P Shukla*, D Mynampati, P Brodin, A Lukaj, R Yaparpalvi, A Basavatia, W Tome, Montefiore Medical Center, Bronx, NY

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  P Shukla

PO-GePV-M-150 (Sunday, 7/10/2022)   [Eastern Time (GMT-4)]

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Purpose: To quantify the feasibility of using the CT electron density calibration curve to perform adaptive dose calculations on the daily iterative cone-beam CT (iCBCT) scans on a Halcyon linear accelerator.

Methods: Three calibration phantoms: CIRS 062, Catphan 604 and QUART DVTap were scanned on a GE lightspeed simulator and on Halcyon2.0 LINAC, with iCBCT pelvis, pelvis large, and head iCBCT protocols. Mean(±SD) of the absolute HU difference between the iCBCT and the CT for all three iCBCT protocols were calculated. A dataset of the mean HUs on the prostate, bladder, and left/right femoral heads for ten prostate patients were obtained on the planning and treatment verification iCBCTs, and subsequently compared between the two.

Results: For all the three iCBCT protocols, the Relative Electron Density vs HU curves for the three phantoms studied were comparable to the CT and the planning system’s calibration curve. Mean(±SD) of the absolute difference between the iCBCT and CT for pelvis, pelvis large and head protocols were 18.4(9.7) HU, 15.5(12.9) HU and 19(13.7) HU respectively. Maximum absolute HU difference between the CT and all three iCBCT protocols were 39.0, 60.0 and 53.8. Mean(±SD) of the absolute HU difference between the planning and iCBCT for prostate, bladder, and the femoral heads were 12.4(9) HU, 7.74(5.7) HU and 60.4(21.4) HU respectively, whereas the maximum differences were 26.1 HU, 16.6 HU and 99.6 HU respectively.

Conclusion: The mean absolute HU difference between the CT and all three iCBCT protocols were all within the generally accepted tolerance of 40HU, and the mean HU difference for anatomical structures was low except for femoral heads. Further investigation will be carried out to evaluate the impact of this absolute HU deviation for femoral heads on the feasibility of using daily iCBCT for adaptive dose calculation.

Keywords

Cone-beam CT, Calibration, Treatment Planning

Taxonomy

IM/TH- Cone Beam CT: General (Most aspects)

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