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Automated Solid Water Phantom Setup Using Light-Dependent-Resistors (LDR)

L Voros*, M Zhang , Y Song , Memorial Sloan Kettering Cancer Center, New York, NY


(Sunday, 7/29/2018) 3:00 PM - 6:00 PM

Room: Exhibit Hall

Purpose: To automate solid water phantom setup with light-dependent-resistors (LDR) utilizing light field as a guide, to build a device for self-aligning setup without user intervention, to replace any light field-related QA tests with an automated system, to eliminate inter-observers variation, and to quantify the device's setup accuracy and precision.

Methods: Linac rooms have no natural light present because they are fully shielded and have a maze. This controlled light environment provides an ideal occasion for LDR application. Our first prototype auto-setup device was constructed with 2 LDRs. It had one-dimensional movement capability achieved with a recycled Varian MLC driving motor (Figure 1a), which has a precision of 0.1 mm and a programmable microcontroller. The physical size of the LDRs is 4 mm in diameter. Its interior design exhibits an inter-digital pattern (Figure 1b). TrueBeam high resolution KV imaging was used to measure and evaluate the device's auto-alignment accuracy (Figure 1c). Also used optical surface monitoring system (OSMS) to record alignment accuracy in both Y (longitudinal) and X (lateral) directions (Figure 1d-e).

Results: Initial experiments indicated that building a self-aligning solid water phantom was feasible. With our device, accurate phantom setup was achieved in seconds. Self-aligned setup accuracy observed with KV imaging (N=10) was 0.33 mm±0.25mm [range, 0-0.7mm], when the device was positioned in the longitudinal direction. Setup accuracy observed with OSMS (N=18) was 0.38 mm±0.32mm [0-1.2mm]. The device was also positioned in the lateral direction for the OSMS measurement, accuracy (N=11) was 0.11mm±0.1mm [0-0.3mm].

Conclusion: This work demonstrates an automated setup device built with LDRs. Preliminary results show that this novel design may replace and even eliminate all user dependent light-radiation field coincidence checks performed by observers and can greatly shorten phantom setup time. This principle can be naturally extended to all dose measuring devices.


Quality Assurance, Autocorrelation Function


IM/TH- Formal quality management tools: General (most aspects)

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