Retrospective Dosimetric Analysis of Occurrence of Radiation Pneumonitis

Purpose: To retrospectively evaluate the impact of dosimetric parameters in treatment planning and dose discrepancies from patient inter-fractional motion on the high radiation pneumonitis (RP) occurrence rate in breast cancer patients receiving radiotherapy at First People's Hospital of Kunshan associated with Duke Kunshan University Medical Physics Graduate Program.Method: Dose-volume parameters were extracted from breast cancer patients’ treatment plans and were compared with corresponding experience-based thresholds associated with RP, including total dose, mean lung dose (MLD), percent of lung volume that receives a dose of 5 Gy or higher (V5), 13 Gy or higher (V13), 20 Gy or higher (V20), and 30 Gy or higher (V30). In addition, an in-house dose calculation system based on MATLAB and Varian Eclipse treatment planning system (TPS) was used to obtain actual dose distributions based on planning computed tomography (CT) scans, cone-beam computed tomography (CBCT) scans and couch shifts. The calculated actual dose was analyzed and compared to the original planning dose to evaluate inter-fractional motion induced dose discrepancies and their impacts on the occurrence of RP. Result: For patients diagnosed with RP, the median MLD is 15.38 Gy and the median V20 is 25.6%, which are higher than corresponding constraints 14 Gy and 24% respectively. Other dose-volume parameters were also much higher than their corresponding constraints for preventing RP. The inter-fractional patient motion induced discrepancies between planning and actual dose-volume parameters. For Patient 1, V20 increases from 23.93% to 28.33% due to the motion, which exceeds the V20 constraint of 24%. V30 increases to 17.99%, which is very close to the V30 constraint of 18%. For Patient 2, V10 increases from 32.00% to 35.43% and V13 increases from 29.86% to 32.99% due to the motion, both becoming to exceed the constraints. For Patient 3, MLD, V10, V13, V20 and V30 all decrease, where MLD and V20 decrease to the values lower than constraints. Summary: Dose-volume parameters in breast cancer treatment plans at First People's Hospital of Kunshan were reviewed. Existing results show that the dose-volume parameters related to RP were higher than internationally recommended constraints, which contributes to the high RP incidence. In addition, an automated MATLAB-based actual dose calculation system was developed and used to analyze the dose discrepancies between planning and actual dose distributions. Inter-fractional patient motions were found to cause discrepancies between the original planning dose and the actual dose.