Radiation-induced acute lung pneumonitis after PBS proton treatment for breast cancer: Correlation with dose-volume parameters and optimization objectives to reduce lung toxicities.

Proton treatment using pencil-beam scanning (PBS) for patients with breast cancer offers advantages in achieving a conformal dose distribution while also reducing the cardiac dose. However, when employing two anterior fields to mitigate the effects of respiratory motion on dose delivery, managing the ipsilateral lung doses becomes critical due to the high linear-energy transfer (LET) at the distal end of the beams. Although the incidence of radiation pneumonitis (RP) following breast radiation therapy is relatively low, it is essential to address the cases that develop RP following proton treatment to minimize lung toxicity. We conducted a retrospective case study analyzing follow-up computed tomography images taken at 1 week, 1.5 months, and 4.5 months after the onset of the patient's pneumonitis symptoms to correlate them with proton doses. The patient's PBS treatment was prescribed at a dose of 50 Gy with an additional 10 Gy boost, using a relative biological effectiveness (RBE) of 1.1, delivered in 2 Gy daily fractions. Our histogram analysis revealed noticeable increases in Hounsfield units at a dose of 40 Gy (RBE = 1.1), underscoring a potential dose-volume parameter that could help minimize the occurrence of RP. Furthermore, the lung volume associated with the RP was encompassed with an iso-LET level greater than 5.0 keV/μm, with a proton dose exceeding 40 Gy (RBE = 1.1). In examining the LET-dependant RBE-weighted dose using the McNamara model in the original treatment plan, we found the volumes receiving more than 50 Gy (V) and 40 Gy (V) were 110 cc and 267 cc, respectively. By incorporating dose objectives of V and V to limit the ipsilateral lung volume into PBS plans, the volumes were successfully reduced to 0 cc and 3 cc, while maintaining target dose coverage and robustness. Optimizing a breast PBS plan (RBE = 1.1) using objectives that addressed both the V and V to minimize lung exposure was shown to be clinically feasible and should be considered as a strategy to reduce lung toxicity when treating breast cancer with PBS proton therapy.