OBJECTIVETo investigate the influence of registration based on different reference markers on the displacement of the geometry consisted of all clips in the cavity for external-beam partial breast irradiation at moderate deep inspiration breath holding assisted by active breathing control device.

METHODSTwenty-seven early stage breast cancer patients feasible for external beam partial breast irradiation (EB-PBI) were selected. The patients undertaken three-dimensional computed tomography (3DCT) simulation scan at moderate deep inspiration breath holding (mDIBH) assisted by active breathing control device, and two sets of mDIBH CT images were got and transferred to the Pinnacle 3 planning system. All of the silver clips were delineated and a geometry consisted of all clips were generated. On the account of automatic registration of mDIBH CT images, manual registration was carried out based separately on the topside clip in the cavity, the labeled skin at anterior surface of the cavity at central level and the metal mark on the body surface near the cavity, then the displacements of center of the geometry in left-right (LR), anterior-posterior (AP) and superior-inferior (SI) directions based separately on the three registrations were measured.

RESULTSThe displacements of center of the geometry in LR, AP and SI directions based on registration of the clips, the labeled skin and the metal mark were (0.61 ± 0.62)mm vs. (1.11 ± 1.21)mm vs. (1.31 ± 1.55)mm, (0.63 ± 0.59)mm vs. (0.92 ± 0.93)mm vs. (1.19 ± 1.24)mm and (0.91 ± 0.96)mm vs. (2.13 ± 2.12)mm vs. (1.93 ± 1.55)mm, respectively. Compared the displacements of center of the geometry in the same direction between the three registrations, significant differences were found only in SI direction between clip registration and skin registration, clip registration and mark registration (t = 5.045, 7.210 and P = 0.025, 0.007) . Compared the displacements of center of the geometry between three dimensional directions for each reference registration, there was no significant difference (all P > 0.05).

CONCLUSIONSWhen EB-PBI is carried out in state of mDIBH, measurement of the intrafraction displacement based on registration of the clip in the cavity is a reasonable selection. Otherwise, excessive margin enlargement of PTV in SI direction will be generated if the regional skin or metal mark is selected as registration reference.