Room: AAPM ePoster Library
Purpose: Treating large (> 5 cm) lung tumors with ablative SBRT doses is challenging given the concern for morbidity. We present a novel dynamic conformal arc (DCA)-based simultaneous integrated boost (SIB) VMAT-SBRT planning technique with flattening filter free (FFF) beams. This fast, safe, and effective method allows for dose escalation while sparing organs at risk (OAR).
Methods: Six patients with large (mean = 6.9 cm, ranged: 6.1-7.7 cm, diameter) solitary stage II non-small-cell lung cancer (NSCLC) tumors underwent SBRT of 40-50 Gy to the planning target volume (PTV) in 5 fractions using 3-6 non-coplanar VMAT (clinical VMAT, isocenter at tumor center) with 6X-FFF beams. Treatments were re-planned using 3-partial arcs (0, ±15-degrees, couch-kicks) for 60 Gy to the gross target volume (GTV) and 50 Gy to the PTV in 5 fractions using a novel DCA-based SIB-VMAT technique (DCA-VMAT) with isocenter at patient midline. DCA-VMAT plans were recalculated using DCA-based dose with user-controlled field aperture shape before VMAT optimization. Clinical VMAT and DCA-VMAT plans were compared for target dose, OAR dose, biological effective dose (BED10), and treatment efficiency.
Results: Compared to clinical VMAT, DCA-based SIB-VMAT plans averaged higher hypoxic tumor center dose [144Gy vs. 113 Gy(BED10)]; and PTV dose [127Gy vs. 92.5Gy(BED10)], with tighter radiosurgical dose-distribution and lower OAR doses. Lower total number of monitor units and small beam modulation factor reduced averaged beam on time by 0.51 min (p <0.001), (maximum up to 0.90 min)-enabling faster treatment.
Conclusion: Combining DCA-based SIB-VMAT technique with midline-isocenter position, provided excellent plans for large lung tumors by escalating hypoxic tumor center dose, reduced intermediate dose-spillage and maintained OAR dose with faster treatment delivery. With less MLC modulation through the target, DCA-based SIB-VMAT plans minimized small-field dosimetry errors and perhaps MLC interplay-effects. If available, DCA-based SIB-VMAT planning is recommended for SBRT of large lung tumors.