Novel Hybrid Volumetric Modulated Arc Therapy (VMAT) for Stereotactic Body Radiation Therapy (SBRT) of Lung Tumors in Proximity to the Chest Wall

Abstract

It can be challenging to optimize SBRT treatment plans of peripheral lung lesions because of the large variation in tissue heterogeneity and close proximity to the chest wall. It has been shown that coplanar volumetric modulated arc therapy (VMAT) is capable of achieving high dose conformity comparable to coplanar static-angle IMRT but with slower dose fall-off than noncoplanar IMRT. To reduce dose to the chest wall and lung, we developed a hybrid VMAT planning strategy to achieve both high dose conformity and steep fall-off beyond the target volume. Seventeen lung lesions in sixteen patients with medically inoperable early stage non-small cell lung cancer or lung oligometastases treated with hybrid VMAT were reviewed. All cases had overlap of the planning target volume (PTV) with the chest wall. Our hybrid VMAT planning approach utilized noncoplanar conformal beams and coplanar intensity modulated arcs. Three to four noncoplanar beams were manually optimized to form the base dose plans, which contribute 25% to 30% of the prescribed dose. The VMAT portion, developed by base plan optimization, consisted of two full rotations. To demonstrate the benefit of the hybrid technique, we compared the hybrid plans with pure VMAT plans. Plan quality was evaluated using: conformity index (CI), ratio of 50% isodose volume to PTV volume (R50%), dose homogeneity, and lung volume receiving 10 and 20 Gy (V10 and V20). All plans in this study were normalized by scaling the 100% isodose line to cover 95% of the PTV with the dose maximum ranging from 124% to 134%. In all cases, the hybrid plans yielded better dose conformity (averaged CI = 1.09; range 1.03 - 1.24) than that of the pure VMAT plans (averaged CI = 1.14; range 1.06 - 1.28). Moreover, the dose gradient was steeper in the hybrid plans: averaged hybrid R50%= 4.90, (range 4.02 - 6.97) vs. pure VMAT 5.36 (range 4.24 - 7.45). Thus the hybrid technique was able to reduce dose to the chest wall and ribs. We noted that R50% is correlated to the size of PTV. No significant difference between the two approaches was found in pulmonary V20 and V10. Treatment delivery time of the hybrid plans ranged from 11 minutes to 14 minutes. At a median follow-up of 6.7 months (range, 1 to 14 months), local control is 95.2% (1 in-field failure in patient with a 4.5 cm mass). One patient experienced grade 2 chest wall pain and 1 patient experienced grade 1 dermatitis, which have resolved. Using the hybrid VMAT technique, we have achieved higher dose conformity with lower dose to the chest wall and ribs compared to a standard coplanar VMAT approach. Longer-term follow up is required to quantify the clinical significance of the hybrid VMAT strategy.