Dosimetric Study Of Flattening Filter-free (FFF) Mode For Fractionated Stereotactic Radiotherapy (FSTR) In Brain Metastasis Carcinoma

Objective:To analyze the difference in dosimetry and therapeutic efficiency between the treatment plan of flattening filter free?FFF?mode and flattening filter?FF?mode in fractional stereotactic radiotherapy for brain metastases,and to provide dosimetric reference for stereotactic radiotherapy of FFF mode in brain metastases.Materials and Methods:From July 2017 to September 2018,a total of 17 patients with brain metastases who underwent stereotactic radiotherapy in the Department of Radiation Oncology of China-Japan Union Hospital of Jilin University were selected for CT simulation and unified target area and organs at risk.A plan for FFF and FF models was designed for each patient and simulated treatment was performed.The 5%target volume relative dose(D_5%);50%target volume relative dose(D_50%);95%target volume relative dose(D_95%);normal brain tissue average dose(D_mean-Brian);normal brain tissue volume with equivalent biological dose over 12Gy(V_>12Gy-Brain);target area homogeneity index?HI?;conformity index?CI?;gradient index?GI?;monitor unit?MU?and total treatment time?TT?were compared between the two model plans.Results:1.Compared with the FF plan,the dose outside the FFF target falls faster and the difference was statistically significant?P<0.05?,and the GI was 3.31±0.97 and 3.45±0.87,respectively.D_5%,D_50%and D_95%of FFF and FF plans were 117.83±2.83%vs.118.21±3.83%,112.63±2.32%vs.112.70±2.83%,101.81±4.16%vs.100.60±2.68%;HI was 1.16±0.05 vs.1.18±0.04;CI was 0.83±0.04 vs.0.83±0.05,and the difference between two groups was not statistically significant?P>0.05?.2.Compared with FF mode,D_mean-Brain and V_>12Gy-Brain are smaller in FFF mode?648.39±411.86Gy vs.667.15±418.01Gy;43.55±35.72 cm³ vs.47.75±38.79 cm³?,the difference between two groups was statistically significant?P<0.05?.3.The MU in FFF mode plan was higher than FF?1320.46±354.99 vs.1220.74±267.86?,and the difference was statistically significant?P<0.05?.TT was?157.35±68.39s vs.160.59±66.65s?,and the difference was not statistically significant?P>0.05?.Conclusion:Both FFF and FF mode plans could achieve the clinical objectives,and the target area conformity and dose uniformity are similar.Moreover,the dose outside the FFF target falls faster,significantly reducing the average dose of normal brain tissue and the brain tissue volume exceeding the equivalent dose of 12Gy,which is beneficial to better protect the important functional areas in normal brain tissue.Although the MU in FFF plan is higher than FF plan,there is no significant difference in the total treatment time between two plans.Therefore,the FFF mode can improve the output dose rate to a certain extent.