Off-target-isocentric approach in non-coplanar Volumetric Modulated Arc Therapy (VMAT) planning for lung SBRT treatments

Abstract

Purpose: Volumetric Modulated Arc Therapy (VMAT) has emerged as an efficient alternative to traditional three-dimensional (3D) non-coplanar conformal (3D-NC-C) beams for lung cancer stereotactic body radiotherapy (SBRT) because of its superior dosimetric properties and native ease in planning and treatment 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 an off-target-isocentric, non-coplanar VMAT (OTI-NC-VMAT) technique that appears to be simple, dosimetrically robust and allows for ample patient/couch-gantry clearance. We compared this technique to a target-isocentric, non-coplanar VMAT (TI-NC-VMAT) technique and the 3D-NC-C beams for dosimetric evaluations.

Methods: Nineteen lung cancer patients previously treated with 3D-NC-C SBRT technique at our institution were selected. For each patient, an OTI-NC-VMAT plan and TI-NC-VMAT plan were created and compared to the original 3D-NC-C treatment plan. All of 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 the TI-NC-VMAT and OTI-NC-VMAT plans, with the couch set at ± 20° and 0°, with each arc rotation in excess of 180°. Progressive Resolution Optimizer (PRO) in Varian Eclipse version 11 was used for all of the treatment planning. Conformity Index (CI), conformity number (CN), gradient index (GI), maximum dose at 2 cm away from the PTV (D2cm), mean lung dose (MLD), V20, V5 and mean target dose (MTD) were analyzed for all of the plans. We also performed statistical analysis to examine differences in the dosimetric indices between 3D and VMAT techniques.

Results: Dosimetric indices CI, CN, GI, V20 and MTD values were similar, within 5%, for all three plans: 3D-NC-C, TI-NC-VMAT and OTI-NC-VMAT. However, both types of VMAT plans were dosimetrically superior to 3D conformal plans in organ-at-risk (OAR) sparing; D2cm, MLD, and V5 values were significantly lower at 6-8%, 9-12% and 26-30% in VMAT plans, respectively. The OTI-NC-VMAT plans showed equivalent plan quality to the TI-NC-VMAT plans and exhibited robust freedom from limiting arc rotation due to potential patient/couch-gantry collision.

Conclusions: The OTI-NC-VMAT plans appear dosimetrically equivalent to TI-NC-VMAT plans for lung SBRT, while permitting large angle arc selection, free from obstructional limitations. Both OTI-NC-VMAT and TI-NC-VMAT plans were dosimetrically superior to 3D-NC-C plans in terms of organ-at-risk (OAR) sparing.

Keywords: 3D conformal; Lung cancer; SBRT; VMAT; hypofractionation; non-coplanar; off-target.

Figure 1

3D-NC-C beam arrangements with standard gantry-couch parameter set for (a) right lung SBRT and (b) left lung SBRT. Note that both arrangements contain a single conformal beam which passes through the contralateral lung to maintain the conformity of target coverage.

Figure 2

TI-NC-VMAT beam arrangement for left lung SBRT. Three dynamic arcs of 210 degree gantry angle with ±20 degree separations were focused to the isocenter that is within a target.

Figure 3

(a) OTI-NC-VMAT beam arrangement for left lung SBRT. Note that the isocenter of three dynamic arcs of 210 degree gantry angle were placed away from the target position. This off-target approach was intended to produce more clearance in gantry-couch collision. (b) Beam’s eye view of a single VMAT beam collimated with HD MLC. As the beam angle changes, the target will oscillate in the beam’s eye view because the isocenter is not located within the target.

Figure 4

Dose distribution of OTI-NC-VMAT plan for left lung SBRT. Note that the maximum dose (122.3% of the prescription dose in this case) was located within the target. Large low dose spillage was also shown from the characteristics of VMAT technique.

Figure 5

Comparison of DVH among three plans, 3D-NC-C, TI-NC-VMAT, and OTI-NC-VMAT, for a typical left lung SBRT plan. Note that both VMAT plans produced lower maximum dose in PTV with lower lung dose, in the dose range of 200-1000 cGy. (Nomenclature: SBRT LT Lung = 3D-NC-C, VMAT1 = TI-NC-VMAT, VMAToff = OTI-NC-VMAT. Legend: Square = 3D-NC-C, Triangle = TI-NC-VMAT, Circle = OTI-NC-VMAT)

Figure 6

Comparison of estimated total treatment delivery time between 3D-NC-C and VMAT plans. Estimated treatment time for 3D-NC-C and VMAT plans were found to be 64 and 49 minutes, respectively.

Figure 7

Example of higher heterogeneities of target dose distribution in a VMAT plan compared to a 3D conformal plan. These heterogeneities may be avoided by using a central inner ring structure to confine the hot spots in the central PTV region. Note that the heterogeneity effect will be dosimetrically minimal due to the target motion.