Dosimetric Comparison of Unmatched Flattening Filter-free and Flattened Beams in Volumetric Arc Therapy Plans for Head-and-neck Cancer

Abstract

Introduction: Conventionally, the fattened beam is being used in radiotherapy for routine clinical cases even after introduction of intensity-modulated radiotherapy with incorporation of multi-leaf collimator system. With the removal of the flattening filter (FF) from the beam's path average energy of the photon gets reduced resulting in reduced scatter, reduction in treatment time, and reduced neutron contamination for high-energy beam and ultimately resulting in treatment plan quality deviations. This study aims to investigate the usefulness of the FF-free (FFF) beam for routine head-and-neck cancer (HNC) cases treated with volumetric arc therapy (VMAT) and dosimetrically compares the result with the FF beam.

Materials and methods: In this study, 20 patients treated on Halcyon^TM (unmatched 6 megavoltage [6MV] FFF beam) medical linear accelerator with VMAT of different HNC selected and for comparison with 6MV FF beam, 20 equivalent treatment plans are created for Truebeam^TM configuration and the plans have been evaluated for target coverage, doses to the organ at risk (OAR), and other dose quality indices.

Results: Comparable target coverage, doses to OARs except for rest right parotid (P = 0.02) between 6MV FFF beam and 6MV FF beam is observed. Insignificant differences in conformity index, homogeneity index, and gradient index have been observed. Higher monitor unit (MU) (P ≤ 0.001) and lesser beam on time (BOT) (P = 0.003) have been observed in 6MV FFF.

Conclusion: 6MV FFF beam provides comparable target coverage and improved dose-sparing effect to most of the OARs. 6MV FFF beam has lesser BOT, but on the other hand number of MUs is higher as compared to 6MV-FF plans.

Keywords: 6 megavoltage flattening filter beam; flattening filter free; head-and-neck cancer; unmatched 6 megavoltage flattening filter-free beam; volumetric arc therapy.

Figure 1

Comparison of V_95% dose color wash of one of the volumetric arc therapy case for 6 megavoltage-flattening filter free and 6megavoltage-flattening filter treatment plans

Figure 2

Dose volume histogram analysis for one of the volumetric arc therapy cases for 6 megavoltage-flattening filter-free and 6 megavoltage-flattening filter treatment plans

Figure 3

Integral dose comparison between 6 megavoltage-flattening filter free and 6megavoltage-flattening filter beams in volumetric arc therapy for head-and-neck cancer

Figure 4

Conformity index comparison between 6 megavoltage-flattening filter-free and 6 megavoltage-flattening filter beams in volumetric arc therapy for head-and-neck cancer

Figure 5

Homogeneity index between 6 megavoltage-flattening filter-free and 6 megavoltage-flattening filter beams in volumetric arc therapy for head-and-neck cancer

Figure 6

Gradient index comparison between 6 megavoltage-flattening filter-free and 6megavoltage-flattening filter beams in volumetric arc therapy for head-and-neck cancer

Figure 7

Monitor unit distribution comparison between 6 megavoltage-flattening filter-free and 6 megavoltage-flattening filter beams in volumetric arc therapy for head-and-neck cancer, where 1-20 represents number of patients and 0-800 represents number of monitor units

Figure 8

Beam on time (in Seconds) comparison between 6 megavoltage-flattening filter-free and 6 megavoltage-flattening filter beams in volumetric arc therapy for head-and-neck cancer, where 1-20 represents number of patients and 0-200 represents beam on time (in Seconds)

Figure 9

Skin dose comparison between 6 megavoltage-flattening filter-free and 6 megavoltage-flattening filter beams in volumetric arc therapy for head-and-neck cancer