A simple, yet novel hybrid-dynamic conformal arc therapy planning via flattening filter-free beam for lung stereotactic body radiotherapy

Purpose: Due to multiple beamlets in the delivery of highly modulated volumetric arc therapy (VMAT) plans, dose delivery uncertainties associated with small-field dosimetry and interplay effects can be concerns in the treatment of mobile lung lesions using a single-dose of stereotactic body radiotherapy (SBRT). Herein, we describe and compare a simple, yet clinically useful, hybrid 3D-dynamic conformal arc (h-DCA) planning technique using flattening filter-free (FFF) beams to minimize these effects. Materials and Methods: Fifteen consecutive solitary early-stage I-II non-small-cell lung cancer (NSCLC) patients who underwent a single-dose of 30 Gy using 3–6 non-coplanar VMAT arcs with 6X-FFF beams in our clinic. These patients’ plans were re-planned using a non-coplanar hybrid technique with 2–3 differentially-weighted partial dynamic conformal arcs (DCA) plus 4–6 static beams. About 60–70% of the total beam weight was given to the DCA and the rest was distributed among the static beams to maximize the tumor coverage and spare the organs-at-risk (OAR). The clinical VMAT and h-DCA plans were compared via RTOG-0915 protocol for conformity and dose to OAR. Additionally, delivery efficiency, accuracy, and overall h-DCA planning time were recorded. Results: All plans met RTOG-0915 requirements. Comparison with clinical VMAT plans h-DAC gave better target coverage with a higher dose to the tumor and exhibited statistically insignificance differences in gradient index, D_2cm, gradient distance and OAR doses with the exception of maximal dose to skin (P = 0.015). For h-DCA plans, higher values of tumor heterogeneity and tumor maximum, minimum and mean doses were observed and were 10%, 2.8, 1.0, and 2.0 Gy, on average, respectively, compared to the clinical VMAT plans. Average beam on time was reduced by a factor of 1.51. Overall treatment planning time for h-DCA was about an hour. Conclusion: Due to no beam modulation through the target, h-DCA plans avoid small-field dosimetry and MLC interplay effects and resulting in enhanced target coverage by improving tumor dose (characteristic of FFF-beam). The h-DCA simplifies treatment planning and beam on time significantly compared to clinical VMAT plans. Additionally, h-DCA allows for the real time target verification and eliminates patient-specific VMAT quality assurance; potentially offering cost-effective, same or next day SBRT treatments. Moreover, this technique can be easily adopted to other disease sites and small clinics with less extensive physics or machine support.