Background

Calculations performed on a single radiotherapy planning CT scan do not accurately represent dose delivery during prostate radiotherapy because of inter- and intra-fraction organ motion. Cone beam CT (CBCT) scanning during the delivery of arc radiotherapy allows organ motion to be monitored. We studied the quality of simultaneous CBCTs.

Method

Forty-nine patients with localised prostate cancer were treated with radical VMAT radiotherapy. Treatment was delivered using a single 8 or 10 MV arc. Standard and simultaneous CBCT's were captured on fractions 1, 6, 11 and 16. The quality of each CBCT image was assessed independently by a clinician and a radiographer using a validated scoring tool. The observers were blinded to whether the scan was a standard or simultaneous CBCT.

Results

392 CBCT scans were performed on 49 patients. 74 simultaneous CBCT scans were performed with a mean arc delivery time of 120 seconds during which 688 CBCT frames were acquired. Following a software upgrade the subsequent 122 simultaneous CBCTs were obtained with a mean arc delivery time of 83 seconds during which 502 CBCT frames were acquired.

There was moderate agreement in image quality scores between the two observers (kappa statistic 0.44). The independent observers agreed that 193 (98%) standard CBCT images and 99 (51%) simultaneous CBCTs were clinically useful. Following the software upgrade the proportion of simultaneous CBCTs which were deemed clinically useful fell from 93% to 67% and from 95% to 85% for observers 1 and 2 respectively.

Conclusion

Simultaneous CBCT scanning can produce clinically useful images. These images could be used to more accurately model the actual dose received by the prostate, rectum and bladder during prostate radiotherapy. Factors which potentially affect simultaneous CBCT image quality include gantry speed, treatment beam energy, CBCT imaging dose and number of CBCT frames acquired. The simultaneous CBCT technique warrants further development.