Dosimetric comparison of the noncoplanar volumetric modulated arc therapy with flattening filter free beams and flattened beams for the whole brain radiotherapy with hippocampus avoidance

Abstract

Objective

A comparative analysis is conducted to examine dosimetric discrepancies between flattening filter-free (FFF) and flattened (FF) beam noncoplanar volumetric modulated arc therapy (NCVMAT) plans for whole brain radiotherapy (WBRT) with hippocampus avoidance.

Methods

Fifteen patients who underwent WBRT with hippocampus avoidance were randomly selected. Individual treatment plans were created on the Infinity accelerator for FF-NCVMAT and FFF-NCVMAT, both with a prescribed dose of 30Gy/10F and identical plan parameters using the Monaco treatment planning system (TPS). The plans were designed to meet the dosimetric requirements specified in RTOG 0933. Both groups achieved a planning target volume (PTV) coverage rate of 95% for the prescribed dose. The dosimetric parameters, including machine units (MU), delivery time (DT), plan modulation factor (MF), and gamma pass rate(GPR) were compared between the two plans.

Results

There were no significant differences in minimum dose (Dmin) and homogeneity index (HI) of the PTV between the FFF-NCVMAT group and the FF-NCVMAT group (P = 0.828, 0.453). However, utilization of the FFF mode effectively reduced maximum dose (Dmax) of the PTV and improved conformity index (CI), with statistical significance observed (P = 0.010, 0.006). The MF and GPR between both plans were similar (P = 0.599,0.130). Within the FFF-NCVMAT group, Dmax, D100%, and mean dose (Dmean) of the hippocampus were significantly smaller compared to those in the FF-NCVMAT group, demonstrating statistically significant differences(P < 0.001, P < 0.001, P < 0.001). In the FFF-NCVMAT group, the Dmax of the left and right optic nerves and optic chiasm were all smaller than those in the FF-NCVMAT group, and the differences were statistically significant (P = 0.027, 0.033, 0.043). The doses of other organs at risk (OARs) such as lens and eyes were all controlled within the clinically safe dose range, and there was no statistically significant difference between the two plans (P = 0.912, 0.179; P = 0.850, 0.855). The MU of the FFF-NCVMAT group was higher than that of the FF-NCVMAT group, but the DT was shorter than that of the FF-VMAT group, and the differences were statistically significant (P < 0.001, P < 0.001).

Conclusion

The FF-NCVMAT and FFF-NCVMAT plans both meet the clinical requirements of RTOG 0933, and their complexity is comparable. However, the FFF-NCVMAT plan offers an advantage in terms of hippocampus and lens protection. Despite having a higher MU, the FFF mode has a shorter DT, resulting in improved treatment efficiency. Therefore, it is recommended to consider using the FFF-NCVMAT plan for HA-WBRT.