Optimizing the CBCT Technique for Linac Synchronized NIPAM 3D Dosimetry
K Lakrad1, 2*, M Oldham1, J Adamson1, (1) Duke University Medical Center, Durham, NC, (2) University Hassan II, Casablanca, Morocco
Presentations
PO-GePV-T-272 (Sunday, 7/10/2022) [Eastern Time (GMT-4)]
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Purpose: Linac-synchronized NIPAM 3D dosimetry is unique because delivered dose causes increased density in the NIPAM dosimeter, which is then manifest as increased intensity in the CBCT. An advantage is that the measured dose is inherently synchronized with the on-board imaging coordinate system; however, one challenge is limited signal strength. Our purpose is to develop a CBCT readout designed to optimize the contrast to noise ratio for Linac Synchronized NIPAM 3D dosimetry.
Methods: NIPAM dosimeters were prepared in 1L PET jars. For the linac-synchronized 3D dosimetry procedure, a dosimeter was placed on the treatment table after which a series of pre-irradiation CBCTs were acquired. The dosimeter was then irradiated using a simple 3 field plan (6MV-FFF, 25Gy), followed by a series of post-irradiation CBCTs (using the same acquisition settings as the pre-irradiation CBCTs). Post analysis included averaging and then subtracting the pre- from the post-irradiation CBCTs. Various CBCT acquisition and reconstruction parameters were investigated including: kVp, mAs setting, number of projections as defined by gantry rotation speed and frame rate, reconstruction technique (standard or iterative), reconstruction filter (standard vs. smooth).
Results: Utilizing an iterative reconstruction algorithm increased contrast to noise ratio (CNR) from 5.8 to 12.3. Even for very high mAs settings (5400mAs x 6 CBCTs) the imaging dose had a negligible effect on dosimeter response as measured by CBCT intensity, although a change in optical density was observed. The slow rotation CBCT technique shows to be improving image high quality and increasing the number of projections from 400 to 5400. Agreement in 3D gamma analysis with pass rates ranged from 92.82% to 96.52% for different criteria (5%2mm, 2%3mm and 3%3mm).
Conclusion: Improved image quality for linac synchronized NIPAM 3D dosimetry shows promise for both dosimetric and spatial verification of complex clinical treatments.
Funding Support, Disclosures, and Conflict of Interest: Dr. Adamson reports ownership of Clearsight RT LLC which is unrelated to this project.
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