Room: AAPM ePoster Library
Purpose: Many approaches to adaptive planning require calculating dose on a CBCT. CBCT scans contain more scatter than fan CT scans, resulting in inaccurate Hounsfield units, which can limit CBCT use for dose calculation. Varian’s new iterative cone beam (iCBCT) reconstruction removes this scatter, has been demonstrated to decrease image noise, and may provide more accurate HU for dose calculations for adaptive planning.
Methods: We used the CIRS electron density phantom and added sheets of 1 cm bolus to give phantoms of diameter, 18, 20, 24, 28 and 32 cm. Each phantom was scanned with the iCBCT pelvis protocol and the non-iterative CBCT pelvis protocol. The HU values of tissue equivalent inserts were found. HU to electron density curves were made using the 18 cm and 32 cm diameter phantoms. The calibration curves were used to calculate dose on CBCT scans of the CIRS thorax phantom, keeping the MU’s fixed. Dose was compared to plans calculated on a planning fan beam CT.
Results: Increasing the diameter of the phantom, the change in HU was much less for the iCBCT reconstruction. Compared to a full arc VMAT plan calculated on a planning CT, using iCBCT calibration curves from 18 and 32 cm diameter phantoms resulted in a higher mean target dose of 0.8% and 1.3% respectively. For the non-iterative CBCT, using calibration curves from 18 and 32 cm diameter phantoms resulted in a higher mean target dose of 0.8% and 2.0% respectively.
Conclusion: Increasing the radial diameter of a phantom has less of an impact on the HU values for the new Varian iCBCT reconstruction compared to the non-iterative reconstruction. For the iCBCT this results in a more accurate dose calculation for phantoms of different sizes. With more investigation iCBCT scans can potentially be used for dose calculation.
Funding Support, Disclosures, and Conflict of Interest: C. Bojechko, Grant funding from Varian Medical Systems.
Not Applicable / None Entered.