The challenge of planning vertebral body SBRT: Optimizing target volume coverage

Stereotactic ablative body radiotherapy for vertebral metastases has been shown to be safe and effective to achieve tumor and pain control. To raise awareness of and build familiarity with vertebral stereotactic ablative body radiation therapy (SBRT) for a multicenter clinical trial including SBRT to vertebral metastases, Trans Tasman Radiation Oncology Cancer Research performed an international planning challenge. A single vertebral case was selected and the computed tomography image and contours were made available. Participants performed a treatment plan according to the NIVORAD clinical trial protocol and uploaded the treatment plan and dose grid Digital Imaging and Communications in Medicine (DICOM) files. A progressive scoring matrix was applied which gave each plan a score based on target and organ at risk dosimetry. The plans were compared based on achieved score and treatment technique details. A total of 149 plans were submitted from 26 countries; the treatment geometry for four plans was deemed to result in collision with the couch and these were removed from analysis. Only one plan exceeded spinal cord constraints; all other plans met protocol constraints. The largest variation in plan quality was observed with the target coverage; the highest scoring plans were able to achieve higher target coverage whilst respecting adjacent organ at risk (OAR) constraints. Consequently, plan score was correlated with the dose gradient at the target-cord interface. We have conducted a large multicenter, international vertebral SBRT planning challenge. The results showed consistent ability to meet protocol constraints, however a large variation in the ability to cover the target volume was observed.