Dosimetric Implication of Neural Planning Organ at Risk Volumes in Stereotactic BodyRadiation Therapy of the Spine

Abstract

To investigate the impact of planning organ at risk volumes (PRV) of neural organs at risk (OAR) (spinal cord (SC) and/or cauda equina (CE)) on dosimetric parameters in stereotactic body radiotherapy (SBRT) of spinal lesions. For 22 spinal lesions, a pair of SBRT plans were generated using either no PRV (OAR-plans) or 1-mm PRV margin (PRV-plans). SC, CE and both SC and CE were near planning target volume (PTV) in 8, 12, and 2 lesions, respectively. Same PTVs were used for planning irrespective of PRV usage. All plans were generated using a volumetric-modulated arc therapy technique with 10-MV flattening filter-free beams. A single dose of 18Gy was prescribed to cover 80% of PTV. Same dose constraints were used for either OAR or PRV: D0.03cc<14Gy and D0.03cc<16Gy for SC and CE. Homogeneity index (HI), conformity index (CI) and PTV coverage by 16Gy were compared between OAR- versus PRV-plans using a paired t-test. To investigate the impact of potential setup errors in OAR doses, D0.03cc of the PRV was analyzed in OAR-plans. In all lesions, both OAR- and PRV-plans were successfully generated that met OAR/PRV dose constraints and achieved 80% PTV coverage by 18Gy. HI and CI were better in OAR-plans than PRV-plans (HI: 0.22 vs. 0.34; CI: 0.94 vs. 0.93, p<0.001). A proportion of PTV receiving 16Gy or greater was significantly higher in OAR-plans compared to PRV-plans (95.3%±1.9% vs. 96.8%±1.4%, p<0.001). In OAR-plans, D0.03ccof the PRV exceeded the constraints in 8 out of 22 lesions (36.4%). Considering setup errors by 1-mm, dose to neural OAR may exceed the constraints in more than a third cases without PRV. Introduction of PRV did not compromise PTV coverage of prescription doses, and minimally affected dose volumetric parameters of PTV. Thus, PRV with an adequate margin should be routinely applied in planning SBRT of spinal lesions to guarantee safe treatment.