Dosimetric Parameters in Hypofractionated Stereotactic Radiotherapy for Brain Metastases: Do Flattening Filter-Free Beams Bring Benefits? A Preliminary Study

Simple Summary The use of flattening filter-free beams (FFF) in stereotactic radiotherapy has increased in recent years due to their dosimetric and clinical benefits. This study evaluated and compared two treatment plans (one employing FFF beams and one with optimized flattened beams) for eighteen patients treated with hypofractionated stereotactic radiotherapy for brain metastasis. Physical and dosimetric parameters were analyzed, resulting in no significant differences regarding the number of monitor units, conformity index (p = 0.28), dose gradient index (p = 0.4) and dose to normal brain tissue (p = 0.51). The average difference in target coverage was 1.26%, with lower doses for treatment plans where FF beams were employed (p = 0.03). Beam on time was significantly reduced for FFF beams (p <= 0.001). Purpose: This study aimed to compare the dosimetric results of flattening filter-free (FFF) vs. flattened (FF) treatment plans for fractionated stereotactic radiotherapy (fSRT), with the goal to highlight potential advantages of FFF beams. Methods: A group of 18 patients with brain metastases treated with fSRT (30 Gy delivered in 5 fractions) were included. The dosimetric parameters evaluated were: (1) physical dosimetric parameters (number of monitor units (MUs), conformity index (CI), dose gradient index (DGI), beam on time (BOT)); (2) clinical dosimetric parameters pertaining to target volume (PTV) and organs at risk (OARs). Two treatment plans were performed for all patients: one used 6 MV FFF beams and the other used 6 MV flattened beams. Results: A slight increase in MUs was observed for the FFF mode (+23.3 MUs). The CI showed a difference of -2.7% for the FF plans (p = 0.28), correlated with a poorer coverage of the PTV. DGI values reported in terms of PTV are in line with international recommendations and showed a +1.9% difference for FFF plans. An average BOT of 90.3 s was reported for FFF plans, which was 2.3 times shorter than that required for FF plans delivery (p <= 0.001). A slight decrease of PTV coverage (-1.26%, p = 0.036) for FF plans can be considered relevant, but no other significant differences were observed between the two optimizations. No statistically significant benefit of using FFF beams to reduce V-20 for normal brain could be demonstrated. Conclusion: These dosimetric results encourage the implementation of fSRT with standard flattened beams in centers where FFF linacs are not available.