Analysis of the inter-and intra-fraction setup errors and residual errors during stereotactic radiotherapy for brain metastasis

Objective: To evaluate the necessity of arc by arc setup verification in patients with brain metastases receiving stereotactic radiotherapy (SRT) by analyzing the inter-and intra-fraction setup errors and residual errors collected from the ExacTrac X-ray portal image. Methods: Clinical data of brain metastases patients treated with SRT in the previous two years were retrospectively analyzed. The ExacTrac X-ray setup images were collected after the normal setup procedure. Setup errors were calculated by registering the cranial bony structures of the ExacTrac X-ray setup images to that of the digitally reconstructed setup images. The inter-and intra-fraction setup errors and residual errors were statistically analyzed. Results: Seventy-five patients from 116 lesions received 337 cycles of SRT of the head. The inter-and intra-fraction translational setup errors in the x, y and z directions were (0.93 ± 0.86) mm and (0.15 ± 0.59) mm; (1.83 ± 1.27) mm and (0.25 ± 0.73) mm; (0.96 ± 0.80) mm and (0.14 ± 0.56) mm, respectively. The inter-and intra-fraction rotational setup errors in the x, y, z directions were (0.65° ± 0.62°) and (0.19° ± 0.40°); (0.97° ± 0.94°) and (0.13° ± 0.25°);(0.92° ± 0.71°) and (0.10° ± 0.29°), respectively. The residual translational setup errors in the x, y, z directions were (0.06 ± 0. 23) mm, (0.08 ± 0.24) mm and (0.08 ± 0.22)mm, and (0.12° ± 0.27°), (0.09° ± 0.18°) and (0.06° ±0.19°) for the residual rotational setup errors, respectively. For a reference setup error threshold of 0.7 mm/0.7°, 99.1% of the SRT exceeded the threshold and required setup correction. For 1006 non-coplanar arcs, rotating the treatment couch from 0° to the treatment angle made 66.4% of arcs exceed the threshold and require at least once setup correction. Conclusions: During SRT for brain metastasis, the inter-and intra-fraction setup errors should be emphasized. It is necessary to perform arc by arc setup error verification. (authors)