Non-isocenteric approach in non-coplanar volumetric modulated arc therapy (VMAT) planning for lung SBRT treatments

S. Kim; T. Tseng; Dimitroyannis, D.
March 2013
Journal of Radiosurgery & SBRT;2013 Supplement 2.1, Vol. 2, p84
Academic Journal
Purpose: Volumetric Modulated Arc Therapy (VMAT) has emerged as an efficient alternative to traditional multi-beam, three-dimensional (3D), conformal techniques for lung cancer stereotactic body radiotherapy (SBRT) because of its superior dosimetric properties and its native ease in planning and in delivery. However, patient immobilization in lung SBRT often presents challenging geometrical clearance issues in the execution of large (in excess of 180°) non-coplanar arcs. In this study, we present a non-isocentric, non-coplanar VMAT technique that appears to be simple, dosimetrically robust and which allows for ample couch-gantry clearance. We compared this technique to an isocentric, non-coplanar VMAT technique, which we have already demonstrated to be superior to multi-beam 3 D conformal lung SBRT. Methods: Nineteen lung cancer patients previously treated with 3D conformal SBRT technique at our institution were selected. For each patient, the non-isocentric, non-coplanar VMAT plan was compared to original non-coplanar 3D treatment plan(s) and an isocentric, non-coplanar VMAT plan. All the plans were created for the same prescription dose of 54 Gy total in 3 fractions covering 95% of the planning target volume (PTV). Nine to ten non-coplanar beams were used for the 3D technique and three non-coplanar arcs were used in both isocentric and non-isocentrtric VMAT plans, with the couch set at ± 20° and at 0°, with each arc rotation in excess of 180°. Progressive Resolution Optimizer (PRO) in Varian Eclipse version 11 was used for all the treatment planning. Conformity Index (CI), conformity number (CN), gradient index (GI), maximum dose at 2 cm away from the PTV (D2cm), V20, V5, mean target dose (MTD) and mean lung dose (MLD) were analyzed for all the plans. We also performed the statistical analysis for the differences of these dosimetric indices between 3D and VMAT techniques. Results: Dosimetric indices CI, CN and GI, V20 and MTD values were similar within 5% for all three plans: 3D, isocentric and non-isocentric VMAT plans, while both types of VMAT plans were dosimetrically superior to 3D conformal plans in organ-at-risk (OAR) sparing; D2cm, V5, and MLD values were significantly lower as 6-8%, 26-30%, 9-12% in VMAT plans, respectively. The non-isocentric VMAT plans showed equivalent plan quality to the isocentric VMAT plans and exhibited robust freedom from limiting arc rotation due to potential couch-gantry collision. Conclusion: Three arc, non-isocentric, non-coplanar lung SBRT VMAT plans appear dosimetrically equivalent to isocentric VMAT plans, while permitting large angle arc selection, free from obstractional limitations. Disclosure: No significant relationships.


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