Research ArticleRole of Volumetric-Modulated Arc Therapy with Flattening Filter Free Delivery in Lung Stereotactic Body Radiotherapy
Introduction
Stereotactic body radiotherapy (SBRT) is a treatment option for patients with stage I non small cell lung cancer (NSCLC) who decline surgery or are medically inoperable [1], [2]. SBRT achieves highly conformal dose distributions with rapid radiation dose falloff for surrounding tissues and comparable local control rates as surgical resection [3]. This has led to a rapid increase in the utilization of SBRT over the past decade [4], [5], [6], [7].
Volumetric-modulated arc therapy (VMAT) is an intensity-modulated radiotherapy technique that operates in combination with variations of the gantry rotation speed, multileaf collimator shape and dose rate. Compared with conventional three-dimensional conformal SBRT, VMAT has shortened both SBRT planning and delivery time [8], [9]. To further increase dose delivery rate, unflattened cone-shaped photon beams (generally known as flattening filter free [FFF]) have been made available with new linear accelerator models by removal of the flattening filter in the beam [10], [11], [12], [13]. VMAT in combination with FFF can increase the rate of dose delivery and reduce RT delivery time. However, FFF modes have an inherent conical beam shape with altered leakage and scatter profile.
Recent availability of deterministic dose calculation algorithm Acuros XB (Varian Medical systems, Palo Alto, CA) has improved dose calculation, especially in lung radiotherapy planning [14], [15], [16], [17]. The combined effects of advances in dose computation, planning technique (eg, VMAT), and delivery mode (ie, FFF) could influence target dose distributions, and the dose received by healthy organs outside the planning target volume (PTV). A recent study reported that VMAT with FFF yielded comparable target dose conformity in lung SBRT but used a three-dimensional pencil beam algorithm–anisotropic analytical algorithm (AAA) in the planning comparison [18]. Using AAA to evaluate lung SBRT has its own challenges, especially for small tumors surrounded by a low density of lung tissue. Previous studies have observed that AAA overestimates the prescription isodose volume, R50%, and D2cm in lung SBRT plans [14], [15]. The Acuros XB algorithm can provide more accurate and reliable dosimetric calculations during VMAT with FFF in lung SBRT. The aim of the present study was to evaluate the dosimetric impact of advanced dose computation during 6 MV VMAT with FFF lung SBRT. We hypothesized that, relative to traditional 6 MV flattened field (FF) VMAT, 6 MV VMAT with FFF, due to its unique beam shape and different scatter and leakage characteristics, would improve the target dose distributions and decrease dose to healthy tissues outside the PTV in lung SBRT.
Section snippets
Study Subject Selection
Ninety-eight patients with early stage lung cancer who previously received lung SBRT at one Cancer Center were included in this replanned dosimetric study. The regimen of 48 Gy in 4 fractions and was used for peripheral tumors; whereas 60 Gy in 8 fractions was used for centrally located tumors (20), or for peripheral tumors whose PTV was close to organs at risk, such as the mediastinum or the diaphragm. The patients’ pulmonary function (volume of air exhaled in the first second [FEV1]) and body
Target Dosimetric Comparison
Detailed dosimetric results are presented in Table 2. A small but significant (P < .05, two tail paired t-test) difference in target dose coverage was observed. 6 MV VMAT with FFF arcs resulted in slightly higher (<1%) mean doses to both the ITV and PTV compared with the 6 MV VMAT with FF technique. The mean differences in R50% and D2cm between FFF and FF beams were +16.2% (range +39.5%–0.4%, P < .001) and +1.0% (+12.5% to −7.1%, P < .001), respectively. There were no differences in HI and CN
Discussion
We observed that FFF beams required significantly higher MUs than FF beams to achieve the same dose constraints and planning priorities. Using univariate and multivariate analysis, the only factor independently associated with increased MUs use was the beam delivery technique (eg, FFF delivery). These results support previous studies [18], [20], [21] which reported that FFF beam delivery required an increase in MUs. A few studies showed that FFF beams did not require more MUs for treatment;
Conclusions
Dosimetrically similar target isodose distributions during lung SBRT can be achieved using 6 MV FFF with VMAT. The FFF technique, however, significantly shortens estimated beam-on time, which is convenient for patients, but has significant impact at the price of 7% higher MUs. In addition, VMAT plus FFF delivery is no more advantageous to healthy organs at risk outside the PTV owing to higher modulation required for target coverage in lung cancer patients.
Acknowledgments
The authors have no research support to declare.
Author's contributions: Hong-Wei Liu has initiated the project, performed data collection, and prepared the draft of the manuscript. Ivo Olivotto has prepared the draft of the manuscript. Harold Lau organized the project and prepared the draft of the manuscript. Zoann Nugent performed data statistic analysis. Rao Khan initiated the project, performed data collection and prepared the draft of the manuscript. All the authors read and approved the
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The author(s) has no financial disclosures or conflicts of interest to declare.