Title: Dosimetric Evaluation of 3-Field Versus 2-Field Approach in Intensity Modulated Proton Therapy Planning of Lung Cancer
Abstract: The main purpose of this study is to compare the dosimetric quality of 3-field versus 2-field approach in lung cancer treatment plans generated by multi-field optimization (MFO) intensity modulated proton therapy (IMPT) technique. Eight lung cancer cases (tumor motion less than 1 cm) previously treated using 3-field uniform scanning proton therapy technique were selected for this retrospective study. All eight lung cases included 4D computed tomography simulation in a supine position. For each case, 3-field and 2-field IMPT plans were generated for a total dose of 74 Gy (relative biologic effectiveness [RBE]) and daily dose of 2 Gy[RBE]. Specifically, a 3-field IMPT plan was generated by adding an extra field in the 2-field IMPT plan. A total of 16 IMPT plans for eight cases were optimized using identical dose-volume constraints. For a comparative purpose, various dosimetric results from the 2-field IMPT plans were compared to that of 3-field IMPT plans. The results are averaged over eight cases in this study. The planning target volume (PTV) coverage (i.e., PTV volume receiving 100% of prescription dose) was higher in the 3-field plans than in the 2-field plans (97.18% vs. 96.21%), whereas the mean PTV dose was similar (75.27 Gy[RBE] vs. 75.24 Gy[RBE]). The mean dose, V20, and V5 for total lung were higher in the 3-field plans when compared to the ones in the 2-field plans (7.23 Gy[RBE] vs. 6.67 Gy[RBE], 14.14% vs. 12.47%, and 20.25% vs. 17.27%, respectively). The mean esophageal dose was comparable (13.95 Gy[RBE] vs. 13.67 Gy[RBE]). The ratio of prescription isodose volume to PTV volume (R100%) was better in the 3-field plans (1.17) than in the 2-field plans (1.20). Similarly, the ratio of 50% prescription isodose volume to PTV (R50%) was superior in the 3-field plans (3.47) when compared to the 2-field plans (3.96). The preliminary results from this study suggest that the 3-field MFO-IMPT technique produces better PTV coverage and high-dose conformality as well as lower intermediate-dose spillage when compared to the 2-field MFO-IMPT technique. However, doses to the organs at risk (OARs), especially normal lung tissue, were slightly higher in the 3-field plans than in the 2-field plans although the OAR results were found to be within the tolerance.