A Dosimetric Study Comparing Two Beam Arrangement Strategies in Fractionated Thoracic Spine Stereotactic Body Radiotherapy (SBRT) Planning.

PURPOSE/OBJECTIVE(S): To compare dosimetric results of two beam arrangement strategies and their robustness to setup uncertainties in fractionated thoracic spine Stereotactic Body Radiotherapy (SBRT) MATERIALS/METHODS: Fifteen patients who received fractionated thoracic spine SBRT were retrospectively studied. All patients received simulation CT scans in body vacuum bag immobilization and multiparametric MRI exams. Clinical target volumes (CTVs) included single vertebral bodies with possible paraspinal space inclusion. Planning target volumes (PTVs) were expanded from CTVs with a 2mm margin but were cropped from spinal cord defined by MRI with a 2mm margin. Two different beam arrangement strategies of volumetric modulated arc therapy planning were studied: 1) 5 full arcs (FA) (360° each arc) with different collimator angles; and 2) 6 partial arcs (PA) (90° each arc) divided into two groups (3 arcs in each group) covering patient left-posterior-oblique (LPO) and right-posterior-oblique (RPO) regions, respectively, with orthogonal collimator angles. Both plans (PlanFA and PlanPA ) were calculated as 24 Gy in 3 fractions using 6xFFF photon energy and a high-definition MLC model. During the inverse optimization of each plan for same patient, a same set of dose-volume constraints and optimization settings was used to exhaust parameter space search. Key dosimetric results of PTV as well as dose-volume parameters of relevant organs-at-risk (OARs) including spinal cord, esophagus, heart, lung and liver were evaluated. Dosimetric impact of on-board patient setup uncertainties to both plans were also simulated. All comparison results were analyzed by Wilcoxon signed-rank tests when the statistical power was sufficient.

RESULTS: Both PlanFA and PlanPA achieved satisfactory spatial dose distribution. After PTV coverage normalization, PlanPA had better PTV dose uniformity (P = 0.026) and PlanFA had better dose fall-off gradient outside PTV. PlanPA had slightly better (18.1 ± 0.8 Gy vs 18.3 ± 0.9 Gy) cord max dose (D0.035cc ) results (P = 0.213) and better cord low dose sparing V12 Gy (P = 0.013) results. PlanPA also achieved lower max dose (D0.035cc ) of esophagus (P = 0.003) and heart, and improved low dose sparing (V5 Gy) of lung (P = 0.002) and liver. In plan parameter comparisons, PlanFA demonstrated stronger beam modulation effect (P < 0.001) but PlanPA had smaller MLC apertures (P < 0.001). In the simulated on-board scenarios with setup uncertainties, while both PlanFA and PlanPA had similar cord max dose increases with simulated pitch and/or roll in setup (< 0.1 Gy differences), PlanPA had minimal cord max dose increase (P < 0.001) with simulated anterior body weight loss.

CONCLUSION: For thoracic spine SBRT plans, the beam arrangement in PlanPA might be favored dosimetrically with better OAR sparing results and could be less sensitive to certain patient uncertainties, while the PlanFA could be acceptable with satisfactory dosimetry results.

AUTHOR DISCLOSURE: C. Wang: None. Y. Xie: None. Z. Hu: None. F. Yin: Research Grant; Varian Medical Systems. Teaching and mentoring graduate students. Administration of graduate program activities; Duke Kunshan University. Board of Directors Member at Large Members; AAPM. organize activities of the SANTRO; SANTRO.Z. Reitman: None. Y. Cui: None.