Comparison of dosimetric characteristics between flattening filter-free and flattening filter mode volumetric-modulated arc therapy plans in rectal cancer

Abstract Objective We aimed to compare the dosimetric characteristics of 6‐MV flattening filter mode (FF) and flattening filter‐free mode (FFF) volumetric‐modulated arc therapy (VMAT) plans in preoperative radiotherapy for rectal cancer using an Edge linear accelerator. Methods Two‐arc FF‐VMAT and FFF‐VMAT plans were generated for 15 patients with rectal cancer using the Philips Pinnacle treatment planning system (version 9.10). The homogeneity index, conformity index, planning target volume covered by 105% isodose lines, organ‐at‐risk dosimetry characteristics, and monitor units (MUs) were compared between the plans. The results were analyzed using paired t‐tests. Results The conformity index of the FF‐VMAT plan and FFF‐VMAT plan were (0.93 ± 0.01) and (0.92 ± 0.02), and the homogeneity index of the two groups were (0.07 ± 0.03) and (0.08 ± 0.01), the differences were not statistically significant (both P > 0.05). The volume covered by 105% isodose lines of the FFF‐VMAT plan was significantly higher than that of the FF‐VMAT plan. The FFF‐VMAT plan required a higher monitor unit (t = 11.49, P = 0.002) and shorter beam‐on time (t = 5.78, P = 0.001) compared with the FF‐VMAT plan. There were no significant differences in the absolute volume receiving 30 Gy and 40 Gy of the bladder between the plans. The absolute volume receiving 30 Gy and absolute volume receiving 40 Gy of the intestine and colon in the FFF‐VMAT plan increased by 5.5% and 2.1% compared with those in the FF‐VMAT plans that increased by 7.2% and 2.2%, respectively (both P > 0.05). The absolute volume receiving 20 Gy of the left and right femoral head in the FFF‐VMAT plan decreased by 3.7% and 4.9% compared with those in the FF‐VMAT plans (P > 0.05). Conclusion Both FF‐VMAT and FFF‐VMAT plans met the clinical requirements.