Room: ePoster Forums
Purpose: To evaluate the dosimetric impact of using the Cone beam CT (CBCT) of head and neck (H&N) patients in daily kV or weekly protocols, versus daily kV imaging only.
Methods: Fifteen H&N patients whose set ups were randomly selected based on the daily kV followed by the daily CBCT. To determine the advantage of the daily CBCT, delivered dose distributions using kV image only are calculated for all of the patients. Geometrical shifts after CBCT images are regarded as the setup error of the kV-based setup. Same amount of shifts but the opposite direction is applied to determine the new isocenter to calculate the delivered dose of the day. Then, dose difference between the intended to the delivered based on kV were calculated. Patients are grouped in two; one Imaging protocol eligible group (EG), the other in-eligible group (IG). Geometrical shifts after CBCT of the EG patients are less than 3 mm for the first 3 consecutive days, otherwise, the others are grouped to be the IG.
Results: There are 13 IG (Average 3-day Couch Delta: ACD=5.33 mm, Ïƒ=2.39 mm) and 2 EG (ACD=2.6 mm, Ïƒ=0.2mm). Dose differences of D95s of PTV are 0.32 % (Ïƒ=0.61%) and 1.57% (Ïƒ=1.88%) for EG and IG, respectively. Those of GTV are 1.12% (Ïƒ=1.63%) and 0% (Ïƒ=0%), respectively. Overall average differences of OARs were generally less than 2.2% for both EG and IG. However, it is worthwhile to pay attention to individual OAR especially which has larger deviation.
Conclusion: EG patients do not need daily CBCT, but daily kV coupled with weekly CBCT may be sufficient, since dose variation due to the set up error from the kV setup is minimal. Weekly CBCT, however, may not be adequate for IG patients.