Detection of intrafractional tumour position error in radiotherapy utilizing cone beam computed tomography.

PURPOSE To study the intrafraction tumour position error utilizing cone beam CT (CBCT) in patients receiving radiotherapy. METHODS AND MATERIALS Fifty-four patients were treated with Elekta Synergy S system, including 19 head and neck, 25 thoracic and 10 abdominal-pelvic tumours. All patients received CBCT after initial setup and some of them received CBCT after correction and after treatment. CBCT were registered to planning CT and errors of isocenter position on the left-to-right (LR), superior-inferior (SI) and anterior-posterior (AP) directions were analyzed. RESULTS After treatment the systematic (Sigma) and random uncertainty (sigma) increased, the increments of tumour Sigma were 0.1-0.3, 0.2-0.5 and 0.2-0.6 mm, respectively, while the increments of sigma were 0.1-0.3, 0.2-0.4 and 0.1-0.4 mm, respectively, for the head and neck, thoracic and abdominal-pelvic tumours. Based on 380 paired pre- and post-treatment CBCT, the intrafraction errors (mean+/-SD) in the LR, SI and AP directions were -0.1+/-0.9, -0.3+/-1.0 and -0.2+/-0.7 mm, respectively, for head and neck tumours, -0.1+/-1.2, -0.1+/-1.9 and 0.1+/-1.3 mm, respectively, for thoracic tumours, -0.1+/-1.1, 0.2+/-1.4 and -0.1+/-1.5 mm, respectively, for abdominal-pelvic tumours. Isotropic planning margins of 3.4, 6.1 and 5.4 mm were generated with linear addition of internal margin (IM) to CTV for the head and neck, thoracic and abdominal-pelvic tumours, respectively, while margins were only 2.4, 4.4 and 3.9 mm, respectively, if IM was added in quadrature. CONCLUSIONS Utilizing CBCT measurements before and after treatment to detect intrafraction tumour position errors was clinically feasible. The detected intrafraction errors could be applied to improve the accuracy of radiation delivery.