Study to Analyze the Dosimetric Characteristics of High  Dose Rate Flattening Filter-Free Beam from different Linear  Accelerators

Palanivelu D; Khanna D; Mohandass P; Thirumal M; Raghul R J; Venugopal S

doi:10.31557/apjcb.2026.11.1.31-39

Authors

Palanivelu D Department of Physical Sciences, School of Sciences, Arts and Media, Karunya Institute of Technology and Sciences, Coimbatore, Tamil Nadu, India. Department of Radiotherapy, Manipal Hospital, Old Airport Road, Bengaluru, Karnataka, India.
Khanna D Department of Physical Sciences, School of Sciences, Arts and Media, Karunya Institute of Technology and Sciences, Coimbatore, Tamil Nadu, India.
Mohandass P Department of Radiation Oncology, Fortis Cancer Institute, Fortis Hospital, Mohali, Punjab, India.
Thirumal M Department of Radiotherapy. All India Institute of Medical Sciences, Bibinagar, Hyderabad, India.
Raghul R J Department of Radiation Oncology, Max Super Specialty Hospital, Noida, Uttar Pradesh, India.
Venugopal S Department of Physical Sciences, School of Sciences, Arts and Media, Karunya Institute of Technology and Sciences, Coimbatore, Tamil Nadu, India.

DOI:

https://doi.org/10.31557/apjcb.2026.11.1.31-39

Keywords:

Flattening Filter, Flattening Filter Free, Linear accelerator, Dosimetry

Abstract

Background: Numerous radiation-generating devices produce flattening filter-free (FFF) beams for treating cancer patients clinically, while all those machines produce FFF beams, they differ in their treatment head, collimation system, Multi leaf collimator (MLC) configurations, MLC speed, gantry speed, and dose rate. Objective of this study was to compare dosimetric characteristics of FFF beams used in different linear accelerators (Linacs) from multiple radiotherapy centres.

Methods: Dosimetric data for 6MV FFF and 10MV FFF beams from Elekta Versa HD™ (EVH1 & EVH2), Varian TrueBeam™ (VT1 & VT2), and 6MV FFF from Varian Halcyon™ (VH1 & VH2) were analyzed. This study compared different dosimetric parameters included depth of maximum dose (Dmax), beam quality index, percentage depth dose (PDD), beam profiles, penumbra, off-axis ratio, percentage surface dose (PSD), head leakage, and multi-leaf collimator (MLC) leakage.

Results: The VH showed significantly lower MLC transmission (0.007–0.03%) compared to VT (1.21–1.23%) and EVH (0.23–0.34%). PSD for a 30×30 cm² field was lower in EVH (37.1–38.4%) than VT (49.3–51.1%) but higher for a 28×28 cm² field in VH (66.2–69.1%). Head leakage showed no major differences, with values in the patient plane of 0.004–0.014% (EVH), 0.005-0.03% (VT), and 0.012–0.10% (VH); and other than patient plane of 0.02–0.10% (EVH), 0.007% (VT), and 0.012–0.209% (VH). Penumbra was slightly lesser in VH (8.9 mm) than VT (9.5 mm) and EVH (9.8 mm). VH exhibited excellent MLC shielding and a narrower penumbra, ensuring better conformity and minimal leakage. EVH had lower PSD, offering improved skin sparing. VT showed higher MLC transmission, indicating slightly higher out-of-field dose. Head leakage was lesser for all machines, confirming effective shielding design.

Conclusion: Overall analysis shows clear performance variations among the evaluated linac platforms. EVH exhibited a higher energy spectrum, while VH and VT demonstrated slightly lower values. VH showed the lowest MLC leakage. Comprehensive dosimetric comparisons are essential when commissioning FFF linacs. Machine specific parameter must be assessed and optimized for clinical needs. Such evaluations are key for advanced radiotherapy quality. Centres should consider these technical details in commissioning and deployment.