A Simulation-free Replacement Solution for Radiation Therapy Immobilization Devices using CNC-Milled Polystyrene Molds

Purpose In radiotherapy, if an immobilization device is lost or damaged, the patient may need to be brought back for re-simulation, device fabrication and treatment planning, causing additional imaging radiation exposure, inconvenience, cost, and delay. We describe a simulation-free method for replacing lost or damaged radiotherapy immobilization devices. Methods and Materials Replacement immobilization devices were fabricated using existing simulation scans as design templates by computerized numerical control (CNC) milling of molds made from extruded polystyrene (XPS). XPS material attenuation and bolusing properties were evaluated, a standard workflow was established, and 12 patients were treated. Setup reproducibility was analyzed post-facto using Dice similarity coefficient (DSC) and mean distance to agreement (MDA) calculations comparing on board treatment imaging with CT sims. Results Results showed that XPS foam material had less dosimetric impact (attenuation and bolusing) than materials used for our standard immobilization devices. The average direct cost to produce each replacement mold was $242.17 compared with over $2,000 for standard re-simulation. Hands-on time to manufacture was 86.3 min, while molds were delivered in as little as 4 hours and mostly within 24 hours compared with a week or more required for standard re-simulation. Each mold was optically scanned after production and was measured to be within 2mm tolerance (pointwise displacement) of design input. All patients were successfully treated using the CNC milled foam mold replacements, and pretreatment imaging verified satisfactory clinical setup reproduction for each case. The external body contours from the setup CBCT and the original CT simulation with matching superior-inferior extent were compared by calculating the Dice similarity coefficient (DSC) and mean distance to agreement (MDA). DSC average was 0.966 (0.011 St Dev) and MDA average was 2.694 mm (St Dev 0.986). Conclusion CNC milling of XPS foam is a quicker and more convenient solution than traditional re-simulation for replacing lost or damaged radiotherapy immobilization devices. Satisfactory patient immobilization, low dosimetric impact compared to standard immobilization devices, and strong correlation of on board contours with CT sims are shown. We share our clinical experience, workflow and manufacturing guide to help other clinicians who may want to adopt this solution.