in volumes receiving >50% and ≤50% of the prescription dose, representing high- and low-dose regions, respectively. Normal Tissue Complication Probability (NTCP) of lungs and esophagus were compared as well.Results: In the high-dose region, mean doses of PTVs were 72.64 GyE, 80.42 GyE and 82.22 GyE and B90 (the ratio of BED>90GyE to the in-field tumor) were 71.17%, 86.59% and 87.32% for Photon-VMAT, Proton-IMPT and Proton-Arc plans, respectively (photon versus proton, p<0.05). In addition, proton plans could keep a similar OARs dose sparing in high-dose region. In stage N2 to N3 group, V50 of esophageal were comparable for the three treatment modalities (p>0.05). In the low-dose region, doses of OARs were reduced dramatically in proton plans compared with Photon-VMAT plans (p<0.05). The potential clinical benefit of utilizing Proton-IMPT will reduce the mean probability of symptomatic radiation pneumonitis by 61.63% and the mean probability of grade≥2 radiation-induced esophagitis by 50.00% in comparison with Photon-VMAT group (p<0.05).Conclusion: Higher BED inside the target could be achieved by using proton beam therapy with similar OARs dose sparing in high-dose region and significantly better OARs low dose sparing compared with photon techniques. Proton techniques has a potential role in the management of large NSCLC to increase the local control rate while reducing the side effect.