Peripheral lung cancers treated with photon, proton, and carbon ion beams: comparative study of dose distribution, robustness, and NTCP analysis

The study aims to compare the dosimetric effects of photon volumetric modulated arc therapy (VMAT), intensity-modulated proton therapy (IMPT), and carbon ion radiotherapy (CIRT) in lung stereotactic body radiation therapy (SBRT) plans. Respiratory-gated IMPT and CIRT plans have been optimized using a set of 20 4DCT DICOM datasets of peripherally located lung lesions. Robust optimization of the clinical target volume (CTV), contoured on the maximum exhalation phase, has been achieved by modeling systematic isotropic shifts, accounting for density uncertainties, and considering the nearest concurrent respiratory phase anatomies. In contrast, the respiratory-gated VMAT plans have been optimized using the maximum inhalation phase on a planning target volume (PTV), which is isotropically expanded from the internal target volume (ITV). In a multicenter approach, VMAT plans have been optimized by the Medical Physics Unit at Fondazione IRCCS San Gerardo dei Tintori (Monza). The comparative analysis includes CTV coverage, sparing of organs at risk in terms of biological equivalent dose (BED), conformity index, homogeneity index, gradient index, and plan robustness. Additionally, radiobiological investigations into normal tissue complication probability (NTCP) are presented to assess the clinical implications of the results. Although previous literature has compared proton and photon techniques for lung SBRT, no studies have employed active spot-scanning IMPT equipped with patient-specific collimators. Furthermore, the use of CTV robust planning for proton therapy represents a novel approach in this type of study.