Passively Scattered Proton Stereotactic Body Radiation Therapy for Spine Metastasis: A Dosimetric Analysis

Abstract

Purpose/Objective(s)Passively scattered proton therapy (PSPT) has been used for stereotactic body radiation therapy (SBRT) of various sites, including the lung and liver. To our knowledge there is no data for PSPT for spinal SBRT. Therefore, we undertook a dosimetric analysis to evaluate the feasibility of using PSPT with minimal beams for SBRT to treat spine metastases per the ongoing clinical trial RTOG 0631.Materials/MethodsIn 2 model patient simulations, vertebral levels were selected in the cervical, thoracic, and lumbar spine, and clinical target volumes (CTVs) were generated according to the guidelines described in RTOG 0631 to a dose of 18 CGE in 1 fraction. Multiple plans were generated for each CTV using passively scattered proton beams. Plans were evaluated based on the dose coverage of the target volume, the ability to meet dose constraints to surrounding organs at risk, as well as dose homogeneity.ResultsRTOG 0631 criteria for target coverage and normal tissue constraints were met for each of the target volumes in the cervical, thoracic, and lumbar spine levels. For upper cervical vertebra, 2 opposed lateral beams were sufficient to achieve dose coverage and meet constraints. The spinal cord received a maximum dose of 13.8 CGE with 0.22 mL receiving 10 CGE, and 2.24 mL of the pharynx received a dose of 10.5 CGE. In the lower cervical, thoracic, and lumbar spine, the shoulders, lungs, and bowel, respectively, impeded the use of lateral beams, and instead 5 beams were required to meet homogeneity and conformity constraints. The 5-beam arrangement included a posterior beam, 2 posterior oblique beams, and 2 posterior oblique patches. For the thoracic target, the spinal cord received a maximum dose of 11.3 CGE with 0.36 mL receiving 10 CGE, and 0.03 mL of the esophagus received 16 CGE. In the lumbar spine, 0.05 mL of the cauda equina received 16 CGE. Distant anterior structures, such as the heart, stomach, and bowel received negligible dose.ConclusionThe treatment of spine metastases per the RTOG 0631 protocol using PSPT is feasible at each spinal level with negligible distant anterior structure dose. For the superior cervical spine we recommend the use of 2 opposed lateral beams, and in the lower cervical, thoracic, and lumbar spine we recommend a 5-beam patch plan using posterior and posterior oblique beams.