Investigating the Dosimetry Potential of the NIPAM Polymer Gel
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Purpose: The aim of this work is to assess the potential of applying NIPAM polymer gel produced at Duke for CT and CBCT on board imaging for radiation therapy. This is after initial progress has been made in establishing the potential of using this gel but produced at a different location to visualize the dose for multitarget radiosurgery using the KV-CBCT system mounted on-board the linear accelerator.
Materials and Methods: A reproducible procedure for manufacturing polymer gel developed by previous researchers in this field was implemented at the 3D Dosimetry and Bio – imaging lab at Duke University. The use of vacuum seal and oil was introduced as an additional means of minimizing oxygen contamination immediately after production. The technique is to apply the gel for CBCT and sheet Dosimetry
CBCT-Dosimetry: The CBCT acquisition and reconstruction parameters were optimized to maximize low contrast resolution. Varian 600C/D linear accelerator whose QA test has been successful and already in clinical use for SRS was used for spot check. Six different angles were made for each of the gantry, collimator and couch rotations. The radiation isocenter was quantified by determining the center of the smallest intersecting circle; the 3D vector coincidence with the imaging coordinate system was also quantified.
Sheet Dosimetry: A Flat shaped gel was prepared in a vacuum sealed bag. Ten different MU values were delivered at different spots of the gel using the Varian 600C/D linear accelerator which was later scanned using the EPSON flatbed scanner and Diagnostic CT. The optical density values were determined in the three color bands; likewise variation of the mean CT number with dose.
Results: CBCT-D: The optimal values for our CBCT setting were full trajectory, smooth reconstruction filter, strong ring suppression and 80KV. For the star shots, the radius of the smallest circle was 0.5mm, 0.3mm, and 0.6mm for the gantry, couch, and collimator, which were within the recommended tolerance limits specified by TG142 and were within 0.2mm of the value obtained from a traditional film star shot. The 2D vector discrepancy from the imaging coordinate system was 0.1mm, 1.3mm and 1.5mm for the gantry, couch, and collimator, respectively.
Sheet: The optical density values increased with increasing MU in the three color channels. The blue channel showed the largest change in optical density (24% difference); while the red color showed the least. The diagnostic scan showed a linear increase of CT number with MU. At lower MU (0 – 200), the increase fluctuated due to background noise and dose scatter. Above 200MU showed a linear increase in polymerization.
Conclusion: All the irradiated spots and spoke polymerized to the degree of radiation dose quantity and easily visible; which supports the feasibility of the NIPAM polymer gel produced at Duke for dose distribution and QA test. Specifically, the calibration potential of the polymer gel when used as sheet for both the optical and diagnostic CT scan has now been established.
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