Dosimetric Implication Of Planning Organ At Risk Volumes In Stereotactic Body Radiation Therapy Of Spine

To investigate the impact of planning organ at risk volumes (PRV) of neural organ at risk (OAR) of spinal cord (SC) and cauda equina (CE) on the safety and efficacy in stereotactic body radiotherapy (SBRT) of spine. Pairs of SBRT plans were generated using either no PRV (OAR-plans) or 1mm PRV margin (PRV-plans) for 22 spinal lesions. An 1-mm planning target volume (PTV) margin was added to clinical target volume (CTV). All plans were generated using a volumetric-modulated arc therapy technique. A single dose of 18Gy was prescribed to cover ≤95% of PTV and dose constraints from RTOG 0631 trial were used. Three types of uncertainties, i.e. pre-treatment residual error, post-treatment residual error, and intrafraction motion, were acquired by matching of pre-/post-treatment cone-beam computed tomography to simulation computed tomography, and used to generate three actual OAR/CTVs representing the actual position of the structures. Using Monte-Carlo simulation with 100 random uncertainties for each type of plan and uncertainty, we estimated the probability of constraint violation. All OAR- and PRV-plans met the dose constraints. Adequate PTV coverage was achieved in all plans (average, OAR-plan: 94.6%; PRV-plan: 94.8%). Coverage of actual CTVs was adequate in OAR plans and PRV plans (97.8±2.1% vs. 97.3±1.6%). Conformity index (0.978 vs. 0.972, p1% in 73% (8/11) and 29% (4/14) of OAR-plans, respectively; however, all PRV-plans resulted in violation probability ≤1% with any type of uncertainty. Spinal SBRT without PRV margin may risk violating constraints for neural OARs due to geometric uncertainties. Introduction of 1-mm PRV margin seems to guarantee safe treatment as well as adequate CTV coverage. Therefore, 1-mm PRV margin should be routinely applied in spinal SBRT planning.