CBCT Guided Interfraction Absolute Displacement and Setup Error Assessment during Breast Prone Radiotherapy

To calculate interfraction absolute displacement/shift and setup error using cone beam computed tomography (CBCT) during breast prone radiotherapy.Fifty-nine patients undergoing prone whole breast-irradiation after breast-conserving surgery (BCS) were studied as part of an institutional review board- approved prospective trial. Setup precision was monitored using a daily online CBCT. Translational shifts in 3 axes (AP: anterior posterior; LR: left right; and SI: superior inferior) and 3 rotations (pitch, roll, and rtn) after CBCT were analyzed for 1062 treatment fractions. The random and systematic setup errors (SE) were calculated and were analyzed for time trends during the course of radiotherapy.For absolute inter-fractional shifts, the numbers of fractions exceeding 10 mm in the AP, LR, and SI directions were 6.5%, 17.42% and 8.92%, respectively; 0%, 0% and 1.31% fractions exceeded 3°for pitch, roll, and Rtn, respectively. The population systematic errors were 1.89/2.91/1.98 mm in AP/SI/LR directions, while the random error were 2.72/3.99/3.31 mm. In pitch, roll, rtn rotations, the population systematic error were 0.64°/0.49°/0.46°, and 0.89°/0.90°/0.93° for the random error. Without correction these would correspond to a clinical to planning target volume margin of 6.64/10.08/7.26 mm in AP/SI/LR directions and 2.22°/1.79°/1.8° in pitch, roll, rtn rotations. The magnitude of inter-fraction motion was not correlated with patient treatment time accept in AP direction (P = 0.000).SE was larger in prone position in breast cancer patients, attributable mostly to random errors, which emphasize the need for on-line imaging guidance in breast prone radiotherapy. 10mm margins would adequately cover the target volume and account for setup errors in the absence of IGRT.