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

Abstract

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 1 006 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. Key words: Neoplasm metastasis, brain/Stereotactic radiotherapy; Setup error; Residual error