Stereotactic Body Radiotherapy (SBRT) for Liver Tumors: A Systematic Review on the Choice of Photon Energy and Linac...

Stereotactic body radiotherapy (SBRT) enables high-precision, high-dose treatment of liver lesions with acceptable toxicity. With the introduction of flattening filter-free (FFF) beams and the use of different photon energies, there is potential for improved dosimetry and treatment efficiency; however, the optimal selection of these parameters remains uncertain. This systematic review evaluated the impact of photon energy (6 MV versus 10 MV) and beam modality (flattened versus FFF) on dosimetric quality, organ-at-risk (OAR) sparing, treatment efficiency, and clinical feasibility in liver SBRT. A comprehensive search of PubMed, Embase, and Scopus databases from January 2010 to April 2025 identified dosimetric and clinical studies comparing FFF and flattened beams as well as different photon energies. Outcomes of interest included target coverage, conformity index (CI), homogeneity index (HI), beam-on time (BOT), monitor units (MUs), and OAR doses. Risk of bias was assessed using the Newcastle-Ottawa Scale. A total of 23 studies met the inclusion criteria with adherence to Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) guidelines. FFF beams consistently reduced beam-on time by approximately 30-60%, improved dose conformity, and enhanced OAR sparing compared with flattened beams. Among FFF modalities, 6FFF demonstrated superior conformity and steeper dose gradients, particularly for small and centrally located lesions, whereas 10FFF provided improved skin sparing and was more suitable for larger or deeper tumors. Although FFF plans required higher MUs, this was offset by significantly faster treatment delivery. Fractionation strategies were influenced by tumor size, anatomical location, and baseline liver function, with a greater number of fractions generally preferred for larger or centrally located lesions. Overall, FFF beams, particularly 6FFF and 10FFF, offer improved treatment efficiency and favorable dosimetric outcomes in liver SBRT. The choice of beam energy and modality should be individualized based on lesion characteristics, including size, depth, and proximity to critical structures. FFF-based SBRT represents a practical and widely implementable approach, although further prospective studies are needed to support standardization of clinical practice.