Dosimetric Comparison of Single-Isocenter and Multiple-Isocenter Techniques for Two-Lesion Lung SBRT Using the RefleXion High-Speed Ring-Gantry System

Abstract

Due to the technical challenges that accompany stereotactic body radiotherapy (SBRT) to multiple lesions, a single-isocenter treatment plan technique for multi-target scenarios may optimize the final, aggregate dose distribution. The RefleXion™ X1 is a new ring-gantry based system with several features that may optimize dosimetry in a multi-target SBRT scenario, including high speed rotation (60 revolutions per minute), a high-speed multi-leaf collimator (100 Hz), and a variable pitch couch. A comprehensive dosimetric comparison for multiple targets that contrasts multiple isocenters versus a single-isocenter technique has not been performed. Therefore, this investigation performed such a comparison in two-lesion lung SBRT cases.Three patients with two synchronous lung lesions with maximum dimension of 5 cm underwent single-isocenter and two-isocenter planning with the RefleXion treatment planning system using a Collapsed Cone Convolution Superposition algorithm. Treatment doses were in accordance with AAPM TG101 criteria (50Gy in 5 fractions) and planned such that 95% of the planning target volume (PTV) would receive at least 95% of the prescription dose. In the single-isocenter plans, the isocenter was placed between the two-lesions, whereas the two-isocenter plans localized the isocenters on each target with a limitation on the IEC-X direction to avoid couch-bore collisions. Conformity index (CI), homogenous index (HI), R50%, and D2cm were calculated. The volume of the normal lung receiving 5 Gy (V5), V10, V15, V20, mean lung dose, and dose to other OARs were evaluated. In addition, the treatment time to deliver each plan was recorded.The mean isocenter to the tumor distance in XYZ-space was 8.53 cm (range: 7.66-10.62 cm). The mean combined PTV was 42.47cc (range: 9.7- 97.8 cc). There was no significant difference in target coverage, maximum target dose, CI, HI, R50%, V20, V15, and V10 between single-isocenter and two-isocenters for lung SBRT plans. On average, single-isocenter plans contributed an increase in OAR dose, especially to the carina and esophagus, by 5.17% (range: 2.91-6.52%) and 15.62% (range: 6.44-22.97%), respectively. With the RefleXion X1 set at 850MU/min, the average difference in total beam-on time was approximately 33 seconds between the two approaches. However, set-up time for the second isocenter in the two-isocenter plan would likely prolong the total treatment delivery time.This study shows that a single-isocenter technique for the scenario of two lung lesions is dosimetrically feasible and clinically efficient with the RefleXion X1 in SBRT mode. Single-isocenter treatment delivery techniques may be a desirable option for efficient radio-ablative therapies directed at multiple metastatic targets.