Abstract
PurposeVolumetric modulated arc therapy (VMAT) is gaining popularity for stereotactic treatment of lung lesions for medically inoperable patients. Due to multiple beamlets in delivery of highly modulated VMAT plans, there are dose delivery uncertainties associated with small‐field dosimetry error and interplay effects with small lesions. We describe and compare a clinically useful dynamic conformal arc (DCA)‐based VMAT (d‐VMAT) technique for lung SBRT using flattening filter free (FFF) beams to minimize these effects.Materials and MethodsTen solitary early‐stage I‐II non‐small‐cell lung cancer (NSCLC) patients were treated with a single dose of 30 Gy using 3–6 non‐coplanar VMAT arcs (clinical VMAT) with 6X‐FFF beams in our clinic. These clinically treated plans were re‐optimized using a novel d‐VMAT planning technique. For comparison, d‐VMAT plans were recalculated using DCA with user‐controlled field aperture shape before VMAT optimization. Identical beam geometry, dose calculation algorithm, grid size, and planning objectives were used. The clinical VMAT and d‐VMAT plans were compared via RTOG‐0915 protocol compliances for conformity, gradient indices, and dose to organs at risk (OAR). Additionally, treatment delivery efficiency and accuracy were recorded.ResultsAll plans met RTOG‐0915 requirements. Comparing with clinical VMAT, d‐VMAT plans gave similar target coverage with better target conformity, tighter radiosurgical dose distribution with lower gradient indices, and dose to OAR. Lower total number of monitor units and small beam modulation factor reduced beam‐on time by 1.75 min (P < 0.001), on average (maximum up to 2.52 min). Beam delivery accuracy was improved by 2%, on average (P < 0.05) and maximum up to 6% in some cases for d‐VMAT plans.ConclusionThis simple d‐VMAT technique provided excellent plan quality, reduced intermediate dose‐spillage, and dose to OAR while providing faster treatment delivery by significantly reducing beam‐on time. This novel treatment planning approach will improve patient compliance along with potentially reducing intrafraction motion error. Moreover, with less MLC modulation through the target, d‐VMAT could potentially minimize small‐field dosimetry errors and MLC interplay effects. If available, d‐VMAT planning approach is recommended for future clinical lung SBRT plan optimization.
Highlights
We describe and compare a clinically useful dynamic conformal arc (DCA)‐based Volumetric modulated arc therapy (VMAT) (d‐VMAT) technique for lung stereotactic body radiation therapy (SBRT) using flattening filter free (FFF) beams to minimize these effects
Clinically desirable tighter 50% isodose distribution was obtained with DCA‐based VMAT (d‐VMAT) compared to the clinical VMAT plan
A simple, yet clinically useful d‐VMAT planning technique was presented for a single dose of lung SBRT treatments
Summary
The use of stereotactic body radiation therapy (SBRT) has become a standard curative treatment for medically inoperable early‐staged non‐small‐cell lung cancer (NSCLC) patients providing a high cure rate and minimal treatment‐related toxicity.[1,2,3,4,5] For the selected peripherally located NSCLC patients, single dose of lung SBRT has become a curative‐intent treatment option as shown by the randomized clinical trials.[6,7,8,9,10,11,12,13] Most recently, clinical use of flattening filter free (FFF) beams has been of interest in delivering lung SBRT treatments due to dosimetric advantages compared to a flattened beam.[14,15,16,17,18] FFF beams can significantly reduce beam‐on time due to their higher dose rates, resulting in better patient compliance, potentially reducing dose delivery uncertainty due to less intrafraction motion error and reduction in out‐of‐field dose with less head scatter and electron contamination.[14,15,16]. Utilizing volumetric modulated arc therapy (VMAT) with FFF‐beams[17,18] resulted in better tumor dose coverage and faster treatment delivery of complex lung SBRT treatments compared to historically used plans with 8–15 non‐ coplanar fixed fields or several coplanar DCA fields with flattened beams.[19,20,21,22] Similar results were observed when compared to linac‐ based intensity modulated radiation therapy (IMRT), VMAT plans, helical TomoTherapy, or optimized robotic CyberKnife plans (showing significant increases in SBRT treatment times).[23,24,25,26] for a single dose of lung SBRT treatments, highly modulated IMRT/ VMAT plans are susceptible to delivery uncertainties due to small‐ field dosimetry error[27] and interplay effects[28] due to MLC modulation of multiple beamlets through the target as a function of lung tumor motion and tissue heterogeneities To minimize these effects, recently, Varian Eclipse treatment planning system (TPS, Varian Medical Systems, Palo Alto CA, Version 15.1 and beyond)[29] has implemented a new multileaf collimator (MLC) optimization algorithm, called Photon Optimizer (PO).
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