OBJECTIVETo explore the overlap ratio of the target volume in different respiratory statuses of active breath control (ABC) and their differences during external-beam partial breast irradiation (EB-PBI), and from the perspective of target volume overlap to determine the influence of the ABC-assisted breathing condition on intra-fractional target movement of EB-PBI.

METHODSThe patients, who received breast-conserving surgery with silver clips marked at the margins of the cavity and were suitable for EB-PBI, were immobilized on the breast bracket to undertake CT simulation assisted by ABC device, six sets of CT simulation images including two sets of image in state of moderate deep inspiration breathing control (mDIBH), two sets of images in state of free breath (FB) and two sets of images in state of deep expiration breathing control (DEBH) were obtained. The six sets of images were transferred to Pinnacle(3) treatment planning system (TPS), then automatic fusion and registration between two sets of mDIBH images, two sets of FB images, two sets of DEBH images and mDIBH image and DEBH image were achieved separately. Thereafter, the overlap ratios of GTV with GTV, CTV with CTV, PTV with PTV were calculated by the Pinnacle(3) TPS. The differences between the overlap ratios of the three kinds of targets in the same registered image and the difference between the overlap ratios of the same kind of target in the different registered images were statistically analyzed using statistical package of SPSS 11.5.

RESULTSBased on mDIBH/mDIBH registration, the overlap ratios of GTV/GTV, CTV/CTV and PTV/PTV were (83.54 ± 11.41)%, (93.00 ± 6.49)%, and (95.26 ± 4.90)%, respectively, and the differences of the overlap ratios between GTV/GTV and CTV/CTV, GTV/GTV and PTV/PTV were all statistically significant (P < 0.05), but statistically not significant between CTV/CTV and PTV/PTV (P > 0.05). Based on FB/FB registration, the overlap ratios of GTV/GTV, CTV/CTV and PTV/PTV were (72.55 ± 29.10)%, (89.36 ± 9.53)% and (92.47 ± 7.25)%, respectively, and the differences of the overlap ratios between GTV/GTV and CTV/CTV, CTV/CTV and PTV/PTV were all not statistically significant (P > 0.05), but statistically significant between GTV/GTV and PTV/PTV (P < 0.05). Based on DEBH/DEBH registration, the overlap ratios of GTV/GTV, CTV/CTV and PTV/PTV were (79.48 ± 22.31)%, (92.83 ± 6.77)% and (95.05 ± 4.81)%, respectively, and the differences of the overlap ratios between GTV/GTV and CTV/CTV (P = 0.000), CTV/CTV and PTV/PTV (P = 0.037), GTV/GTV and PTV/PTV (P = 0.000) were statistically all significant (P = 0.000). The differences of the overlap ratios of GTV/GTV, CTV/CTV, and PTV/PTV (P = 0.000) between mDIBH/mDIBH and DEBH/DEBH, mDIBH/mDIBH and FB/FB, FB/FB and DEBH/DEBH were all statistically significant (P = 0.000), and not statistically significant between mDIBH/mDIBH and mDIBH/DEBH, FB/FB and mDIBH/DEBH.

CONCLUSIONSDuring the delivering of EB-PBI assisted by ABC, the intra-fractional overlap ratios of the target volume between the same breathing state is increasing in the order of GTV/GTV → CTV/CTV → PTV/PTV. The difference of the overlap ratios of the target volumes between mDIBH and mDIBH, FB and FB, DEBH and DEBH is not significant, and the overlap ratios of PTV/PTV in the three breathing statuses of mDIBH, FB and DEBH reaches a higher level. Therefore, from the perspective of target volume overlap, if the setup error is corrected online before delivering, the necessity of breathing control during delivering of EB-PBI is worthy discussing.