Purpose: Small field dosimetry and MLC/lung tumor interplay effects are major concerns for highly modulated IMRT/VMAT planning in treatments of single-dose lung SBRS. Herein, we describe and compare a simple hybrid 3D-dynamic conformal arc (h-DCA) therapy plan for SBRS of lung lesions that minimize these effects.
Methods: Thirteen solitary early-stage non-small-cell lung cancer patients underwent a single-dose of 30 Gy using highly conformal 3-6 non-coplanar VMAT arcs plan with 6X-FFF beam. These plans were re-planned using a non-coplanar hybrid technique with 3 differentially-weighted partial DCAs plus 4-6 static beams. About 60% of the beam weight was given to the DCA and the rest of was distributed among the static beams to minimize the dose to the organs-at-risk (OAR). VMAT and h-DCA plans were compared via RTOG-0915 protocol compliance, target conformity (CI), heterogeneity index (HI), gradient index (GI) and dose to OARs. Additionally, delivery efficiency and overall 3D-planning time were recorded.
Results: All plans met RTOG-0915 requirements. Comparison with clinical-VMAT plans gave better target coverage and exhibited no statistically significant difference in GI, D2cm and OAR doses with the exception of max and 10 cc skin doses (P=0.01 and 0.02). For the h-DCA plans HI, GTV maximum and mean doses were on average 10%, 2.9Â±2.4Gy and 2.2Â±1.9Gy, respectively, higher than VMAT plans. Overall, treatment-planning time was about an hour. Average beam-on time was reduced by a factor of 1.5.
Conclusion: h-DCA avoids small field dosimetry and interplay effects (open static fields) resulting in enhanced target coverage by improving HI (characteristic of FFF-beam), simplifying treatment planning and reducing planning and beam-on time significantly compared to VMAT plan. Additionally, h-DCA allows for real time target verification and eliminates patient-specific IMRT QA; potentially offering cost- effective, same or next day SBRS; it can be easily adopted to small clinics with less extensive physics support.