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Evaluation of a Transmission Detector On IMRT QA Using Structure Similarity Index (SSIM)

C Shi*, S Lim , M Chan , Memorial Sloan-Kettering Cancer Center, Basking Ridge, NJ


(Tuesday, 7/31/2018) 10:30 AM - 11:00 AM

Room: Exhibit Hall | Forum 6

Purpose: Patient-specific IMRT QA is an important work in radiation oncology. Current practice has multiple choices for 2D fluence map measurements, such as GafChromic film, EPID, MapCHECK etc. However, most of those devices need build up materials to do the measurement, which would introduce unnecessary scatter into the measurement device. On the other hand, fluence based detectors are available in clinics, such as Dolphin device by IBA company. How the Dolphin device performs in a clinic is still unknown, especially comparing to EPID and MapCHECK. Also, how to compare different devices performance for patient-specific IMRT QA is still not straightforward. In this study, we investigate all those issues.

Methods: Dolphin device was used to measure patient-specific QA for different sites (2 brain, 2 lung, 1 GI) and the results were compared with MapCHECK, EPID, and calculated fluence map. In order cross-compare those devices, a new QA metric parameter, Structure SIMilarity index (SSIM) was used. SSIM was widely used in industry to compare different video image quality. Here we transformed the concept for dose map comparison.

Results: SSIM map and values were derived for EPID, MapCHECK, and Dolphin. Although the absolute values were close to each other, the EPID shows better matching with calculated fluence map, followed by Dolphin and MapCHECK. The major reason was the detector resolution and scatter dose effect on the final 2D map. Since Dolphin has no buildup, the scatter dose for lower dose is much less than MapCHECK. On the other hand, Dolphin has 5 mm resolution, which is coarser than EPID, resulting in a lower SSIM.

Conclusion: SSIM was used to compare different 2D measurement devices for patient-specific QA. Dolphin shows better performance than MapCHECK but worse than EPID considering SSIM values and resolution factors.


Quality Assurance, Intensity Modulation, Dosimetry


TH- Radiation dose measurement devices: Development (new technology and techniques)

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