SU‐E‐J‐103: Preliminary Evaluation of the Dosimetric Accuracy of the Cone‐Beam CT for the Cases with the Respiratory Motion

Abstract

Purpose: Cone‐beam computed tomography (CBCT) images are currently used for patient positioning and adaptive dose calculation. However, it is still interesting to know how large the uncertainty of CBCT for the cases of respiratory motion. This study evaluated the uncertainty of the dose calculation based on CBCT for the cases of the respiratory motion. Methods: The geometrical and Hounsfield Unit (HU) differences estimated for CT and CBCT by using the caphan phantom (The Phantom Laboratory, Salem, NY). The ECLIPSE (Varian Medical Systems, Palo Alto, CA) radiation treatment planning system was used the dose calculations both of CT and CBCT image to compare the dose distributions. HU and dose calculation Result are compared between CT and CBCT to evaluate the effect by respiratory motion. Results: A geometrical uncertainties of CT and CBCT image of the catphan phantom were measured less than 0.15 cm. The CBCT gives a relatively large uncertainty, but absolute value of the uncertainty was not significant. The HU difference between CT and CBCT with standard‐dose‐head mode, high‐quality‐head mode, normal pelvis mode and low‐dose‐thorax mode measured as 31, 36, 23 and 33 HU. The dose calculation difference estimated as gamma (3%, 0.3 cm) as less than 1 in 99% of area. The HU difference with motion phantom was ranged from 5 to 450 HU. The dose calculation difference was estimated as gamma as less than 1 in 98% of area. Conclusion: We evaluated the uncertainty of CBCT based dose calculation for cases of the respiratory motion. The dosimetric uncertainty by motion is not significant in the case of the planning CT with 4DCT MIPS for whole phase, because of CBCT gives similar scanning Result of their scan time (about 1minute). On the other hand, it is expected that the CBCT base adaptive planning with respiratory gate treatment will give the relatively large uncertainty.