SU-E-T-539: The Effect of the Scattering Volume of Phantom on Dose Calculation Accuracy Using Elekta's Cone-Beam...

To analyze the scattering volume effect of phantom on HU values and dose calculation accuracy utilizing Elekta's cone-beam computed tomography (CBCT) for head and neck radiotherapy.

A water phantom was designed to simulate different scattering volume by varying its lengthfrom 5cm to 30cm.A CIRS density reference phantom combined with the water phantom was used to measure the CBCT HU values. HU-ED correction curves for 30cm length (Correction-A) and 5cm length (Correction-B) were generated for CBCT dose calculation, respectively. A four-field-box plan was designed both on CBCT and conventional CT of a head-neck anatomical phantom to compare the dosimetric difference.

The maximum HU variance in the CBCT of the water phantom was up to 10% with varying phantom length. For the combined CIRS phantom, the HU value in CBCT was found to decrease in high-density inserts, but increase in low-density inserts with increasing scatter length.For dense-bone insert, the change in HU values was up to 1422HU. For head-neck anatomical phantom, the dose results showed an average difference of 0.3% of cGy /MU, 2-3mm of isodose discrepancy,and 97% of 2%/2mm DTA index for each beam with Correction-A; and 3.7% of cGy /MU,1.5-3cm of isodose discrepancy,and 59.5% of 2%/2mm DTA index for each beam with Correction-B.

The scattering volume of phantom has a significant impact on the HU value and further HU-D calibration of Elekta's CBCT. Excellent dosimetric agreement between CBCT and conventional CT was found when the HU-D calibration phantom volume is close to patient volume imaged by CBCT.