A study on the necessity of kV-CBCT imaging compared to kV-orthogonal portal imaging based on setup errors: considering other socioeconomical factors.

Purpose: Evaluation of setup accuracy in kV-orthogonal portal imaging (OPI)-based and kV-CBCT-based radiotherapy treatment and to find out the necessity of cone-beam computed tomography (CBCT) compared to OPI.

Materials And Methods: A retrospective study was carried out on 30 patients, who received radiotherapy to the Brain, Head and Neck, and Pelvis. In the OPI technique, anterior-posterior and right-lateral portal images were taken by the On Board Imaging (OBI) system and were superimposed on the reference images. Similarly, in the kV-CBCT technique, CBCT was performed by the OBI system and CBCT images were superimposed on the reference CT images. A total of 150 comparison sets of kV-OPI and kV-CBCT images were analyzed and evaluated. Shifts in the Lateral, Longitudinal, and Vertical directions were noted in both techniques. The iso displacement vector (IDV) was calculated for all imaging.

Results: The mean IDV (in cm) are found to be 0.3395 (SD: 0.1477) and 0.3088 (SD: 0.1593) in cases of the brain, 0.4266 (SD: 0.1511) and 0.3666 (SD: 0.1533) in cases of the head and neck, and 1.0339 (SD: 0.5893) and 0.9498 (SD: 0.6047) in cases of the pelvis for the CBCT and OPI techniques, respectively. The P values were 0.3201, 0.0515, and 0.4829 for the brain, head and neck, and pelvic cases, respectively.

Conclusions: There is statistically no significant difference between both the imaging techniques. As the dose delivered by the CBCT technique is higher than that by the OPI technique, from the socioeconomical and radiation safety point of view, the OPI technique is possibly better than the CBCT technique. Hence, it can be concluded that CBCT is not a mandatory technique compared to the OPI technique in routine brain, head and neck, and pelvic cases, except in those cases where better information about interfraction movements of soft tissue is necessarily required for positioning of the target, as is the case in prostate carcinoma.